Re: [PATCH bpf-next v7 05/10] bpf: Account R2 of register-pair returns in live register analysis

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

On 8/18/26 11:32 PM, bot+bpf-ci@kernel.org wrote:
>>      bpf: Account R2 of register-pair returns in live register analysis
>>
>>      A BPF_EXIT of a subprogram returning a value larger than 8 bytes (a
>>      struct/union or an __int128) reads R2 as well as R0, since the second half
>>      of the return value is passed back in R2. compute_insn_live_regs() only
>>      marked R0 used at exit, so a callee's R2 could be considered dead and
>>      cleaned from checkpointed states, which would allow unsound state pruning.
>>
>>      Mark R2 as read at the BPF_EXIT of a subprogram that does return a register
>>      pair. bpf_compute_live_registers() now loops over the subprograms and, for
>>      each, over the [start, end) instruction range from env->subprog_info[], so
>>      the return convention is queried once per subprogram through
>>      bpf_ret_reg_pair() rather than once per instruction.
>>
>>      Marking R2 at every exit instead would be simpler, but R2 would then stay
>>      live backwards across any call that is not followed by a write to R2, which
>>      is nearly every program, and would needlessly hurt state pruning.
>>
>>      Acked-by: Eduard Zingerman <eddyz87@gmail.com>
>>      Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> This commit fixes a bug where R2 was not marked as live at BPF_EXIT for
> subprograms returning register pairs, which could allow unsound state
> pruning. The bug was introduced when register-pair return support was
> added. Should this include:
>
>    Fixes: b176d222c14a ("bpf: Add helpers to describe the R0:R2 return register pair")

No need to have above Fixes in the same series.

>
>
> ---
> 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/32221968339