Re: [PATCH bpf-next v2 06/13] bpf: Reject callbacks returning more than 8 bytes

[Yonghong Song <yonghong.song@linux.dev>]

On 8/4/26 2:54 PM, bot+bpf-ci@kernel.org wrote:
>> commit 9c43eef7a91393c06822d44c361ee806f383cf97
>> Author: Yonghong Song <yonghong.song@linux.dev>
>>
>>      bpf: Reject callbacks returning more than 8 bytes
>>
>>      A callback handed to a helper or a kfunc (bpf_loop(),
>>      bpf_timer_set_callback(), bpf_for_each_map_elem(), ...) is invoked
>>      through bpf_callback_t, and an exception callback is invoked by
>>      bpf_throw() through
>>
>>        u64 (*bpf_exception_cb)(u64 cookie, u64 sp, u64 bp, u64, u64);
>>
>>      Both prototypes yield a single u64 in R0, and neither caller has any
>>      notion of a second return register, so a callback returning a value in
>>      the R0:R2 pair would have the upper half of its return value silently
>>      dropped.
>>
>>      Reject both at load time:
>>
>>       - check_ld_imm(): a callback is materialized as PTR_TO_FUNC by an
>>         ld_imm64 pointing at its subprogram, so the subprogram's return
>>         convention can be checked where the callback pointer is created,
>>         before it ever reaches a helper or kfunc argument.
>>
>>       - do_check_common(): an exception callback is not referenced by a
>>         PTR_TO_FUNC, it is named by a BTF decl_tag and verified on its own,
>>         so check it as its frame is set up, next to the existing "cannot
>>         return void" and single-argument checks.
>>
>>      Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> This looks like a bug fix. Should this include:
>
>    Fixes: 63b5669f8a0d ("bpf: Add helpers to describe the R0:R2 return register pair")

There is no need to the bug fix. The actual R2 usage is in later patch
   Add verifier support for 16-byte returns in R0:R2

>
>
> ---
> 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/30949310795