Re: [PATCH bpf 1/2] libbpf: Fix usdt attach failure when nop10 crosses page boundary

On 8/24/26 6:03 PM, [email protected] wrote:
>> Fixes: 41a5c7df4466 ("libbpf: Add support to detect nop,nop5 instructions combo for usdt probe")
>> Signed-off-by: Jiayuan Chen <[email protected]>
> Does the Fixes tag point at the right commit? The -ENOTSUPP attach
> failure could not happen at 41a5c7df4466.
>
> At 41a5c7df4466 the uprobe was shifted onto a nop5 (0f 1f 44 00 00), and
> arch_uprobe_analyze_insn() ran uprobe_init_insn() unconditionally:
>
>      ret = uprobe_init_insn(auprobe, &insn, is_64bit_mm(mm));
>      if (ret)
>          return ret;
>      if (can_optimize(&insn, addr))
>          set_bit(ARCH_UPROBE_FLAG_CAN_OPTIMIZE, &auprobe->flags);
>
> uprobe_init_insn() accepts nop5: is_prefix_bad() finds no prefix, and
> test_bit(OPCODE1 == 0x0f, good_insns_64) is set. So a nop5 that crossed
> a page boundary simply produced a normal, working int3 uprobe. The attach
> succeeded.
>
> The failure was introduced by the nop10 switch:
>
> 554ba38456da ("uprobes/x86: Move optimized uprobe from nop5 to nop10")
> made can_optimize() failure fall through to uprobe_init_insn(), and the
> nop10's 0x2e byte is a CS prefix that is_prefix_bad() rejects:
>
>      if (can_optimize(&insn, addr)) {
>          set_bit(ARCH_UPROBE_FLAG_CAN_OPTIMIZE, &auprobe->flags);
>      } else {
>          ret = uprobe_init_insn(auprobe, &insn);
>          if (ret)
>              return ret;
>      }
>
> ee2862439e5c ("libbpf: Change has_nop_combo to work on top of nop10")
> made libbpf shift the uprobe onto that nop10.
>
> Should the tag be:
>
>      Fixes: ee2862439e5c ("libbpf: Change has_nop_combo to work on top of nop10")


ee2862439e5c is correct.

Waiting for more feedback.

> This also matters for backport scope: a tree that has 41a5c7df4466 but
> not ee2862439e5c still matches the 6-byte nop,nop5 pattern, where the
> new hardcoded +10 in nop10_within_page() would be over-strict by 5 bytes
> while the bug being fixed does not exist there.
>
>
>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/32712703327