Re: [PATCH bpf-next v4 12/13] selftests/bpf: Add tests for callbacks returning more than 8 bytes

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2026-08-10 at 17:10 -0700, Yonghong Song wrote:
> Add two __failure tests covering the callback return-size checks:
> 
>  - timer_ret_pair_fail: a bpf_timer callback declared to return more than
>    8 bytes, rejected by check_ld_imm() where the callback's PTR_TO_FUNC is
>    created, with "callback function with >8-byte return value is not
>    supported".
> 
>  - exceptions_ret_pair_fail: an exception callback declared to return more
>    than 8 bytes, rejected by do_check_common() when the callback
>    subprogram is verified, with "exception cb cannot return value larger
>    than 8 bytes".
> 
> Both callback bodies are written in inline asm so that the tests do not
> depend on LLVM 23 R0:R2 codegen and run on any compiler. The verifier reads
> the return type from BTF rather than from the instructions, so the >8 byte
> return prototype is supplied through __btf_func_path(), pointing at a
> companion btf__*.c program that exists only to carry that BTF.
> 
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> ---
>  .../selftests/bpf/prog_tests/exceptions.c     |  2 +
>  .../testing/selftests/bpf/prog_tests/timer.c  |  2 +
>  .../bpf/progs/btf__exceptions_ret_pair_fail.c | 10 ++++
>  .../bpf/progs/btf__timer_ret_pair_fail.c      | 10 ++++
>  .../bpf/progs/exceptions_ret_pair_fail.c      | 30 ++++++++++++
>  .../selftests/bpf/progs/timer_ret_pair_fail.c | 49 +++++++++++++++++++
>  6 files changed, 103 insertions(+)

Dropping callback checks avoids adding all this code.

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