PR13513: undo PR11759 PREEMPT_RT hack

[systemtap.git] / NEWS

* What's new in version 1.7

- The stap-server service (initscript) now supports four new options:
    -D MACRO[=VALUE]
    --log LOGFILE
    --port PORT-NUMBER
    --SSL CERT-DATABASE
  These allow the specification of macro definitions to be passed to stap
  by the server, the location of the log file, network port number and
  NSS certificate database location respectively. These options are also
  supported within individual server configuration files. See stap-server
  and initscript/README.stap-server for details.

- process("PATH").[library("PATH")].function("NAME").exported probes are now
  supported to filter function() to only exported instances.

- The translator supports a new --suppress-handler-errors option, which
  causes most runtime errors to be turned into quiet skipped probes.  This
  also disables the MAXERRORS and MAXSKIPPED limits.

- The translator supports a new --modinfo NAME=VALUE option to emit additional
  MODULE_INFO(n,v) macros into the generated code.

- There is no more fixed maximum number of VMA pages that will be tracked
  at runtime. This reduces memory use for those scripts that don't need any,
  or only limited target process VMA tracking and allows easier system
  wide probes inspecting shared library variables and/or user backtraces.
  stap will now silently ignore -DTASK_FINDER_VMA_ENTRY_ITEMS.

- The tapset functions remote_id() and remote_uri() identify the member of a
  swarm of "stap --remote FOO --remote BAR baz.stp" concurrent executions.

- Systemtap now supports a new privilege level and group, "stapsys", which
  is equivalent to the privilege afforded by membership in the group "stapdev",
  except that guru mode (-g) functionality may not be used. To support this, a
  new option, --privilege=[stapusr|stapsys|stapdev] has been added.
  --privilege=stapusr is equivalent to specifying the existing --unprivileged
  option. --privilege=stapdev is the default. See man stap(1) for details.

- Scripts that use kernel.trace("...") probes compile much faster.

- SDT can now define up to 12 arguments in a probe point.

- Parse errors no longer generate a cascade of false errors.  Instead, a
  parse error skips the rest of the current probe or function, and resumes
  at the next one.  This should generate fewer and better messages.

- Global array wrapping is now supported for both associative and statistics typed
  arrays using the '%' character to signify a wrapped array. For example,
  'global foo%[100]' would allow the array 'foo' to be wrapped if more than 100
  elements are inserted.

- process("PATH").library("PATH").plt("NAME") probes are now supported.
  Wildcards are supported in the plt-name part, to refer to any function in the
  program linkage table which matches the glob pattern and the rest of the
  probe point.

- A new option, --dump-probe-types, will dump a list of supported probe types.
  If --unprivileged is also specified, the list will be limited to probe types
  which are available to unprivileged users.

- Systemtap can now automatically download the required debuginfo
  using abrt. The --download-debuginfo[=OPTION] can be used to
  control this feature. Possible values are: 'yes', 'no', 'ask',
  and a positive number representing the timeout desired. The 
  default behavior is to not automatically download the debuginfo.

- On 2.6.29+ kernels, systemtap can now probe kernel modules that
  arrive and/or depart during the run-time of a session.  This allows
  probing of device driver initialization functions, which had been
  blacklisted.

- The following probe types are deprecated in release 1.7 and will be
  removed in release 1.8:
      kernel.function(number).inline
      module(string).function(number).inline
      process.function(number).inline
      process.library(string).function(number).inline
      process(string).function(number).inline
      process(string).library(string).function(number).inline

- The systemtap-grapher is deprecated in release 1.7 and will be removed in
  release 1.8.

- The task_backtrace() tapset function was deprecated in 1.6 and has been
  removed in 1.7.

- MAXBACKTRACE did work in earlier releases, but has now been documented
  in the stap 1 manual page.

- New tapset function probe_type(). Returns a short string describing
  the low level probe handler type for the current probe point.

- Both unwind and symbol data is now only collected and emitted for
  scripts actually using backtracing or function/data symbols.
  Tapset functions are marked with /* pragma:symbols */ or
  /* pragma:unwind */ to indicate they need the specific data.

- Kernel backtraces can now be generated for non-pt_regs probe context
  if the kernel support dump_trace(). This enables backtraces from
  certain timer probes and tracepoints.

- ubacktrace() should now also work for some kernel probes on x86 which can
  use the dwarf unwinder to recover the user registers to provide
  more accurate user backtraces.

- For s390x the systemtap runtime now properly splits kernel and user
  addresses (which are in separate address spaces on that architecture)
  which enable user space introspection.

- ppc and s390x now supports user backtraces through the DWARF unwinder.

- ppc now handles function descriptors as symbol names correctly.

- arm support kernel backtraces through the DWARF unwinder.

- arm now have a uprobes port which enables user probes. This still
  requires some kernel patches (user_regsets and tracehook support for
  arm).

- Starting in release 1.7, these old variables will be deprecated:
  - The 'pid' variable in the 'kprocess.release' probe has been
    deprecated in favor of the new 'released_pid' variable.
  - The 'args' variable in the
    '_sunrpc.clnt.create_client.rpc_new_client_inline' probe has been
    deprecated in favor of the new internal-only '__args' variable.

* What's new in version 1.6, 2011-07-25

- Security fixes for CVE-2011-2503: read instead of mmap to load modules,
  CVE-2011-2502: Don't allow path-based auth for uprobes

- The systemtap compile-server no longer uses the -k option when calling the
  translator (stap). As a result, the server will now take advantage of the
  module cache when compiling the same script more than once. You may observe
  an improvement in the performance of the server in this situation.

- The systemtap compile-server and client now each check the version of the
  other, allowing both to adapt when communicating with a down-level
  counterpart. As a result, all version of the client can communicate
  with all versions of the server and vice-versa. Client will prefer newer
  servers when selecting a server automatically.

- SystemTap has improved support for the ARM architecture.  The
  kread() and kwrite() operations for ARM were corrected allowing many
  of the tapsets probes and function to work properly on the ARM
  architecture.

- Staprun can now rename the module to a unique name with the '-R' option before 
  inserting it. Systemtap itself will also call staprun with '-R' by default. 
  This allows the same module to be inserted more than once, without conflicting 
  duplicate names.

- Systemtap error messages now provide feedback when staprun or any other
  process fails to launch.  This also specifically covers when the user 
  doesn't have the proper permissions to launch staprun.

- Systemtap will now map - to _ in module names.  Previously, 
  stap -L 'module("i2c-core").function("*")' would be empty.  It now returns
  a list had stap -L 'module("i2c_core").function("*") been specified.

- Systemtap now fills in missing process names to probe points, to
  avoid having to name them twice twice:
  % stap -e 'probe process("a.out").function("*") {}' -c 'a.out ...'
  Now the probed process name is inferred from the -c CMD argument.
  % stap -e 'probe process.function("*") {}' -c 'a.out ...'

- stap -L 'process("PATH").syscall' will now list context variables

- Depends on elfutils 0.142+.

- Deprecated task_backtrace:string (task:long). This function will go
  away after 1.6. Please run your scripts with stap --check-version.

* What's new in version 1.5, 2011-05-23

- Security fixes for CVE-2011-1781, CVE-2011-1769: correct DW_OP_{mod,div}
  division-by-zero bug

- The compile server and its related tools (stap-gen-ert, stap-authorize-cert,
   stap-sign-module) have been re-implemented in C++. Previously, these
   components were a mix of bash scripts and C code. These changes should be
   transparent to the end user with the exception of NSS certificate database
   password prompting (see below). The old implementation would prompt more
   than once for the same password in some situations.
  
- eventcount.stp now allows for event counting in the format of
  'stap eventcount.stp process.end syscall.* ...', and also reports
  corresponding event tid's.

- Systemtap checks that the build-id of the module being probed matches the
  build-id saved in the systemtap module.  Invoking systemtap with
  -DSTP_NO_BUILDID_CHECK will bypass this build-id runtime verification.  See
  man ld(1) for info on --build-id.

- stapio will now report if a child process has an abnormal exit along with
  the associated status or signal.

- Compiler optimization may sometimes result in systemtap not being able to
  access a user-space probe argument.  Compiling the application with
  -DSTAP_SDT_ARG_CONSTRAINT=nr will force the argument to be an immediate or
  register value which should enable systemtap to access the argument.

- GNU Gettext has now been intergrated with systemtap.  Our translation
  page can be found at http://www.transifex.net/projects/p/systemtap/ .
  "make update-po" will generate the necessary files to use translated
  messages.  Please refer to the po/README file for more info and 
  please consider contributing to this I18N effort!

- The new addr() function returns the probe's instruction pointer.

- process("...").library("...") probes are now supported.  Wildcards
  are supported in the library-name part, to refer to any shared
  library that is required by process-name, which matches the glob
  pattern and the rest of the probe point.

- The "--remote USER@HOST" functionality can now be specified multiple times
  to fan out on multiple targets.  If the targets have distinct kernel and
  architecture configurations, stap will automatically build the script
  appropriately for each one.  This option is also no longer considered
  experimental.

- The NSS certificate database generated for use by the compile server is now
  generated with no password. Previously, a random password was generated and
  used to access the database. This change should be transparent to most users.
  However, if you are prompted for a password when using systemtap, then
  running $libexecdir/stap-gen-cert should correct the problem.

- The timestamp tapset includes jiffies() and HZ() for lightweight approximate
  timekeeping.

- A powerful new command line option --version has been added.

- process.mark now supports $$parms for reading probe parameters.

- A new command line option, --use-server-on-error[=yes|no] is available
  for stap.  It instructs stap to retry compilation of a script using a
  compile server if it fails on the local host.  The default setting
  is 'no'.

- The following deprecated tools have been removed:
      stap-client
      stap-authorize-server-cert
      stap-authorize-signing-cert
      stap-find-or-start-server
      stap-find-servers
  Use the --use-server, --trust-server and --list-servers options of stap
  instead.

* What's new in version 1.4, 2011-01-17

- Security fixes for CVE-2010-4170, CVE-2010-4171: staprun module
  loading/unloading

- A new /* myproc-unprivileged */ marker is now available for embedded C
  code and and expressions. Like the /* unprivileged */ marker, it makes
  the code or expression available for use in unprivileged mode (see
  --unprivileged). However, it also automatically adds a call to
  assert_is_myproc() to the code or expression, thus, making it available
  to the unprivileged user only if the target of the current probe is within
  the user's own process.

- The experimental "--remote USER@HOST" option will run pass 5 on a given
  ssh host, after building locally (or with --use-server) for that target.

- Warning messages from the script may now be suppressed with the stap
  and/or staprun -w option.  By default, duplicate warning messages are
  suppressed (up to a certain limit).  With stap --vp 00002 and above,
  the duplicate elimination is defeated.

- The print_ubacktrace and usym* functions attempt to print the full
  path of the user-space binaries' paths, instead of just the basename.
  The maximum saved path length is set by -DTASK_FINDER_VMA_ENTRY_PATHLEN,
  default 64.  Warning messages are produced if unwinding fails due to
  a missing 'stap -d MODULE' option, providing preloaded unwind data.

- The new tz_ctime() tapset function prints times in the local time zone.

- More kernel tracepoints are accessible to the kernel.trace("...") mechanism,
  if kernel source trees or debuginfo are available.  These formerly "hidden"
  tracepoints are those that are declared somewhere other than the usual
  include/linux/trace/ headers, such as xfs and kvm.

- debuginfo-based process("...").function/.statement/.mark probes support
  wildcards in the process-name part, to refer to any executable files that
  match the glob pattern and the rest of the probe point.

- The -t option now displays information per probe-point rather than a summary
  for each probe.  It also now shows the derivation chain for each probe-point.

- A rewrite of the sys/sdt.h header file provides zero-cost startup (few or
  no ELF relocations) for the debuginfo-less near-zero-cost runtime probes.
  Binaries compiled with earlier sdt.h versions remain supported.  The
  stap -L (listing) option now lists parameters for sys/sdt.h markers.

- The implementation of the integrated compile-server client has been
  extended.
  o --use-server now accepts an argument representing a particular server and
    may be specified more than once.
  o --list-servers now accepts an expanded range of arguments.
  o a new --trust-servers option has been added to stap to replace several
    old certificate-management scripts.
  o The following tools are now deprecated and will be removed in release 1.5:
      stap-client
      stap-authorize-server-cert
      stap-authorize-signing-cert
      stap-find-or-start-server
      stap-find-servers
  See man stap(1) for complete details.

- The compile-server now returns the uprobes.ko to the client when it is
  required by the script being compiled. The integrated compile-server client
  now makes it available to be loaded by staprun. The old (deprecated)
  stap-client does not do this.

- process probes with scripts as the target are recognized by stap and the
  interpreter would be selected for probing.

- Starting in release 1.5, these old variables/functions will be deprecated
  and will only be available when the '--compatible=1.4' flag is used:

  - In the 'syscall.add_key' probe, the 'description_auddr' variable
    has been deprecated in favor of the new 'description_uaddr'
    variable.
  - In the 'syscall.fgetxattr', 'syscall.fsetxattr',
    'syscall.getxattr', 'syscall.lgetxattr', and
    'syscall.lremovexattr' probes, the 'name2' variable has been
    deprecated in favor of the new 'name_str' variable.
  - In the 'nd_syscall.accept' probe the 'flag_str' variable
    has been deprecated in favor of the new 'flags_str' variable.
  - In the 'nd_syscall.dup' probe the 'old_fd' variable has been
    deprecated in favor of the new 'oldfd' variable.
  - In the 'nd_syscall.fgetxattr', 'nd_syscall.fremovexattr',
    'nd_syscall.fsetxattr', 'nd_syscall.getxattr', and
    'nd_syscall.lremovexattr' probes, the 'name2' variable has been 
    deprecated in favor of the new 'name_str' variable.
  - The tapset alias 'nd_syscall.compat_pselect7a' was misnamed.  It should
    have been 'nd_syscall.compat_pselect7' (without the trailing 'a').
  - The tapset function 'cpuid' is deprecated in favor of the better known 
    'cpu'.
  - In the i386 'syscall.sigaltstack' probe, the 'ussp' variable has
    been deprecated in favor of the new 'uss_uaddr' variable.
  - In the ia64 'syscall.sigaltstack' probe, the 'ss_uaddr' and
    'oss_uaddr' variables have been deprecated in favor of the new
    'uss_uaddr' and 'uoss_uaddr' variables.
  - The powerpc tapset alias 'syscall.compat_sysctl' was deprecated
    and renamed 'syscall.sysctl32'.
  - In the x86_64 'syscall.sigaltstack' probe, the 'regs_uaddr'
    variable has been deprecated in favor of the new 'regs' variable.

* What's new in version 1.3, 2010-07-21

- The uprobes kernel module now has about half the overhead when probing
  NOPs, which is particularly relevant for sdt.h markers.

- New stap option -G VAR=VALUE allows overriding global variables
  by passing the settings to staprun as module options.

- The tapset alias 'syscall.compat_pselect7a' was misnamed.  It should
  have been 'syscall.compat_pselect7' (without the trailing 'a').
  Starting in release 1.4, the old name will be deprecated and
  will only be available when the '--compatible=1.3' flag is used.

- A new procfs parameter .umask(UMASK) which provides modification of
  file permissions using the proper umask value.  Default file
  permissions for a read probe are 0400, 0200 for a write probe, and
  0600 for a file with a read and write probe.

- It is now possible in some situations to use print_ubacktrace() to
  get a user space stack trace from a kernel probe point. e.g. for
  user backtraces when there is a pagefault:
  $ stap -d /bin/sort --ldd -e 'probe vm.pagefault {
      if (pid() == target()) {
         printf("pagefault @0x%x\n", address); print_ubacktrace();
      } }' -c /bin/sort
  [...]
  pagefault @0x7fea0595fa70
   0x000000384f07f958 : __GI_strcmp+0x12b8/0x1440 [libc-2.12.so]
   0x000000384f02824e : __gconv_lookup_cache+0xee/0x5a0 [libc-2.12.so]
   0x000000384f021092 : __gconv_find_transform+0x92/0x2cf [libc-2.12.so]
   0x000000384f094896 : __wcsmbs_load_conv+0x106/0x2b0 [libc-2.12.so]
   0x000000384f08bd90 : mbrtowc+0x1b0/0x1c0 [libc-2.12.so]
   0x0000000000404199 : ismbblank+0x39/0x90 [sort]
   0x0000000000404a4f : inittables_mb+0xef/0x290 [sort]
   0x0000000000406934 : main+0x174/0x2510 [sort]
   0x000000384f01ec5d : __libc_start_main+0xfd/0x1d0 [libc-2.12.so]
   0x0000000000402509 : _start+0x29/0x2c [sort]
  [...]

- New tapset functions to get a string representation of a stack trace:
  sprint_[u]backtrace() and sprint_[u]stack().

- New tapset function to get the module (shared library) name for a
  user space address umodname:string(long). The module name will now
  also be in the output of usymdata() and in backtrace addresses even
  when they were not given with -d at the command line.

- Kernel backtraces are now much faster (replaced a linear search
  with a binary search).

- A new integrated compile-server client is now available as part of stap.

  o 'stap --use-server ...' is equivalent to 'stap-client ...'
  o 'stap --list-servers' is equivalent to 'stap-find-servers'
  o 'stap --list-servers=online' is equivalent to 'stap-find-servers --all'
  o stap-client and its related tools will soon be deprecated.
  o the nss-devel and avahi-devel packages are required for building stap with
    the integrated client (checked during configuration).
  o nss and avahi are required to run the integrated client.

- A new operator @entry is available for automatically saving an expression
  at entry time for use in a .return probe.
   probe foo.return { println(get_cycles() - @entry(get_cycles())) }

- Probe $target variables and @cast() can now use a suffix to print complex
  data types as strings.  Use a single '$' for a shallow view, or '$$' for a
  deeper view that includes nested types.  For example, with fs_struct:
   $fs$ : "{.users=%i, .lock={...}, .umask=%i,
            .in_exec=%i, .root={...}, .pwd={...}}"
   $fs$$ : "{.users=%i, .lock={.raw_lock={.lock=%u}}, .umask=%i, .in_exec=%i,
             .root={.mnt=%p, .dentry=%p}, .pwd={.mnt=%p, .dentry=%p}}"

- The <sys/sdt.h> user-space markers no longer default to an implicit
  MARKER_NAME_ENABLED() semaphore check for each marker.  To check for
  enabled markers use a .d declaration file, then:
     if (MARKER_NAME_ENABLED()) MARKER_NAME()

- Hyphenated <sys/sdt.h> marker names such as process(...).mark("foo-bar")
  are now accepted in scripts.  They are mapped to the double-underscore
  form ("foo__bar").

- More robust <sys/sdt.h> user-space markers support is included.  For
  some platforms (x86*, ppc*), this can let systemtap probe the markers
  without debuginfo.  This implementation also supports preserving
  the "provider" name associated with a marker:
    probe process("foo").provider("bar").mark("baz") to match
    STAP_PROBE<n>(bar, baz <...>)
  (Compile with -DSTAP_SDT_V1 to revert to the previous implementation.
  Systemtap supports pre-existing or new binaries using them.)

- Embedded-C may be used within expressions as values, when in guru mode:
     num = %{ LINUX_VERSION_CODE %}               // int64_t
     name = %{ /* string */ THIS_MODULE->name %}  // const char*
     printf ("%s %x\n", name, num)
  The usual /* pure */, /* unprivileged */, and /* guru */ markers may be used 
  as with embedded-C functions.

- By default the systemtap-runtime RPM builds now include a shared
  library, staplog.so, that allows crash to extract systemtap data from
  a vmcore image.

- Iterating with "foreach" can now explicitly save the value for the loop.
     foreach(v = [i,j] in array)
       printf("array[%d,%s] = %d\n", i, j, v /* array[i,j] */)

- The new "--ldd" option automatically adds any additional shared
  libraries needed by probed or -d-listed userspace binaries to the -d
  list, to enable symbolic backtracing through them.  Similarly, the
  new "--all-modules" option automatically adds any currently loaded
  kernel modules (listed in /proc/modules) to the -d list.

- A new family of set_kernel_* functions make it easier for gurus to write
  new values at arbitrary memory addresses.

- Probe wildcards can now use '**' to cross the '.' separator.
     $ stap -l 'sys**open'
     syscall.mq_open
     syscall.open

- Backward compatibility flags (--compatible=VERSION, and matching
  script preprocessing predicate %( systemtap_v CMP "version" %)
  and a deprecation policy are being introduced, in case future
  tapset/language changes break valid scripts.

* What's new in version 1.2, 2010-03-22

- Prototype support for "perf events", where the kernel supports the
  2.6.33 in-kernel API.  Probe points may refer to low-level 
  perf_event_attr type/config numbers, or to a number of aliases
  defined in the new perf.stp tapset:
     probe perf.sw.cpu_clock, perf.type(0).config(4) { }

- Type-casting can now use multiple headers to resolve codependencies.
     @cast(task, "task_struct",
           "kernel<linux/sched.h><linux/fs_struct.h>")->fs->umask

- Tapset-related man pages have been renamed.  'man -k 3stap' should show
  the installed list, which due to prefixing should no longer collide over
  ordinary system functions.

- User space marker arguments no longer use volatile if the version of gcc,
  which must be at least 4.5.0, supports richer DWARF debuginfo.  Use cflags
  -DSTAP_SDT_VOLATILE=volatile or -DSTAP_SDT_VOLATILE= when building
  the sys/sdt.h application to override this one way or another.

- A new construct for error handling is available.  It is similar to c++
  exception catching, using try and catch as new keywords.  Within a handler
  or function, the following is valid and may be nested:
     try { /* arbitrary statements */ }
     catch (er) { /* e.g. println("caught error ", er) */ }

- A new command line flag '-W' forces systemtap to abort translation of
  a script if any warnings are produced.  It is similar to gcc's -Werror.
  (If '-w' is also supplied to suppress warnings, it wins.)

- A new predicate @defined is available for testing whether a
  particular $variable/expression is resolvable at translate time:
  probe foo { if (@defined($bar)) log ("$bar is available here") }

- Adjacent string literals are glued together, making this
  construct valid:
     probe process("/usr" @1 "/bin").function("*") { ... }

- In order to limit potential impact from future security problems, 
  the stap-server process does not permit its being launched as root.

- On recent kernels, for some architectures/configurations, hardware
  breakpoint probes are supported.  The probe point syntax is:

     probe kernel.data(ADDRESS).write
     probe kernel.data(ADDRESS).length(LEN).write
     probe kernel.data("SYMBOL_NAME").write

* What's new in version 1.1, 2010-01-15

- New tracepoint based tapset for memory subsystem.

- The loading of signed modules by staprun is no longer allowed for
  ordinary, unprivileged users.  This means that only root, members of
  the group 'stapdev' and members of the group 'stapusr' can load
  systemtap modules using staprun, stap or stap-client.  The minimum
  privilege required to run arbitrary --unprivileged scripts is now
  'stapusr' membership.

- The stap-server initscript is available. This initscript allows you
  to start systemtap compile servers as a system service and to manage
  these servers as a group or individually. The stap-server initscript
  is installed by the systemtap-server rpm.  The build directory for
  the uprobes module (/usr/share/systemtap/runtime/uprobes) is made
  writable by the 'stap-server' group. All of the files generated when
  building the uprobes module, including the digital signature, are
  also writable by members of stap-server.

  See initscript/README.stap-server for details.

- Some of the compile server client, server and certificate management
  tools have been moved from $bindir to $libexecdir/systemtap.
  You should use the new stap-server script or the stap-server initscript
  for server management where possible. The stap-server script provides the same
  functionality as the stap-server initscript except that the servers are
  run by the invoking user by default as opposed to servers started by the
  stap-server initscript which are run by the user stap-server
  by default. See stap-server(8) for more information.

  You may continue to use these tools by adding $libexecdir/systemtap to
  your path. You would need to do this, for example, if you are not root,
  you want to start a compile server and you are not running systemtap from a
  private installation. In this case you still need to use stap-start-server.

- Any diagnostic output line that starts with "ERROR", as in
  error("foo"), will promote a "Pass 5: run failed", and the return
  code is 1.

- Systemtap now warns about global variables being referenced from other
  script files.  This aims to protect against unintended local-vs-global 
  namespace collisions such as:

     % cat some_tapset.stp
     probe baz.one = bar { foo = $foo; bar = $bar }
     % cat end_user_script.stp
     global foo # intended to be private variable
     probe timer.s(1) { foo ++ }
     probe baz.* { println(foo, pp()) }
     % stap end_user_script.stp
     WARNING: cross-file global variable reference to foo from some_tapset.stp

- Preprocessor conditional for kernel configuration testing:
  %( CONFIG_foo == "y" %? ... %) 

- ftrace(msg:string) tapset function to send strings to the system-wide
  ftrace ring-buffer (if any).

- Better support for richer DWARF debuginfo output from GCC 4.5
  (variable tracking assignments). Kernel modules are now always resolved
  against all their dependencies to find any info referring to missing
  symbols. DW_AT_const_value is now supported when no DW_AT_location
  is available.

* What's new in verson 1.0, 2009-09-22

- process().mark() probes now use an enabling semaphore to reduce the
  computation overhead of dormant probes.

- The function spec for dwarf probes now supports C++ scopes, so you can
  limit the probes to specific namespaces or classes.  Multiple scopes
  can be specified, and they will be matched progressively outward.
      probe process("foo").function("std::vector<*>::*") { }
      probe process("foo").function("::global_function") { }

- It is now possible to cross-compile systemtap scripts for foreign
  architectures, using the new '-a ARCH' and '-B OPT=VALUE' flags.
  For example, put arm-linux-gcc etc. into your $PATH, and point
  systemtap at the target kernel build tree with:
     stap -a arm -B CROSS_COMPILE=arm-linux- -r /build/tree  [...]
  The -B option is passed to kbuild make.  -r identifies the already
  configured/built kernel tree and -a its architecture (kbuild ARCH=...).
  Systemtap will infer -p4.

- Cross compilation using the systemtap client and server
  - stap-start-server now accepts the -r, -R, -I, -B and -a options in
    order to start a cross compiling server. The server will correctly
    advertise itself with respect to the kernel release and architecture
    that it compiles for.
  - When specified on stap-client, the -r and -a options will be
    considered when searching for a suitable server.

- When using the systemtap client and server udp port 5353 must be open
  in your firewall in order for the client to find servers using
  avahi-browse.  Also the systemtap server will choose a random port in
  the range 1024-63999 for accepting ssl connections.

- Support for unprivileged users:
  ***********************************************************************
  * WARNING!!!!!!!!!!                                                   *
  * This feature is EXPERIMENTAL at this time and should be used with   *
  * care. This feature allows systemtap kernel modules to be loaded by  *
  * unprivileged users. The user interface and restrictions will change *
  * as this feature evolves.                                            *
  ***********************************************************************
  - Systemtap modules generated from scripts which use a restricted
    subset of the features available may be loaded by staprun for
    unprivileged users. Previously, staprun would load modules only for
    root or for members of the groups stapdev and stapusr.
  - Using the --unprivileged option on stap enables translation-time
    checking for use by unprivileged users (see restrictions below).
  - All modules deemed suitable for use by unprivileged users will be
    signed by the systemtap server when --unprivileged is specified on
    stap-client. See module signing in release 0.9.8 and stap-server in
    release 0.9 below.
  - Modules signed by trusted signers (servers) and verified by staprun
    will be loaded by staprun regardless of the user's privilege level.
  - The system administrator asserts the trustworthiness of a signer
    (server) by running stap-authorize-signing-cert <cert-file> as root,
    where the <cert-file> can be found in
    ~<user>/.systemtap/ssl/server/stap.cert for servers started by
    ordinary users and in $sysconfdir/systemtap/ssl/server/stap.cert for
    servers started by root.
  - Restrictions are intentionally strict at this time and may be
    relaxed in the future:
     - probe points are restricted to:
         begin, begin(n), end, end(n), error, error(n), never,
         timer.{jiffies,s,sec,ms,msec,us,usec,ns,nsec}(n)*, timer.hz(n),
         process.* (for processes owned by the user).
     - use of embedded C code is not allowed.
     - use of tapset functions is restricted.
       - some tapset functions may not be used at all. A message will be
         generated at module compilation time.
       - some actions by allowed tapset functions may only be performed
         in the context of the user's own process. A runtime fault will
         occur in these situations, for example, direct memory access.
       - The is_myproc() tapset function has been provided so that
         tapset writers for unprivileged users can check that the
         context is of the users own process before attempting these
         actions.
     - accessing the kernel memory space is not allowed.
     - The following command line options may not be used by stap-client
       -g, -I, -D, -R, -B
     - The following environment variables are ignored by stap-client:
       SYSTEMTAP_RUNTIME, SYSTEMTAP_TAPSET, SYSTEMTAP_DEBUGINFO_PATH
  - nss and nss-tools are required to use this feature.

- Support output file switching by SIGUSR2. Users can command running
  stapio to switch output file by sending SIGUSR2.

- Memory consumption for scripts involving many uprobes has been
  dramatically reduced.

- The preprocessor now supports || and && in the conditions.
  e.g. %( arch == "x86_64" || arch == "ia64" %: ... %)

- The systemtap notion of "architecture" now matches the kernel's, rather
  than that of "uname -m".  This means that 32-bit i386 family are all
  known as "i386" rather than "i386" or "i686"; "ppc64" as "powerpc";
  "s390x" as "s390", and so on.  This is consistent between the new
  "-a ARCH" flag and the script-level %( arch ... %) conditional.

- It is now possible to define multiple probe aliases with the same name.
  A probe will expand to all matching aliases.
    probe foo = bar { }
    probe foo = baz { }
    probe foo { } # expands twice, once to bar and once to baz

- A new experimental transport mechanism, using ftrace's ring_buffer,
  has been added.  This may become the default transport mechanism in
  future versions of systemtap.  To test this new transport mechanism,
  define 'STP_USE_RING_BUFFER'.

- Support for recognizing DW_OP_{stack,implicit}_value DWARF expressions
  as emitted by GCC 4.5.

* What's new in version 0.9.9, 2009-08-04

- Systemwide kernel .function.return (kretprobe) maxactive defaults may
  be overridden with the -DKRETACTIVE=nnn parameter.

- Translation pass 2 is significantly faster by avoiding unnecessary
  searching through a kernel build/module directory tree.

- When compiled against elfutils 0.142 systemtap now handles the new
  DW_OP_call_frame_CFA generated by by GCC.

- uprobes and ustack() are more robust when used on applications that
  depend on prelinked/separate debuginfo shared libraries.

- User space PROBE marks are not always found with or without separate
  debuginfo. The .probes section itself is now always put in the main
  elf file and marked as allocated. When building pic code the section
  is marked writable. The selinux memory check problems seen with
  programs using STAP_PROBES is fixed.

- statement() probes can now override "address not at start of statement"
  errors in guru mode. They also provide alternative addresses to use
  in non-guru mode.

- The stapgraph application can generate graphs of data and events
  emitted by systemtap scripts in real time.  Run "stapgraph
  testsuite/systemtap.examples/general/grapher.stp" for an example of
  graphing the system load average and keyboard events.
  
- Dwarf probes now show parameters and local variables in the verbose
  listing mode (-L).

- Symbol aliases are now resolved to their canonical dwarf names.  For
  example, probing "malloc" in libc resolves to "__libc_malloc".

- The syntax for dereferencing $target variables and @cast() gained new
  capabilities:
  - Array indexes can now be arbitrary numeric expressions.
  - Array subscripts are now supported on pointer types.
  - An '&' operator before a @cast or $target returns the address of the
    final component, especially useful for nested structures.

- For reading all probe variables, kernel.mark now supports $$vars and
  $$parms, and process.syscall now supports $$vars.

- The SNMP tapset provides probes and functions for many network
  statistics.  See stapprobes.snmp(3stap) for more details.

- The dentry tapset provides functions to map kernel VFS directory entries
  to file or full path names: d_path(), d_name() and reverse_path_walk().

- SystemTap now has userspace markers in its own binaries, and the stap
  tapset provides the available probepoints and local variables.

- Miscellaneous new tapset functions:
  - pgrp() returns the process group ID of the current process
  - str_replace() performs string replacement

* What's new in version 0.9.8, 2009-06-11

- Miscellaneous new tapset functions:
  - sid() returns the session ID of the current process
  - stringat() indexes a single character from a string.

- Using %M in print formats for hex dumps can now print entire buffers,
  instead of just small numbers.

- Dwarfless syscalls: The nd_syscalls tapset is now available to probe
  system calls without requiring kernel debugging information.  All of
  the same probepoints in the normal syscalls tapset are available with
  an "nd_" prefix, e.g. syscall.open becomes nd_syscall.open.  Most
  syscall arguments are also available by name in nd_syscalls.

- Module signing: If the appropriate nss libraries are available on your
  system, stap-server will sign each compiled module using a self-generated
  certificate.  This is the first step toward extending authority to
  load certain modules to unprivileged users. For now, if the system
  administrator adds a certificate to a database of trusted signers
  (stap-authorize-signing-cert), modules signed using that certificate
  will be verified by staprun against tampering.  Otherwise, you should
  notice no difference in the operation of stap or staprun.

* What's new in version 0.9.7, 2009-04-23

- @cast can now determine its type information using an explicit header
  specification.  For example:
    @cast(tv, "timeval", "<sys/time.h>")->tv_sec
    @cast(task, "task_struct", "kernel<linux/sched.h>")->tgid

- The overlapping process.* tapsets are now separated.  Those probe points
  documented in stapprobes(3stap) remain the same.  Those that were formerly
  in stapprobes.process(3stap) have been renamed to kprocess, to reflect
  their kernel perspective on processes.

- The --skip-badvars option now also suppresses run-time error
  messages that would otherwise result from erroneous memory accesses.
  Such accesses can originate from $context expressions fueled by
  erroneous debug data, or by kernel_{long,string,...}() tapset calls.

- New probes kprobe.function(FUNCTION) and kprobe.function(FUNCTION).return
  for dwarfless probing. These postpone function address resolution to
  run-time and use the kprobe symbol-resolution mechanism.
  Probing of absolute statements can be done using the
  kprobe.statement(ADDRESS).absolute construct.

- EXPERIMENTAL support for user process unwinding. A new collection of
  tapset functions have been added to handle user space backtraces from
  probe points that support them (currently process and timer probes -
  for timer probes test whether or not in user space first with the
  already existing user_mode() function). The new tapset functions are:
    uaddr - User space address of current running task.
    usymname - Return the symbol of an address in the current task.
    usymdata - Return the symbol and module offset of an address.
    print_ustack - Print out stack for the current task from string.
    print_ubacktrace - Print stack back trace for current task.
    ubacktrace - Hex backtrace of current task stack.
  Please read http://sourceware.org/ml/systemtap/2009-q2/msg00364.html
  on the current restrictions and possible changes in the future and
  give feedback if you want to influence future developments.

* What's new in version 0.9.5, 2009-03-27

- New probes process().insn and process().insn.block that allows
  inspection of the process after each instruction or block of
  instructions executed. So to count the total number of instructions
  a process executes during a run do something like:
    $ stap -e 'global steps; probe process("/bin/ls").insn {steps++}
               probe end {printf("Total instructions: %d\n", steps);}' \
           -c /bin/ls
  This feature can slow down execution of a process somewhat.

- Systemtap probes and function man pages extracted from the tapsets
  are now available under 3stap. To show the page for probe vm.pagefault
  or the stap function pexecname do:
    $ man 3stap vm.pagefault
    $ man 3stap pexecname

- Kernel tracepoints are now supported for probing predefined kernel
  events without any debuginfo.  Tracepoints incur less overhead than
  kprobes, and context parameters are available with full type
  information.  Any kernel 2.6.28 and later should have defined
  tracepoints.  Try the following to see what's available:
    $ stap -L 'kernel.trace("*")'

- Typecasting with @cast now supports modules search paths, which is
  useful in case there are multiple places where the type definition
  may be found.  For example:
    @cast(sdev, "scsi_device", "kernel:scsi_mod")->sdev_state

- On-file flight recorder is supported. It allows stap to record huge
  trace log on the disk and to run in background.
  Passing -F option with -o option runs stap in background mode. In this
  mode, staprun is detached from console, and stap itself shows staprun's
  pid and exits.
  Specifying the max size and the max number of log files are also available
  by passing -S option. This option has one or two arguments seperated by
  a comma. The first argument is the max size of a log file in MB. If the
  size of a log file exceeds it, stap switches to the next log file
  automatically. The second is how many files are kept on the disk. If the
  number of log files exceeds it, the oldest log file is removed
  automatically. The second argument can be omitted.

  For example, this will record output on log files each of them is smaller
  than 1024MB and keep last 3 logs, in background.
    % stap -F -o /tmp/staplog -S 1024,3 script.stp

- In guru mode (-g), the kernel probing blacklist is disabled, leaving 
  only a subset - the kernel's own internal kprobe blacklist - to attempt
  to filter out areas unsafe to probe.  The differences may be enough to
  probe more interrupt handlers.

- Variables unavailable in current context may be skipped by setting a 
  session level flag with command line option --skip-badvars now available.
  This replaces any dwarf $variable expressions that could not be resolved
  with literal numeric zeros, along with a warning message.

- Both kernel markers and kernel tracepoint support argument listing
  through stap -L 'kernel.mark("*")' or stap -L 'kernel.trace("*")'

- Users can use -DINTERRUPTIBLE=0 to prevent interrupt reentrancy in
  their script, at the cost of a bit more overhead to toggle the
  interrupt mask.

- Added reentrancy debugging. If stap is run with the arguments
  "-t -DDEBUG_REENTRANCY", additional warnings will be printed for
  every reentrancy event, including the probe points of the
  resident and interloper probes.

- Default to --disable-pie for configure.
  Use --enable-pie to turn it back on.

- Improved sdt.h compatibility and test suite for static dtrace
  compatible user space markers.

- Some architectures now use syscall wrappers (HAVE_SYSCALL_WRAPPERS).
  The syscall tapset has been enhanced to take care of the syscall
  wrappers in this release.

- Security fix for CVE-2009-0784: stapusr module-path checking race.

* What's new in version 0.9, 2009-02-19

- Typecasting is now supported using the @cast operator.  A script can
  define a pointer type for a "long" value, and then access type members
  using the same syntax as with $target variables.  For example, this will
  retrieve the parent pid from a kernel task_struct:
    @cast(pointer, "task_struct", "kernel")->parent->pid

- process().mark() probes are now possible to trace static user space
  markers put in programs with the STAP_PROBE macro using the new
  sys/sdt.h include file. This also provides dtrace compatible markers
  through DTRACE_PROBE and an associated python 'dtrace' script that
  can be used in builds based on dtrace that need dtrace -h or -G
  functionality.

- For those that really want to run stap from the build tree there is
  now the 'run-stap' script in the top-level build directory that sets
  up the SYSTEMTAP_TAPSET, SYSTEMTAP_RUNTIME, SYSTEMTAP_STAPRUN, and
  SYSTEMTAP_STAPIO environment variables (installing systemtap, in a
  local prefix, is still recommended for common use).

- Systemtap now comes with a new Beginners Guide that walks the user
  through their first steps setting up stap, understanding how it all
  works, introduces some useful scripts and describes some common
  pitfalls.  It isn't created by default since it needs a Publican
  setup, but full build instructions can be found in the wiki:
  http://sourceware.org/systemtap/wiki/PublicanQuikHowto
  An online version can be found at:
  http://sourceware.org/systemtap/SystemTap_Beginners_Guide.pdf

- Standard tapsets included with Systemtap were modified to include
  extractable documentation information based on the kernel-doc
  infrastructure. When configured --enabled-docs a HTML and PDF
  version of the Tapset Reference Manual is produced explaining probes
  defined in each tapset.

- The systemtap client and compile server are now available.
  These allow you to compile a systemtap module on a host other than
  the one which it will be run, providing the client and server
  are compatible. Other than using a server for passes 1 through
  4, the client behaves like the 'stap' front end itself. This
  means, among other things, that the client will automatically
  load the resulting module on the local host unless -p[1234]
  was specified.  See stap-server(8) for more details.
  The client/server now use SSL for network connection security and
  for signing.

  The systemtap client and server are prototypes only. Interfaces, options
  and usage may change at any time.

- function("func").label("label") probes are now supported to allow matching
  the label of a function.

- Systemtap initscript is available. This initscript allows you to run
  systemtap scripts as system services (in flight recorder mode) and
  control those scripts individually.
  See README.systemtap for details.

- The stap "-r DIR" option may be used to identify a hand-made kernel
  build directory.  The tool determines the appropriate release string
  automatically from the directory.

- Serious problems associated with user-space probing in shared libraries
  were corrected, making it now possible to experiment with probe shared
  libraries.  Assuming dwarf debugging information is installed, use this
  twist on the normal syntax:

    probe process("/lib64/libc-2.8.so").function("....") { ... }

  This would probe all threads that call into that library.  Running
  "stap -c CMD" or "stap -x PID" naturally restricts this to the target
  command+descendants only.  $$vars etc. may be used.

- For scripts that sometimes terminate with excessive "skipped" probes,
  rerunning the script with "-t" (timing) will print more details about
  the skippage reasons.

- Symbol tables and unwind (backtracing) data support were formerly
  compiled in for all probed modules as identified by the script
  (kernel; module("name"); process("file")) plus those listed by the
  stap "-d BINARY" option.  Now, this data is included only if the systemtap
  script uses tapset functions like probefunc() or backtrace() that require
  such information.  This shrinks the probe modules considerably for the rest.

- Per-pass verbosity control is available with the new "--vp {N}+" option.
  "stap --vp 040" adds 4 units of -v verbosity only to pass 2.  This is useful
  for diagnosing errors from one pass without excessive verbosity from others.

- Most probe handlers now run with interrupts enabled, for improved
  system responsiveness and less probing overhead.  This may result
  in more skipped probes, for example if a reentrant probe handler
  is attempted from within an interrupt handler.  It may also make the
  systemtap overload detection facility more likely to be triggered, as
  interrupt handlers' run time would be included in the self-assessed
  overhead of running probe handlers.

* What's new in version 0.8, 2008-11-13

- Cache limiting is now available.  If the compiled module cache size is
  over a limit specified in the $SYSTEMTAP_DIR/cache/cache_mb_limit file,
  some old cache entries will be unlinked.  See man stap(1) for more.

- Error and warning messages are now followed by source context displaying
  the erroneous line/s and a handy '^' in the following line pointing to the
  appropriate column.

- A bug reporting tool "stap-report" is now available which will quickly
  retrieve much of the information requested here: 
  http://sourceware.org/systemtap/wiki/HowToReportBugs

- The translator can resolve members of anonymous structs / unions:
    given            struct { int foo; struct { int bar; }; } *p;
    this now works:  $p->bar

- The stap "-F" flag activates "flight recorder" mode, which consists of
  translating the given script as usual, but implicitly launching it into
  the background with staprun's existing "-L" (launch) option.  A user
  can later reattach to the module with "staprun -A MODULENAME".

- Additional context variables are available on user-space syscall probes.
  - $argN ($arg1, $arg2, ... $arg6) in process(PATH_OR_PID).syscall
    gives you the argument of the system call.
  - $return in process(PATH_OR_PID).syscall.return gives you the return
    value of the system call.

- Target process mode (stap -c CMD or -x PID) now implicitly restricts all
  "process.*" probes to the given child process.  (It does not affect
  kernel.* or other probe types.)  The CMD string is normally run directly,
  rather than via a /bin/sh -c subshell, since then utrace/uprobe probes
  receive a fairly "clean" event stream.  If metacharacters like
  redirection operators were present in CMD, then "sh -c CMD" is still
  used, and utrace/uprobe probes will receive events from the shell.

     % stap -e 'probe process.syscall, process.end {
                   printf("%s %d %s\n", execname(), pid(), pp())}'\
            -c ls
     ls 2323 process.syscall 
     ls 2323 process.syscall 
     ls 2323 process.end

- Probe listing mode is improved: "-L" lists available script-level variables

     % stap -L 'syscall.*open*'
     syscall.mq_open name:string name_uaddr:long filename:string mode:long u_attr_uaddr:long oflag:long argstr:string
     syscall.open name:string filename:string flags:long mode:long argstr:string
     syscall.openat name:string filename:string flags:long mode:long argstr:string

- All user-space-related probes support $PATH-resolved executable
  names, so

     probe process("ls").syscall {}
     probe process("./a.out").syscall {}

  work now, instead of just

     probe process("/bin/ls").syscall {}
     probe process("/my/directory/a.out").syscall {}

- Prototype symbolic user-space probing support:

     # stap -e 'probe process("ls").function("*").call {
                   log (probefunc()." ".$$parms)
                }' \
            -c 'ls -l'

  This requires:
  - debugging information for the named program
  - a version of utrace in the kernel that is compatible with the "uprobes"
    kernel module prototype.  This includes RHEL5 and older Fedora, but not
    yet current lkml-track utrace; a "pass 4a"-time build failure means
    your system cannot use this yet.

- Global variables which are written to but never read are now
  automatically displayed when the session does a shutdown.  For example:

      global running_tasks
      probe timer.profile {running_tasks[pid(),tid()] = execname()}
      probe timer.ms(8000) {exit()}

- A formatted string representation of the variables, parameters, or local
  variables at a probe point is now supported via the special $$vars,
  $$parms, and $$locals context variables, which expand to a string
  containing a list "var1=0xdead var2=0xbeef var3=?".  (Here, var3 exists
  but is for some reason unavailable.)  In return probes only, $$return
  expands to an empty string for a void function, or "return=0xf00".


* What's new in version 0.7, 2008-07-15

- .statement("func@file:*") and .statement("func@file:M-N") probes are now
  supported to allow matching a range of lines in a function.  This allows
  tracing the execution of a function.

- Scripts relying on probe point wildcards like "syscall.*" that expand
  to distinct kprobes are processed significantly faster than before.

- The vector of script command line arguments is available in a
  tapset-provided global array argv[].  It is indexed 1 ... argc,
  another global.  This can substitute for of preprocessor
  directives @NNN that fail at parse time if there are not
  enough arguments.

      printf("argv: %s %s %s", argv[1], argv[2], argv[3])

- .statement("func@file+line") probes are now supported to allow a
  match relative to the entry of the function incremented by line
  number.  This allows using the same systemtap script if the rest
  of the file.c source only changes slightly.

- A probe listing mode is available.
  % stap -l vm.*
  vm.brk
  vm.mmap
  vm.munmap
  vm.oom_kill
  vm.pagefault
  vm.write_shared

- More user-space probe types are added:

  probe process(PID).begin { }
  probe process("PATH").begin { }
  probe process(PID).thread.begin { }
  probe process("PATH").thread.begin { }
  probe process(PID).end { }
  probe process("PATH").end { }
  probe process(PID).thread.end { }
  probe process("PATH").thread.end { }
  probe process(PID).syscall { }
  probe process("PATH").syscall { }
  probe process(PID).syscall.return { }
  probe process("PATH").syscall.return { }

- Globals now accept ; terminators

  global odds, evens;
  global little[10], big[5];

* What's new in version 0.6, 2007-12-15

- A copy of the systemtap tutorial and language reference guide
  are now included.

- There is a new format specifier, %m, for the printf family of
  functions.  It functions like %s, except that it does not stop when
  a nul ('\0') byte is encountered.  The number of bytes output is
  determined by the precision specifier.  The default precision is 1.
  For example:

      printf ("%m", "My String") // prints one character: M
      printf ("%.5", myString)   // prints 5 bytes beginning at the start
                                 // of myString

- The %b format specifier for the printf family of functions has been enhanced
  as follows:

  1) When the width and precision are both unspecified, the default is %8.8b.
  2) When only one of the width or precision is specified, the other defaults
     to the same value.  For example, %4b == %.4b == %4.4b
  3) Nul ('\0') bytes are used for field width padding.  For example,

     printf ("%b", 0x1111deadbeef2222) // prints all eight bytes
     printf ("%4.2b", 0xdeadbeef)      // prints  \0\0\xbe\xef

- Dynamic width and precision are now supported for all printf family format
  specifiers.  For example:

     four = 4
     two = 2
     printf ("%*.*b", four, two, 0xdeadbbeef) // prints  \0\0\xbe\xef
     printf ("%*d", four, two)                // prints  <space><space><space>2

- Preprocessor conditional expressions can now include wildcard style
  matches on kernel versions.
  %( kernel_vr != "*xen" %? foo %: bar %)

- Prototype support for user-space probing is showing some progress.
  No symbolic notations are supported yet (so no probing by function names,
  file names, process names, and no access to $context variables), but at
  least it's something:

    probe process(PID).statement(ADDRESS).absolute { }

  This will set a uprobe on the given process-id and given virtual address.
  The proble handler runs in kernel-space as usual, and can generally use
  existing tapset functions.

- Crash utility can retrieve systemtap's relay buffer from a kernel dump
  image by using staplog which is a crash extension module. To use this
  feature, type commands as below from crash(8)'s command line:

    crash> extend staplog.so
    crash> help systemtaplog

  Then, you can see more precise help message.

- You can share a relay buffer amoung several scripts and merge outputs from
  several scripts by using "-DRELAY_HOST" and "-DRELAY_GUEST" options.
  For example:

    # run a host script
    % stap -ve 'probe begin{}' -o merged.out -DRELAY_HOST &
    # wait until starting the host.
    % stap -ve 'probe begin{print("hello ");exit()}' -DRELAY_GUEST
    % stap -ve 'probe begin{print("world\n");exit()}' -DRELAY_GUEST

  Then, you'll see "hello world" in merged.out.

- You can add a conditional statement for each probe point or aliase, which
  is evaluated when the probe point is hit. If the condition is false, the
  whole probe body(including aliases) is skipped. For example:

    global switch = 0;
    probe syscall.* if (switch) { ... }
    probe procfs.write {switch = strtol($value,10)} /* enable/disable ctrl */

- Systemtap will warn you if your script contains unused variables or
  functions.  This is helpful in case of misspelled variables.  If it
  doth protest too much, turn it off with "stap -w ...".

- You can add error-handling probes to a script, which are run if a
  script was stopped due to errors.  In such a case, "end" probes are
  not run, but "error" ones are.

    probe error { println ("oops, errors encountered; here's a report anyway")
                  foreach (coin in mint) { println (coin) } }

- In a related twist, one may list probe points in order of preference,
  and mark any of them as "sufficient" beyond just "optional".  Probe
  point sequence expansion stops if a sufficient-marked probe point has a hit.
  This is useful for probes on functions that may be in a module (CONFIG_FOO=m)
  or may have been compiled into the kernel (CONFIG_FOO=y), but we don't know
  which.  Instead of

    probe module("sd").function("sd_init_command") ? ,
          kernel.function("sd_init_command") ? { ... }

  which might match neither, now one can write this:

    probe module("sd").function("sd_init_command") ! , /* <-- note excl. mark */
          kernel.function("sd_init_command")  { ... }

- New security model.  To install a systemtap kernel module, a user
  must be one of the following: the root user; a member of the
  'stapdev' group; or a member of the 'stapusr' group.  Members of the
  stapusr group can only use modules located in the
  /lib/modules/VERSION/systemtap directory (where VERSION is the
  output of "uname -r").

- .statement("...@file:line") probes now apply heuristics to allow an
  approximate match for the line number.  This works similarly to gdb,
  where a breakpoint placed on an empty source line is automatically
  moved to the next statement.  A silly bug that made many $target
  variables inaccessible to .statement() probes was also fixed.

- LKET has been retired.  Please let us know on <systemtap@sourceware.org>
  if you have been a user of the tapset/tools, so we can help you find
  another way.

- New families of printing functions println() and printd() have been added.
  println() is like print() but adds a newline at the end;
  printd() is like a sequence of print()s, with a specified field delimiter.

* What's new since version 0.5.14?, 2007-07-03

- The way in which command line arguments for scripts are substituted has
  changed.  Previously, $1 etc. would interpret the corresponding command
  line argument as an numeric literal, and @1 as a string literal.  Now,
  the command line arguments are pasted uninterpreted wherever $1 etc.
  appears at the beginning of a token.  @1 is similar, but is quoted as
  a string.  This change does not modify old scripts, but has the effect
  of permitting substitution of arbitrary token sequences.

  # This worked before, and still does:
  % stap -e 'probe timer.s($1) {}'        5
  # Now this also works:
  % stap -e 'probe syscall.$1 {log(@1)}'  open
  # This won't crash, just signal a recursion error:
  % stap -e '$1'                          '$1'
  # As before, $1... is recognized only at the beginning of a token
  % stap -e 'probe begin {foo$1=5}'

* What's new since version 0.5.13?, 2007-03-26

- The way in which systemtap resolves function/inline probes has changed:
   .function(...) - now refers to all functions, inlined or not
   .inline(...)   - is deprecated, use instead:
   .function(...).inline - filters function() to only inlined instances
   .function(...).call - filters function() to only non-inlined instances
   .function(...).return - as before, but now pairs best with .function().call
   .statement() is unchanged.

* What's new since version 0.5.12?, 2007-01-01

- When running in -p4 (compile-only) mode, the compiled .ko file name
  is printed on standard output.

- An array element with a null value such as zero or an empty string
  is now preserved, and will show up in a "foreach" loop or "in" test.
  To delete such an element, the scripts needs to use an explicit
  "delete array[idx]" statement rather than something like "array[idx]=0".

- The new "-P" option controls whether prologue searching heuristics
  will be activated for function probes.  This was needed to get correct
  debugging information (dwarf location list) data for $target variables.
  Modern compilers (gcc 4.1+) tend not to need this heuristic, so it is
  no longer default.  A new configure flag (--enable-prologues) restores
  it as a default setting, and is appropriate for older compilers (gcc 3.*).

- Each systemtap module prints a one-line message to the kernel informational
  log when it starts.  This line identifies the translator version, base
  address of the probe module, a broken-down memory consumption estimate, and
  the total number of probes.  This is meant as a debugging / auditing aid.

- Begin/end probes are run with interrupts enabled (but with
  preemption disabled).  This will allow begin/end probes to be
  longer, to support generating longer reports.

- The numeric forms of kernel.statement() and kernel.function() probe points
  are now interpreted as relocatable values - treated as relative to the
  _stext symbol in that kernel binary.  Since some modern kernel images
  are relocated to a different virtual address at startup, such addresses
  may shift up or down when actually inserted into a running kernel.

     kernel.statement(0xdeadbeef): validated, interpreted relative to _stext,
                                   may map to 0xceadbeef at run time.

  In order to specify unrelocated addresses, use the new ".absolute"
  probe point suffix for such numeric addresses.  These are only
  allowed in guru mode, and provide access to no $target variables.
  They don't use debugging information at all, actually.

     kernel.statement(0xfeedface).absolute: raw, unvalidated, guru mode only

* What's new since version 0.5.10?, 2006-10-19

- Offline processing of debugging information, enabling general
  cross-compilation of probe scripts to remote hosts, without
  requiring identical module/memory layout.  This slows down
  compilation/translation somewhat.

- Kernel symbol table data is loaded by staprun at startup time
  rather than compiled into the module.

- Support the "limit" keyword for foreach iterations:
    foreach ([x,y] in ary limit 5) { ... }
  This implicitly exits after the fifth iteration.  It also enables
  more efficient key/value sorting.

- Support the "maxactive" keyword for return probes:
    probe kernel.function("sdfsdf").maxactive(848) { ... }
  This allows up to 848 concurrently outstanding entries to
  the sdfsdf function before one returns.  The default maxactive
  number is smaller, and can result in missed return probes.

- Support accessing of saved function arguments from within
  return probes.  These values are saved by a synthesized
  function-entry probe.

- Add substantial version/architecture checking in compiled probes to
  assert correct installation of debugging information and correct
  execution on a compatible kernel.

- Add probe-time checking for sufficient free stack space when probe
  handlers are invoked, as a safety improvement.

- Add an optional numeric parameter for begin/end probe specifications,
  to order their execution.
     probe begin(10) { } /* comes after */ probe begin(-10) {}

- Add an optional array size declaration, which is handy for very small
  or very large ones.
     global little[5], big[20000]

- Include some example scripts along with the documentation.

- Change the start-time allocation of probe memory to avoid causing OOM
  situations, and to abort cleanly if free kernel memory is short.

- Automatically use the kernel DWARF unwinder, if present, for stack
  tracebacks.

- Many minor bug fixes, performance, tapset, and error message
  improvements.