bpf, x86: Convert struct_ops arena arguments in the trampoline

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.

Keep arena and nullable classification in btf_func_model.
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 <tj@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20260808003938.3486067-8-memxor@gmail.com
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

0 files changed, 0 insertions, 0 deletions