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

Masami Hiramatsu (Google)
2025-09-01T14:44:40Z

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.

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>