Re: [PATCH bpf-next v3 3/9] bpf, x86: JIT __arena kfunc argument rebasing

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2026-08-03 at 14:51 +0200, Kumar Kartikeya Dwivedi wrote:
> From: Tejun Heo <tj@kernel.org>
> 
> Implement arena argument rebasing for kfunc calls on x86. R12 already
> holds kern_vm_start whenever the prog has an arena, so each tagged
> argument costs two instructions emitted right before the call:
> 
>   movl %eN, %eN         /* truncate, clear the upper 32 bits */
>   addq %r12, %rN
> 
> A nullable argument tests the truncated value and jumps over the add:
> 
>   movl  %eN, %eN
>   testl %eN, %eN
>   jz    1f
>   addq  %r12, %rN
> 1:
> 
> addq carries a REX prefix for every argument register and is always
> three bytes, so the jz displacement is constant. The sequence is native
> code generated after constant blinding has run on the BPF instruction
> stream, so blinding never sees the rebase and needs no special handling.
> 
> bpf_jit_supports_arena_args() is not flipped yet; that happens when the
> struct_ops trampoline side is in place as well.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

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