Re: [PATCH bpf-next v3 1/9] bpf: Support __arena and __arena_nullable kfunc argument suffixes

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

On Tue Aug 4, 2026 at 7:42 PM CEST, Amery Hung wrote:
> On Mon, Aug 3, 2026 at 6:12 AM Kumar Kartikeya Dwivedi <memxor@gmail.com> wrote:
>>
>> From: Tejun Heo <tj@kernel.org>
>>
>> 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.
>>
>> 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 <tj@kernel.org>
>> Co-developed-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
>> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
>> ---
>>  Documentation/bpf/kfuncs.rst | 29 ++++++++++++++++++++++++++
>>  include/linux/bpf.h          |  6 ++++++
>>  include/linux/filter.h       |  1 +
>>  kernel/bpf/btf.c             | 18 +++++++++++++++-
>>  kernel/bpf/core.c            |  5 +++++
>>  kernel/bpf/verifier.c        | 40 +++++++++++++++++++++++++++++++++++-
>>  6 files changed, 97 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
>> index cbde86d082cc..8e84484636c5 100644
>> --- a/Documentation/bpf/kfuncs.rst
>> +++ b/Documentation/bpf/kfuncs.rst
>> @@ -278,6 +278,33 @@ An example is given below::
>>                  ...
>>          }
>>
>> +2.3.8 __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); 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.
>> +
>>  .. _BPF_kfunc_nodef:
>>
>>  2.4 Using an existing kernel function
>> @@ -515,6 +542,8 @@ In order to accommodate such requirements, the verifier will enforce strict
>>  PTR_TO_BTF_ID type matching if two types have the exact same name, with one
>>  being suffixed with ``___init``.
>>
>> +.. _BPF_kfunc_arena_access:
>> +
>>  2.8 Accessing arena memory through kfunc arguments
>>  --------------------------------------------------
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 356884587ae1..6da8a6be930c 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -1213,6 +1213,12 @@ struct bpf_prog_offload {
>>  /* The argument is signed. */
>>  #define BTF_FMODEL_SIGNED_ARG          BIT(1)
>>
>> +/* The argument is an arena pointer. */
>> +#define BTF_FMODEL_ARENA_ARG           BIT(2)
>> +
>> +/* The argument is nullable. */
>> +#define BTF_FMODEL_NULLABLE_ARG                BIT(3)
>> +
>>  struct btf_func_model {
>>         u8 ret_size;
>>         u8 ret_flags;
>> diff --git a/include/linux/filter.h b/include/linux/filter.h
>> index 32d5297c557e..36ce3403fe59 100644
>> --- a/include/linux/filter.h
>> +++ b/include/linux/filter.h
>> @@ -1183,6 +1183,7 @@ bool bpf_jit_supports_subprog_tailcalls(void);
>>  bool bpf_jit_supports_percpu_insn(void);
>>  bool bpf_jit_supports_kfunc_call(void);
>>  bool bpf_jit_supports_stack_args(void);
>> +bool bpf_jit_supports_arena_args(void);
>>  bool bpf_jit_supports_far_kfunc_call(void);
>>  bool bpf_jit_supports_exceptions(void);
>>  bool bpf_jit_supports_ptr_xchg(void);
>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>> index 5e8ac45ce56a..bf86df91a70b 100644
>> --- a/kernel/bpf/btf.c
>> +++ b/kernel/bpf/btf.c
>> @@ -7541,6 +7541,22 @@ static u8 __get_type_fmodel_flags(const struct btf_type *t)
>>         return flags;
>>  }
>>
>> +static u8 __get_arg_fmodel_flags(const struct btf *btf,
>> +                                const struct btf_param *arg,
>> +                                const struct btf_type *t)
>> +{
>> +       u8 flags = __get_type_fmodel_flags(t);
>> +
>> +       if (btf_param_match_suffix(btf, arg, "__arena") ||
>> +           btf_param_match_suffix(btf, arg, "__arena_nullable"))
>> +               flags |= BTF_FMODEL_ARENA_ARG;
>> +       if (btf_param_match_suffix(btf, arg, "__nullable") ||
>> +           btf_param_match_suffix(btf, arg, "__arena_nullable"))
>> +               flags |= BTF_FMODEL_NULLABLE_ARG;
>> +
>> +       return flags;
>> +}
>> +
>>  int btf_distill_func_proto(struct bpf_verifier_log *log,
>>                            struct btf *btf,
>>                            const struct btf_type *func,
>> @@ -7606,7 +7622,7 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
>>                         return -EINVAL;
>>                 }
>>                 m->arg_size[i] = ret;
>> -               m->arg_flags[i] = __get_type_fmodel_flags(t);
>> +               m->arg_flags[i] = __get_arg_fmodel_flags(btf, &args[i], t);
>>         }
>>         m->nr_args = nargs;
>>         return 0;
>> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
>> index e2076667b245..a3e1fae32eac 100644
>> --- a/kernel/bpf/core.c
>> +++ b/kernel/bpf/core.c
>> @@ -3308,6 +3308,11 @@ bool __weak bpf_jit_supports_stack_args(void)
>>         return false;
>>  }
>>
>> +bool __weak bpf_jit_supports_arena_args(void)
>> +{
>> +       return false;
>> +}
>> +
>>  bool __weak bpf_jit_supports_far_kfunc_call(void)
>>  {
>>         return false;
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index b274004fccfd..4c50237f49f1 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -10907,6 +10907,16 @@ static bool is_kfunc_arg_irq_flag(const struct btf *btf, const struct btf_param
>>         return btf_param_match_suffix(btf, arg, "__irq_flag");
>>  }
>>
>> +static bool is_kfunc_arg_arena(const struct btf *btf, const struct btf_param *arg)
>> +{
>> +       return btf_param_match_suffix(btf, arg, "__arena");
>> +}
>> +
>> +static bool is_kfunc_arg_arena_nullable(const struct btf *btf, const struct btf_param *arg)
>> +{
>> +       return btf_param_match_suffix(btf, arg, "__arena_nullable");
>> +}
>> +
>
> From the verifier's point of view, __arena and __arena_nullable all
> maps to KF_ARG_PTR_TO_ARENA and require the same check so maybe
> is_kfunc_arg_arena() alone with two suffix matching is good enough,
> but it also doesn't hurt to have two separated functions.
>

Missed in previous reply; I consolidated both into the same is_kfunc_arg_arena()
predicate.

> [...]