Re: [PATCH bpf] bpf: Fix a verifier bug due to incorrect branch offset comparison with cpu=v4

[Stanislav Fomichev <sdf@google.com>]

On 11/28, Yonghong Song wrote:
> Bpf cpu=v4 support is introduced in [1] and Commit 4cd58e9af8b9
> ("bpf: Support new 32bit offset jmp instruction") added support for new
> 32bit offset jmp instruction. Unfortunately, in function
> bpf_adj_delta_to_off(), for new branch insn with 32bit offset, the offset
> (plus/minor a small delta) compares to 16-bit offset bound
> [S16_MIN, S16_MAX], which caused the following verification failure:
>   $ ./test_progs-cpuv4 -t verif_scale_pyperf180
>   ...
>   insn 10 cannot be patched due to 16-bit range
>   ...
>   libbpf: failed to load object 'pyperf180.bpf.o'
>   scale_test:FAIL:expect_success unexpected error: -12 (errno 12)
>   #405     verif_scale_pyperf180:FAIL
> 
> Note that due to recent llvm18 development, the patch [2] (already applied
> in bpf-next) needs to be applied to bpf tree for testing purpose.
> 
> The fix is rather simple. For 32bit offset branch insn, the adjusted
> offset compares to [S32_MIN, S32_MAX] and then verification succeeded.
> 
>   [1] https://lore.kernel.org/all/20230728011143.3710005-1-yonghong.song@linux.dev
>   [2] https://lore.kernel.org/bpf/20231110193644.3130906-1-yonghong.song@linux.dev
> 
> Fixes: 4cd58e9af8b9 ("bpf: Support new 32bit offset jmp instruction")
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>

Acked-by: Stanislav Fomichev <sdf@google.com>