KGTP (Linux debugger and tracer) 20130508 release (add single step support)

Hui Zhu teawater@gmail.com
Wed May 8 09:42:00 GMT 2013


Hi guys,

KGTP (http://code.google.com/p/kgtp/) is a flexible, lightweight and
realtime Linux debugger and tracer.
To use it, you don't need patch or rebuild the Linux kernel. Just
build KGTP module and insmod it is OK.

Please go to http://code.google.com/p/kgtp/wiki/HOWTO or
http://code.google.com/p/kgtp/wiki/HOWTO (Chinese) to get more info
about howto use KGTP.

Please goto https://code.google.com/p/kgtp/wiki/HOWTO#Table_of_different_between_GDB_debug_normal_program_and_KGTP
or https://code.google.com/p/kgtp/wiki/HOWTOCN#GDB调试普通程序和KGTP的区别表
(Chinese) get the table of different between GDB debug normal program
and KGTP if you have experience using GDB debug normal program.

Now, KGTP 20130508 release.
You can get the package for it from
http://kgtp.googlecode.com/files/kgtp_20130508.tar.bz2
or
svn co https://kgtp.googlecode.com/svn/tags/20130508

The main change of this release is complete the support of
while-stepping.  KGTP can let Linux kernel do single step now.
Please note that while-stepping is just support by X86 and X86_64 now.
Howto use while-stepping:
while-stepping is a special tracepoint action that include some actions with it.
When tracepoints that its actions include "while-stepping n" execute,
it will do n times single steps and executes the actions of
while-stepping. For example:
trace vfs_read
#Because single step will make system slow, so use passcount or
condition to limit the execution times of tracepoint is better.
passcount 1
  commands
    collect $bt
    collect $step_count
    #do 2000 times single steps.
    while-stepping 2000
      #Following part is actions of "while-stepping 2000".
      #Because step maybe execute to other functions, so does not
access local variables is better.
      collect $bt
      collect $step_count
    end
  end
You can switch KGTP to replay mode to select all the traceframe of a
while-stepping tracepoint with GDB execution and reverse-execution
commands. For example:
Use tfind select one the traceframe of a while-stepping tracepoint.
(gdb) tfind
Found trace frame 0, tracepoint 1
#0  vfs_read (file=0xffff8801f7bd4c00, buf=0x7fff74e4edb0 <Address
0x7fff74e4edb0 out of bounds>, count=16,
    pos=0xffff8801f4b45f48) at /build/buildd/linux-3.2.0/fs/read_write.c:365
365     {
Following commands will swith KGTP to replay mode.

(gdb) monitor replay
(gdb) tfind -1
No longer looking at any trace frame
#0  vfs_read (file=0xffff8801f7bd4c00, buf=0x7fff74e4edb0 <Address
0x7fff74e4edb0 out of bounds>, count=16,
    pos=0xffff8801f4b45f48) at /build/buildd/linux-3.2.0/fs/read_write.c:365
365     {
Then you can use execution commands.

(gdb) n
368             if (!(file->f_mode & FMODE_READ))
(gdb) p file->f_mode
$5 = 3
Set breakpoints (Just valid in replay mode, will not affect Linux
kernel execution).

(gdb) b 375
Breakpoint 2 at 0xffffffff81179b75: file
/build/buildd/linux-3.2.0/fs/read_write.c, line 375.
(gdb) c
Continuing.

Breakpoint 2, vfs_read (file=0xffff8801f7bd4c00, buf=0x7fff74e4edb0
<Address 0x7fff74e4edb0 out of bounds>, count=16,
    pos=0xffff8801f4b45f48) at /build/buildd/linux-3.2.0/fs/read_write.c:375
375             ret = rw_verify_area(READ, file, pos, count);
(gdb) s
rw_verify_area (read_write=0, file=0xffff8801f7bd4c00,
ppos=0xffff8801f4b45f48, count=16)
    at /build/buildd/linux-3.2.0/fs/read_write.c:300
300             inode = file->f_path.dentry->d_inode;
Use reverse-execution commands.

(gdb) rs

Breakpoint 2, vfs_read (file=0xffff8801f7bd4c00, buf=0x7fff74e4edb0
<Address 0x7fff74e4edb0 out of bounds>, count=16,
    pos=0xffff8801f4b45f48) at /build/buildd/linux-3.2.0/fs/read_write.c:375
375             ret = rw_verify_area(READ, file, pos, count);
(gdb) rn
372             if (unlikely(!access_ok(VERIFY_WRITE, buf, count)))
GDB commands tstart, tfind or quit can auto close the replay mode.

Please goto https://code.google.com/p/kgtp/wiki/HOWTO#Use_while-stepping_let_Linux_kernel_do_single_step
or https://code.google.com/p/kgtp/wiki/HOWTOCN#使用while-stepping让Linux内核做单步
(Chinese version) get more info about it.

Please goto http://code.google.com/p/kgtp/wiki/UPDATE get more info
about this release.

According to the comments of Christoph, Geoff and Andi.  I make lite
patch for review.  Please goto https://lkml.org/lkml/2012/5/9/90 to
see it.

Thanks,
Hui



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