Re: [PATCH bpf-next v10 00/24] bpf: Run exception cleanup landing pads when bpf_unwind() unwinds

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

# CARRIED OVER from the unsent v9 draft (nothing was sent on v9, v10 has only bot fixes). Delete this line, or the file if you dropped it on purpose.
On Thu, Oct 08, 2026 at 09:14 PM Yonghong Song <yonghong.song@linux.dev> wrote:
>   - A subprog an unwind passes through must have an exit.
>   - Until the Rust toolchain supports BPF exception handling, the selftests
>     write their tables by hand in inline asm.

rustc can do it already.
For example,
take RUSTC_BPF from selftests/bpf/Makefile.buildvars, replace
panic=abort with panic=unwind, build core, then --emit=llvm-bc and
clang -c like Makefile.skel does for progs/*.rs.

  #[inline(never)]
  fn fail() -> ! { unsafe { bpf_unwind() }; loop {} }

  #[inline(never)]
  fn check(i: u32) -> u32 { if i > 7 { fail() } i }

  pub extern "C-unwind" fn prog(ctx: *const u32) -> u32 {
      let _g = RcuReadGuard::new();
      check(unsafe { *ctx })
  }

The object has .bpf_cleanup with the right record and this code:

  fail:
      call bpf_unwind
  L0: goto L0
  check:
      if w1 > 0x7 goto L0
      w0 = w1
      exit
  L0: call fail
  prog:
      r6 = r1
      call bpf_rcu_read_lock
      w6 = *(u32 *)(r6 + 0x0)
      w1 = w6
      call check
      call bpf_rcu_read_unlock
      w0 = w6
      exit
      r6 = r0
      call bpf_rcu_read_unlock
      r1 = r6
      call _Unwind_Resume

They won't pass the verifier,
because
_Unwind_Resume() and fail() don't return, so LLVM emits nothing after
the call. check_subprogs() refuses prog() and check() with
"last insn is not an exit or jmp".
rust-bpf hack adds 'exit' and replaces call with 'call bpf_unwind_resume'.
We need to get rid of rust-bpf IR rewrite,
so let's try to do so now?
'call _Unwind_Resume' can be replaced by LLVM, by libbpf, or by the verifier
with an exit. I feel that the verifier is right place to do it,
but open to suggestions.

but since rustc doesn't emit debug info for extern fn, so there is no BTF
for _Unwind_Resume, so libbpf may be a better place?
Not sure, let's discuss.

pw-bot: cr