Re: [PATCH bpf-next v4 07/13] bpf, x86: Convert struct_ops arena arguments in the trampoline

On Wed, 2026-08-05 at 23:04 +0200, Kumar Kartikeya Dwivedi wrote:
> From: Tejun Heo <[email protected]>
> 
> Implement the struct_ops arena argument conversion on x86. save_args()
> receives the arena base from bpf_tramp_arena_base() and consults the
> btf_func_model argument flags as it copies each native argument into the
> BPF ctx, routing a marked argument through RAX:
> 
>   movl %esrc, %eax      /* truncate and clear the upper 32 bits */
>   subl $base_lo, %eax
>   movq %rax, ctx_slot
> 
> A nullable argument tests the full 64-bit kernel pointer first:
> 
>   movq  %rsrc, %rax
>   testq %rax, %rax
>   jz    1f
>   subl  $base_lo, %eax
> 1:
>   movq  %rax, ctx_slot
> 
> The 32-bit subtraction is sufficient since (u32)(kaddr - base) ==
> (u32)kaddr - (u32)base, and it clears the upper half as the JITs require
> of arena pointer registers. Stack-passed arguments already reload
> through RAX, so only the subtraction (and the NULL test) is inserted
> there.
> 
> Keeping arena and nullable classification in btf_func_model avoids a
> parallel trampoline slot bitmap. bpf_tramp_arena_base() returns a base
> only for a single-program struct_ops indirect trampoline; other
> trampolines pass zero and perform no conversion. The size probe reruns
> the same emission with the same model and nodes, so the image size
> matches by construction.
> 
> With both the kfunc and struct_ops directions implemented, flip
> bpf_jit_supports_arena_args() on for x86.
> 
> Signed-off-by: Tejun Heo <[email protected]>
> Signed-off-by: Kumar Kartikeya Dwivedi <[email protected]>
> ---

[But please fix the botched squash discussed earlier]

Acked-by: Eduard Zingerman <[email protected]>

...