Re: [RFC PATCH 0/6] tracing: wprobe: Add wprobe for watchpoint

Jinchao Wang
2025-09-05T09:02:32Z

On 9/5/25 10:08, Masami Hiramatsu (Google) wrote:
> On Thu, 4 Sep 2025 14:35:33 +0800
> Jinchao Wang <wangjinchao600@gmail.com> wrote:
>
>> On 9/2/25 22:02, Masami Hiramatsu (Google) wrote:
>>> (Adding Jinchao)
>>>
>>> On Mon, 1 Sep 2025 23:44:35 +0900
>>> "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
>>>
>>>> Hi,
>>>>
>>>> Here is an RFC series for adding new wprobe (watch probe) which
>>>> provides memory access tracing event. Moreover, this can be used via
>>>> event trigger. Thus it can trace memory access on a dynamically
>>>> allocated objects too.
>>>
>>> BTW, this series is on the top of probes/for-next branch in the
>>> linux-trace tree.
>>>
>>
>> Hi, Masami
>>
>> Thanks for including me. I only received the cover letter, so I'm
>> providing my feedback here:
>
> Thanks for your feedback! I'll send v2.
>
>>
>> - trigger_data issue:
>> it appears that the instance being removed is not the same as
>> the one that was originally set.
>
> Did you mean it will free invalid struct event_trigger_data by
> wprobe_trigger_free(), or hw_breakpoint instance?
>
> When trace_event_enable_disable(wprobe_data->file, 1, 1) at
> wprobe_trigger_cmd_parse(), the trace_wprobe is enabled == call
> enable_trace_wprobe(), which eventually call register_wide_hw_breakpoint()
> with dummy address. IOW, the (wide)hw_breakpoint is allocated here.
>
> When the trigger is hit, modify_wide_hw_breakpoint_local() is called
> for updating local target address on the same hw_breakpoint.
> So latter instance should be the same.
In wprobe_trigger_cmd_parse(), consider removing:
echo '!set_wprobe:watch:dentry' >> events/fprobes/truncate/trigger
The trigger_data passed to event_trigger_unregister() is freshly allocated.
>
>>
>> - the function call issue:
>> `trace_wprobe_update_local()` is called twice, once in the trigger
>> callback and again in `wprobe_work_func`.
>
> Yes, I found it can happen. Maybe I need a cpumask to skip the first one.
>
>> I also noticed that the Watchpoint probe and KStackWatch implementations
>> share very similar logic for managing hardware breakpoints/watchpoints
>> (`hwbp/watch`):
>> - `watch_init(unsigned long &place_holder)`
>> - `watch_on(struct perf_event_attr *attr)`
>> - `watch_off()` (or reset to the `place_holder` value)
>> - `watch_uninit()`
>
> Hmm, I agree that we are using hwbp in a similar way but
> I think current interface is enough.
>
>>
>> Their primary difference lies in their handler functions, specifically
>> the `perf_overflow_handler_t triggered` callback.
>>
>> I believe we could work together to unify this logic. I am open to
>> either approach: I can refactor my watch.c, or you can introduce new
>> helpers. This would help us save duplicated work and review time.
>
> I'm not sure what you mean. Is there any concern to use the same
> (current hwbp) interfaces?
>
It is not the _current_ HWBP interface, it needs review. We will write
the same logic except for the HWBP interface regarding atomic context
watch switch (addr/type/len) and the process to sync with other CPUs.

The only difference is the HWBP handler.

I am not familiar with the process of how we usually do roughly the
same thing. Maybe I am thinking about it the wrong way; I will look
at it later. > Thank you,
>
>>
>>> Thanks,
>>>
>>>>
>>>> In this version, I reuse Jinchao's arch_reinstall_hw_breakpoint()
>>>> patch[1].
>>>>
>>>> [1] https://lore.kernel.org/all/20250828073311.1116593-6-wangjinchao600@gmail.com/
>>>>
>>>> The basic usage of this wprobe is similar to other probes;
>>>>
>>>> w:[GRP/][EVENT] [r|w|rw]@<ADDRESS|SYMBOL[+OFFS]> [FETCHARGS]
>>>>
>>>> This defines a new wprobe event. For example, to trace jiffies update,
>>>> you can do;
>>>>
>>>> echo 'w:my_jiffies w@jiffies:8 value=+0($addr)' >> dynamic_events
>>>> echo 1 > events/wprobes/my_jiffies/enable
>>>>
>>>> Moreover, this can be combined with event trigger to trace the memory
>>>> accecss on slab objects. The trigger syntax is;
>>>>
>>>> set_wprobe:WPROBE_EVENT:FIELD[+ADJUST]
>>>> clear_wprobe:WPROBE_EVENT
>>>>
>>>> For example, trace the first 8 byte of the dentry data structure passed
>>>> to do_truncate() until it is deleted by __dentry_kill().
>>>> (Note: all tracefs setup uses '>>' so that it does not kick do_truncate())
>>>>
>>>> # echo 'w:watch rw@0:8 address=$addr value=+0($addr)' > dynamic_events
>>>>
>>>> # echo 'f:truncate do_truncate dentry=$arg2' >> dynamic_events
>>>> # echo 'set_wprobe:watch:dentry' >> events/fprobes/truncate/trigger
>>>>
>>>> # echo 'f:dentry_kill __dentry_kill dentry=$arg1' >> dynamic_events
>>>> # echo 'clear_wprobe:watch' >> events/fprobes/dentry_kill/trigger
>>>>
>>>> # echo 1 >> events/fprobes/truncate/enable
>>>> # echo 1 >> events/fprobes/dentry_kill/enable
>>>>
>>>> # echo aaa > /tmp/hoge
>>>> # echo bbb > /tmp/hoge
>>>> # echo ccc > /tmp/hoge
>>>> # rm /tmp/hoge
>>>>
>>>> Then, the trace data will show;
>>>>
>>>> # tracer: nop
>>>> #
>>>> # entries-in-buffer/entries-written: 16/16 #P:8
>>>> #
>>>> # _-----=> irqs-off/BH-disabled
>>>> # / _----=> need-resched
>>>> # | / _---=> hardirq/softirq
>>>> # || / _--=> preempt-depth
>>>> # ||| / _-=> migrate-disable
>>>> # |||| / delay
>>>> # TASK-PID CPU# ||||| TIMESTAMP FUNCTION
>>>> # [ 7.026136] sh (113) used greatest stack depth: 12912 bytes left
>>>> | | | ||||| | |
>>>> sh-113 [002] ..... 7.024402: truncate: (do_truncate+0x4/0x120) dentry=0xffff8880069194b8
>>>> sh-113 [002] ..Zff 7.024822: watch: (lookup_fast+0xaa/0x150) address=0xffff8880069194b8 value=0x200008
>>>> sh-113 [002] ..Zff 7.024830: watch: (step_into+0x82/0x360) address=0xffff8880069194b8 value=0x200008
>>>> sh-113 [002] ..Zff 7.024834: watch: (step_into+0x9f/0x360) address=0xffff8880069194b8 value=0x200008
>>>> sh-113 [002] ..Zff 7.024839: watch: (path_openat+0xb3a/0xe70) address=0xffff8880069194b8 value=0x200008
>>>> sh-113 [002] ..Zff 7.024843: watch: (path_openat+0xb9a/0xe70) address=0xffff8880069194b8 value=0x200008
>>>> sh-113 [002] ..... 7.024847: truncate: (do_truncate+0x4/0x120) dentry=0xffff8880069194b8
>>>> sh-113 [002] ...1. 7.025364: dentry_kill: (__dentry_kill+0x0/0x220) dentry=0xffff888006919380
>>>> sh-113 [002] ...1. 7.025511: dentry_kill: (__dentry_kill+0x0/0x220) dentry=0xffff8880069195f0
>>>> rm-118 [003] ...1. 7.027543: dentry_kill: (__dentry_kill+0x0/0x220) dentry=0xffff8880069194b8
>>>> sh-113 [002] ...2. 7.027825: dentry_kill: (__dentry_kill+0x0/0x220) dentry=0xffff8880044429c0
>>>> sh-113 [002] ...2. 7.027833: dentry_kill: (__dentry_kill+0x0/0x220) dentry=0xffff888004442270
>>>>
>>>>
>>>> Thank you,
>>>>
>>>> ---
>>>>
>>>> Jinchao Wang (1):
>>>> x86/HWBP: introduce arch_reinstall_hw_breakpoint() for atomic context
>>>>
>>>> Masami Hiramatsu (Google) (5):
>>>> tracing: wprobe: Add watchpoint probe event based on hardware breakpoint
>>>> HWBP: Add modify_wide_hw_breakpoint_local() API
>>>> tracing: wprobe: Add wprobe event trigger
>>>> selftests: tracing: Add a basic testcase for wprobe
>>>> selftests: tracing: Add syntax testcase for wprobe
>>>>
>>>>
>>>> Documentation/trace/index.rst | 1
>>>> Documentation/trace/wprobetrace.rst | 129 ++
>>>> arch/Kconfig | 10
>>>> arch/x86/Kconfig | 1
>>>> arch/x86/include/asm/hw_breakpoint.h | 3
>>>> arch/x86/kernel/hw_breakpoint.c | 61 +
>>>> include/linux/hw_breakpoint.h | 6
>>>> include/linux/trace_events.h | 3
>>>> kernel/events/hw_breakpoint.c | 36 +
>>>> kernel/trace/Kconfig | 24
>>>> kernel/trace/Makefile | 1
>>>> kernel/trace/trace.c | 9
>>>> kernel/trace/trace.h | 5
>>>> kernel/trace/trace_probe.c | 20
>>>> kernel/trace/trace_probe.h | 8
>>>> kernel/trace/trace_wprobe.c | 1111 ++++++++++++++++++++
>>>> .../ftrace/test.d/dynevent/add_remove_wprobe.tc | 68 +
>>>> .../test.d/dynevent/wprobes_syntax_errors.tc | 20
>>>> 18 files changed, 1513 insertions(+), 3 deletions(-)
>>>> create mode 100644 Documentation/trace/wprobetrace.rst
>>>> create mode 100644 kernel/trace/trace_wprobe.c
>>>> create mode 100644 tools/testing/selftests/ftrace/test.d/dynevent/add_remove_wprobe.tc
>>>> create mode 100644 tools/testing/selftests/ftrace/test.d/dynevent/wprobes_syntax_errors.tc
>>>>
>>>> --
>>>> Masami Hiramatsu (Google) <mhiramat@kernel.org>
>>>
>>>
>>
>>
>> --
>> Best regards,
>> Jinchao
>
>

--
Best regards,
Jinchao