Re: [PATCH bpf-next v3 05/15] compiler_types: Add __arena and __nullable BTF type tag qualifiers

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Thu Oct 15, 2026 at 11:41 PM UTC, Kumar Kartikeya Dwivedi wrote:
> Kernel structures increasingly want to publish pointers into a BPF arena
> to the BPF programs they are shared with, and some pointer members that
> reach programs as trusted are legitimately NULL at times. Neither can be
> expressed today: a pointer member reaches the program as an ordinary
> kernel pointer, and sched_ext resorts to a plain u64 holding a
> pre-translated arena address that programs cast before use.
>
> Define __arena and __nullable as BTF type tags, the way __user, __rcu
> and __percpu already are, so that tagged members survive into vmlinux
> and module BTF and the verifier can act on them. To kernel code and to
> sparse the qualifiers are empty: the pointers are ordinary kernel
> pointers. The verifier side follows in the next patches.
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---
>  include/linux/compiler_types.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index c5921f139007..e97ee2d4d1df 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -51,6 +51,8 @@
>  # define __iomem	__attribute__((noderef, address_space(__iomem)))
>  # define __percpu	__attribute__((noderef, address_space(__percpu)))
>  # define __rcu		__attribute__((noderef, address_space(__rcu)))
> +# define __arena
> +# define __nullable
>  static inline void __chk_user_ptr(const volatile void __user *ptr) { }
>  static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
>  /* other */
> @@ -70,6 +72,14 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
>  # define __iomem
>  # define __percpu	__percpu_qual BTF_TYPE_TAG(percpu)
>  # define __rcu		BTF_TYPE_TAG(rcu)
> +/*
> + * Qualifiers for the BPF verifier rather than address spaces: __arena marks
> + * a pointer into a BPF arena, __nullable a pointer that may be NULL. They
> + * reach BPF programs as BTF type tags on kernel struct members; to kernel
> + * code the pointers are ordinary.
> + */
> +# define __arena	BTF_TYPE_TAG(arena)
> +# define __nullable	BTF_TYPE_TAG(nullable)

I feel this is too early to put this for the whole kernel to use.
Only couple fields in sched-ext need it, right?
So sched-ext can define them locally.