[PATCH 0/8] Get rid of the whole moribund breakpoints mechanism, use TRAP_BRKPT

[Pedro Alves]

Well, not really get rid of it (at least not yet), but add a reliable,
non-heuristic, but still efficient alternative.

Note: the "remote" patch includes manual and NEWS changes.

The non-stop-fair-events.exp test is frequently failing currently.
E.g., see https://sourceware.org/ml/gdb-testers/2015-q1/msg03148.html.

The root cause is a fundamental problem with the whole moribund
breakpoint locations mechanism.

This series teaches GDB about targets that can reliably tell whether a
trap was caused by a software or hardware breakpoint, and makes use of
that information instead as reliable mechanism to detect delayed
events.  With that in place, we don't handle moribund locations
anymore (on such targets).

On Linux, we'll get that information out of the si_code of the SIGTRAP
siginfo.  E.g., for software breakpoint hits, Linux sets that field to
TRAP_BRKPT.

For years I assumed that x86 just didn't include this information in
the siginfo, because when a breakpoint trap is raised, si_code does
not show TRAP_BRKPT, as one sees on other architectures.  Because of
that, I had added an entry to gdb's kernel wishlist at
https://sourceware.org/gdb/wiki/LinuxKernelWishList for TRAP_BRKPT in
2012...

But taking a closer look, even though the x86 port puts odd values in
SIGTRAP's si_code, those _are_ usable as is.

Based on reading the kernel's source and experimenting things, this is
how x86's si_code works (and has "always" worked):

| what                                     | si_code    |
|------------------------------------------+------------|
| software breakpoints (int3)              | SI_KERNEL  |
| single-steps                             | TRAP_TRACE |
| single-stepping a syscall                | TRAP_BRKPT |
| user sent SIGTRAP                        | 0          |
| exec SIGTRAP (when no PTRACE_EVENT_EXEC) | 0          |
| hardware breakpoints/watchpoints         | TRAP_HWBPT |

It's almost as if TRAP_BRKPT and SI_KERNEL were mistakenly reversed...

Both user-sent traps and execs get the same si_code, but that's not
really a problem given PTRACE_EVENT_EXEC.

If the kernel is ever "fixed" to use the right values, we can always
detect it like we detect support for ptrace features (by having gdb
debug itself a bit at startup, and looking at the si_code of a
breakpoint hit).

As I can't be sure all supported architectures get si_code right, I
left the old code paths in place, guarded behind a macro.  I prefer
keeping that for a little longer, until we're reasonably sure no
target is messed up by this change.  IOW, I propose enabling this by
default everywhere, and only disabling if problems are reported.  If
we only enable on an arch-by-arch basis, it's almost certain that even
if all architectures can use this, we'll never end up enabling it on
all architectures...  If after a while, no problem is ever reported,
we can remove the fallback code then.  And if some architecture does
indeed trip on some problem, we'll disable the new code on that arch,
but I'll make some loud noises about fixing it in the kernel too, so
later GDB releases can again consider dropping the old code.

Tested on:

. x86-64 Fedora 20 (decr_pc_after_break target)
. s390 RHEL 7  (decr_pc_after_break)
. PPC64 Fedora 18 (!decr_pc_after_break)

An earlier version was tested on:

. ARM Fedora 21 (!decr_pc_after_break).

I've tested the series incrementally at each patch on x86-64 Fedora
20, and I also hacked away the new feature manually in different
manners to exercise different combinations of GDB vs GDBserver
support, to make sure GDB and GDBserver coped with different
combinations of GDB, GDBserver and backend support.

Pedro Alves (8):
  enum lwp_stop_reason -> enum target_stop_reason
  Teach GDB about targets that can tell whether a trap is a breakpoint
    event
  record-full/record-btrace: software/hardware breakpoint trap
  remote+docs: software/hardware breakpoint traps
  Linux native: Use TRAP_BRKPT/TRAP_HWBPT
  gdbserver: Support the "swbreak"/"hwbreak" stop reasons
  gdbserver/Linux: Use TRAP_BRKPT/TRAP_HWBPT
  garbage collect target_decr_pc_after_break

 gdb/NEWS                      |  10 +++
 gdb/aix-thread.c              |   2 +-
 gdb/breakpoint.c              |  91 +++++++++++++++++--------
 gdb/breakpoint.h              |   5 ++
 gdb/btrace.h                  |   4 ++
 gdb/darwin-nat.c              |   4 +-
 gdb/doc/gdb.texinfo           |  80 ++++++++++++++++++++++
 gdb/gdbserver/linux-low.c     | 154 ++++++++++++++++++++++++++++++++++++------
 gdb/gdbserver/linux-low.h     |  21 +-----
 gdb/gdbserver/linux-x86-low.c |   2 +-
 gdb/gdbserver/remote-utils.c  |  10 +++
 gdb/gdbserver/server.c        |  25 +++++++
 gdb/gdbserver/server.h        |  11 +++
 gdb/gdbserver/target.h        |  31 +++++++++
 gdb/infrun.c                  |  72 +++++++++++++++++++-
 gdb/linux-nat.c               | 131 ++++++++++++++++++++++++++++++-----
 gdb/linux-nat.h               |  20 +-----
 gdb/linux-thread-db.c         |   6 +-
 gdb/nat/linux-ptrace.h        |  51 ++++++++++++++
 gdb/record-btrace.c           |  69 ++++++++++++++++---
 gdb/record-full.c             | 103 ++++++++++++++++++----------
 gdb/record.c                  |  19 ++++++
 gdb/record.h                  |  13 ++++
 gdb/remote.c                  | 119 +++++++++++++++++++++++++++++---
 gdb/target-delegates.c        | 151 +++++++++++++++++++++++++++++++++--------
 gdb/target.c                  |  20 ------
 gdb/target.h                  |  54 ++++++++++++---
 gdb/target/waitstatus.h       |  19 ++++++
 gdb/x86-linux-nat.c           |   2 +-
 29 files changed, 1071 insertions(+), 228 deletions(-)

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1.9.3