The potential issue of kprobe_write_ctx+freplace was mentioned in
"bpf: Disallow !kprobe_write_ctx progs tail-calling kprobe_write_ctx progs" [1].

It is true issue, that the test in patch #2 verifies that kprobe_write_ctx=false
kprobe progs can be abused to modify struct pt_regs via kprobe_write_ctx=true
freplace progs.

When struct pt_regs is modified, bpf_prog_test_run_opts() gets -EFAULT instead
of 0.

test_freplace_kprobe_write_ctx:FAIL:bpf_prog_test_run_opts unexpected error: -14 (errno 14)

We will disallow attaching freplace programs on kprobe programs with different
kprobe_write_ctx values.

Links:
[1] https://lore.kernel.org/bpf/CAP01T74w4KVMn9bEwpQXrk+bqcUxzb6VW1SQ_QvNy0A4EY-9Jg@mail.gmail.com/

Changes:
v2 -> v3:
* Add comment to the rejection of kprobe_write_ctx (per Jiri).
* Use libbpf_get_error() instead of errno in test (per Jiri).
* Collect Acked-by tags from Jiri and Song, thanks.
v2: https://lore.kernel.org/bpf/20260326141718.17731-1-leon.hwang@linux.dev/

v1 -> v2:
* Drop patch #1 in v1, as it wasn't an issue (per Toke).
* Check kprobe_write_ctx value at attach time instead of at load time, to
  prevent attaching kprobe_write_ctx=true freplace progs on
  kprobe_write_ctx=false kprobe progs (per Gemini/sashiko).
* Move kprobe_write_ctx test code to attach_probe.c and kprobe_write_ctx.c.
v1: https://lore.kernel.org/bpf/20260324150444.68166-1-leon.hwang@linux.dev/

Leon Hwang (2):
  bpf: Fix abuse of kprobe_write_ctx via freplace
  selftests/bpf: Add test to verify the fix of kprobe_write_ctx abuse

 kernel/bpf/syscall.c                          | 17 +++++
 .../selftests/bpf/prog_tests/attach_probe.c   | 64 +++++++++++++++++++
 .../selftests/bpf/progs/kprobe_write_ctx.c    | 19 ++++++
 3 files changed, 100 insertions(+)

-- 
2.53.0