Re: [PATCH bpf-next v4 04/13] bpf: Support __arena and __arena__nullable kfunc argument suffixes

On Wed, 2026-08-05 at 23:04 +0200, Kumar Kartikeya Dwivedi wrote:
> From: Tejun Heo <[email protected]>
> 
> Passing an arena pointer to a kfunc takes two steps today. There is no
> arena pointer argument type, so the pointer crosses the boundary as a
> bare scalar, and the kfunc then offsets it by the arena base and casts
> it before it can touch the memory. Every such kfunc open-codes the same
> translation.
> 
> Add the __arena and __arena__nullable argument suffixes to make this more
> convenient. The kfunc declares the parameter by its real pointer type
> and dereferences it directly, with the JIT rebasing the value at the
> call site, rN = kern_vm_start + (u32)rN. No bounds check is needed: the
> u32 offset stays within the guard-padded arena kernel mapping, and a
> fault on an unpopulated page recovers through the per-arena scratch
> page. A suffixed argument accepts a PTR_TO_ARENA or scalar register,
> matching global subprog arena arguments.
> 
> __arena rebases unconditionally, so the kfunc never sees NULL and a
> value with zero in the low 32 bits arrives as the arena base.
> __arena__nullable preserves NULL for optional arguments by skipping the
> rebase when the truncated value, arena offset 0, is zero. Keeping the
> plain form NULL-free saves the NULL test on every call.
> 
> The double separator makes the annotations composable:
> __arena__nullable also ends in __nullable. Match the composite suffix
> first when classifying kfunc arguments and function-model flags so it
> retains arena semantics while carrying the nullable flag.
> 
> This patch adds the verifier side: the suffixes are recognized in
> check_kfunc_args() and distilled into argument flags in the function
> model stored in the kfunc descriptor. JITs retrieve the model while
> emitting the call, avoiding per-call state in insn_aux_data.
> 
> JITs declare support with bpf_jit_supports_arena_args() and verification
> fails with -ENOTSUPP elsewhere.
> 
> Signed-off-by: Tejun Heo <[email protected]>
> Co-developed-by: Kumar Kartikeya Dwivedi <[email protected]>
> Signed-off-by: Kumar Kartikeya Dwivedi <[email protected]>
> ---

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