1035645 - [Flame] Enable gdb hardware watchpoint

Status: RESOLVED FIXED

(Firefox OS Graveyard :: General, defect)

Description

[Ting-Yu Chou [:ting] (away)]

Tried to enable gdb hardware watchpoint on Flame by the steps at [1], but gdb is not stopped when the address is written by the program, rwatch works fine though. The results are the same on both linarno 7.6.1-2013.10 [2] and jimdb 7.5 [3].

[1] https://intranet.mozilla.org/B2G_Team/Unagi#Alternatively_building_your_own_Unagi_kernel
[2] https://releases.linaro.org/13.10/components/toolchain/binaries/gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_linux.tar.bz2
[3] http://people.mozilla.org/~nchen/jimdb/jimdb-arm-linux_x64.tar.bz2

Comment 1

[Ting-Yu Chou [:ting] (away)]

Even it is the instruction "str r2, [r3, #0]" triggers do_DataAbort(), the bit WnR of DFSR is not set, which fails the access type checking below and return from watchpoint_handler():

  access = (fsr & ARM_FSR_ACCESS_MASK) ? HW_BREAKPOINT_W : HW_BREAKPOINT_R;
  if (!(access & hw_breakpoint_type(wp)))
      goto unlock;

This causes infinite do_DataAbort(), and gdb is never stopped. But if you interrupt gdb and continue several times, you should see $pc is not changing.

There is a patch for this [1], watch works ok after applying it. There's another patch [2] also worths merging.

[1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=bf8801145c01ab600f8df66e8c879ac642fa5846
[2] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=d968d2b801d877601d54e35e6dd0f52d9c797c99

Comment 2

[Ting-Yu Chou [:ting] (away)]

Attachment: Link to PR

Comment 3

[Ting-Yu Chou [:ting] (away)]

Here are the steps I used to enable HW watchpoint:

1. Enable HAVE_HW_BREAKPOINT by modifying kernel/arch/arm/Kconfig:

   diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
   index 9dd7949..04f3b99 100644
   --- a/arch/arm/Kconfig
   +++ b/arch/arm/Kconfig
   @@ -29,7 +29,8 @@ config ARM
           select HAVE_PERF_EVENTS
           select PERF_USE_VMALLOC
           select HAVE_REGS_AND_STACK_ACCESS_API
   -       select HAVE_HW_BREAKPOINT if (PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7))
   +#      select HAVE_HW_BREAKPOINT if (PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7))
   +       select HAVE_HW_BREAKPOINT
           select HAVE_C_RECORDMCOUNT
           select HAVE_GENERIC_HARDIRQS
           select HAVE_SPARSE_IRQ
2. Replace prebuilts/misc/android-arm/gdbserver/gdbserver by linaro/gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_linux/arm-linux-gnueabihf/debug-root/usr/bin/gdbserver.
3. $ ./build.sh && ./flash.sh
4. $ export GDB=linaro/gcc-linaro-arm-linux-gnueabihf-4.8-2013.10_linux/bin/arm-linux-gnueabihf-gdb
   $ ./run-gdb.sh

Comment 4

[Michael Wu [:mwu]]

CCing m1 in case he is interested in getting hardware watchpoints working.

Comment 5

[Ting-Yu Chou [:ting] (away)]

Thanks mwu.

Comment 6

[Ryan VanderMeulen [:RyanVM]]

t2m-flame-3.4-jb: https://github.com/mozilla-b2g/codeaurora_kernel_msm/commit/30d40a5636ff28a554f1d8e9d975bfd04c2463d3

Comment 7

[Michael Vines [:m1] [:evilmachines]]

(In reply to Michael Wu [:mwu] from comment #4)
> CCing m1 in case he is interested in getting hardware watchpoints working.

Cool, thanks for the heads up