[PATCH v4 0/5] tracing: add ref_trace_final_put tracing

[Eugene Mavick <m@mavick.dev>]

When debugging use-after-free(UAF) bugs, knowing when the object reaches
0 references and enters final release can significantly aid the
debugging process.

There is currently no universal way to trace this information.

This patch series implements tracing of the final puts in the
most widely used refcounting implementations,
refcount_t(and thus kref which uses it), and percpu-ref.

The tracepoint records three fields:
- caller: function that called the refcounting
  function(refcount_sub_and_test, percpu_ref_put_many)
- ip: return address of the trace wrapper macro call
- obj: refcount object(struct percpu_ref, refcount_t)

Signed-off-by: Eugene Mavick <m@mavick.dev>
---
Changes in v4:
ref-trace:
-remove fn
-add ip variable
-change trace wrapper macro respectively, _THIS_IP_ is used for ip variable
-change relevant code respect to fn removal and ip addition
-fix style issues in include/linux/ref_trace.h
-add new macro do_trace_ref_final_put_cond that only calls tracepoint if cond is
 true
lib/refcount.c:
-change from do_trace_ref_final_put to *_cond
-remove if statement above since _cond already performs the check
KUnit:
-change relevant code respect to fn removal and ip addition
-check if caller and ip are valid addresses
-change timeout from 10 jiffies to 10 seconds
-move didn't timeout assertion from before to after probe
 unregistration, to prevent it from impacting next test

Changes in v3:
include/trace/events/ref_trace.h kernel doc comments:
-caller of refcount function -> return address of refcount function
-ref_trace_final_put->do_ref_trace_final_put
lib/ref_trace.c: add include trace/events/ref_trace.h
kunit:
-change Kconfig depends from FTRACE->TRACEPOINTS
-EXPECT_FALSE->ASSERT_FALSE for calling percpu_ref_init
-add tracepoint_synchronise_unregister to test_exit macro
-added timeout to capture.count waiting
-remove noinline and __always_inline from function attributes
 (added for testing, but accidentally submitted)
-add period to the end of Kconfig help text
v2 link:
https://lore.kernel.org/all/20260710-refcount-final-put-trace-v2-0-557cfce860a2@mavick.dev/

Changes in v2:
-include/linux/ref_trace.h: change macro name, use direct tracepoint
 call in macro to avoid double check
-add tracepoint to refcount_dec_if_one
-kunit: make significant improvements to design, fix critical bug, add test case for
 refcount_dec_if_one()
-Link to v1: https://lore.kernel.org/r/20260705-refcount-final-put-trace-v1-0-0ae936edb750@mavick.dev

---
Eugene Mavick (5):
      tracing: add ref_trace_final_put tracepoint
      refcount: add ref_trace_final_put tracepoint
      percpu-refcount: add ref_trace_final_put trace
      kunit: add test for ref_trace_final_put
      MAINTAINERS: add entries for ref_trace_final_put

 MAINTAINERS                      |   3 +
 include/linux/percpu-refcount.h  |   5 +-
 include/linux/ref_trace.h        |  40 +++++++++++
 include/linux/refcount.h         |   2 +
 include/trace/events/ref_trace.h |  51 ++++++++++++++
 lib/Kconfig                      |  10 +++
 lib/Makefile                     |   2 +
 lib/ref_trace.c                  |  13 ++++
 lib/refcount.c                   |   6 +-
 lib/tests/Makefile               |   1 +
 lib/tests/ref_trace_kunit.c      | 141 +++++++++++++++++++++++++++++++++++++++
 11 files changed, 272 insertions(+), 2 deletions(-)
---
base-commit: df685633c3dbc67441cc86f1c3fee58de4652ba2
change-id: 20260624-refcount-final-put-trace-49bd7c39bd5a

Best regards,
-- 
Eugene Mavick <m@mavick.dev>