Re: [PATCH bpf-next v1 1/9] bpf: Support __arena and __arena_nullable kfunc argument suffixes

["Kumar Kartikeya Dwivedi" <memxor@gmail.com>]

On Thu Jul 16, 2026 at 1:05 AM CEST, bot+bpf-ci wrote:
>> diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
>> index c801a330aece..a980266ec788 100644
>> --- a/Documentation/bpf/kfuncs.rst
>> +++ b/Documentation/bpf/kfuncs.rst
>> @@ -250,6 +250,33 @@ Or::
>>                  ...
>>          }
>>
>> +2.3.7 __arena and __arena_nullable Annotations
>> +----------------------------------------------
>> +
>> +Both annotations indicate that the pointer argument points into the
>> +calling program's arena. The JIT rebases the value at the call site so
>> +the kfunc receives a directly dereferenceable kernel address, subject to
>> +the access rules described in :ref:`BPF_kfunc_arena_access` (at most
>> +``GUARD_SZ / 2``, 32 KiB, past the pointer in a single unchecked access).
>> +
>> +With ``__arena`` the rebase is unconditional and the argument is never
>> +NULL: a value whose lower 32 bits are zero arrives as the arena base
>> +address (arena offset 0). The kfunc must not check the argument for NULL.
>> +With ``__arena_nullable`` such a value arrives as NULL instead and the
>> +kfunc must check before dereferencing.
>> +
>> +An example is given below::
>> +
>> +        __bpf_kfunc int bpf_process_item(struct item *item__arena)
>> +        {
>> +        ...
>> +        }
>> +
>> +Calling such a kfunc requires the program to use an arena map and a JIT
>> +with arena argument support (currently x86-64 and arm64); verification
>> +fails otherwise. The program can pass any value without compromising the
>> +kernel. A value that does not point into the arena is a program bug.
>
> The documentation says arena argument support is available on "currently
> x86-64 and arm64", but does arm64 actually support it here?
>
> bpf_jit_supports_arena_args() is only defined to return true in the
> x86-64 JIT. The arm64 JIT is not touched, so on arm64
> bpf_jit_supports_arena_args() falls through to the __weak stub added in
> this commit:
>
> kernel/bpf/core.c:
> bool __weak bpf_jit_supports_arena_args(void)
>
> which returns false.
>
> On arm64, check_kfunc_args() would then reject any __arena or
> __arena_nullable argument with -ENOTSUPP. Should the documentation drop
> arm64 from the list of supported architectures until the arm64 JIT gains
> support?
>

Thanks, fixed the documentation.

>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/29454748905