bpf, arm64: JIT __arena kfunc argument rebasing

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Implement arena argument rebasing for kfunc calls on arm64. x28 already
holds kern_vm_start whenever the prog has an arena, and the newly added
extended-register add zero-extends the 32-bit arena offset in place, so
an unconditional argument costs a single instruction emitted right
before the call:

  add xN, x28, wN, uxtw

A nullable argument first truncates into wN so that a zero offset leaves
xN holding a real NULL, then tests it and jumps over the add:

  mov wN, wN
  cbz wN, 1f
  add xN, x28, wN, uxtw
1:

The rebase is native code generated after constant blinding has run on
the BPF instruction stream, so blinding never sees it and needs no
special handling. The emitted count depends only on the kfunc model, so
it is identical across JIT passes.

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: Puranjay Mohan <puranjay@kernel.org>
Reviewed-by: Xu Kuohai <xukuohai@huawei.com>
Link: https://lore.kernel.org/bpf/20260813190356.335181-4-puranjay@kernel.org
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

...

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Puranjay Mohan 2026-08-13 12:03:50 -07:00 • committed by Kumar Kartikeya Dwivedi

parent f4adb983ef

commit 760cb40cfd

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11

arch/arm64/net/bpf_jit.h

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42

arch/arm64/net/bpf_jit_comp.c

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