Re: [PATCH bpf-next v2] bpf: Add __prog tag to pass in prog->aux

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2025-05-12 at 19:57 -0700, Kumar Kartikeya Dwivedi wrote:

[...]

In Documentation/bpf/kfuncs.rst there are descriptions for several
suffixes that are supported, "__prog" should be added there.

Do we want to add a separate test case for this feature?
It looks like there are no tests for other suffixes,
so relying on wq tests passing is probably fine.

Implementation lgtm, a few nits below.

>  include/linux/bpf_verifier.h |  1 +
>  kernel/bpf/helpers.c         |  4 ++--
>  kernel/bpf/verifier.c        | 33 +++++++++++++++++++++++++++------
>  3 files changed, 30 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index 9734544b6957..7dd85ed6059e 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -606,6 +606,7 @@ struct bpf_insn_aux_data {
>  	bool calls_callback;
>  	/* registers alive before this instruction. */
>  	u16 live_regs_before;
> +	u16 arg_prog;

Nit: there is a 4-bit hole after `fastcall_spills_num`,
     `arg_prog` field could be put there and 2 bytes at the tail
     would be remain for future extension.

>  };
> 
>  #define MAX_USED_MAPS 64 /* max number of maps accessed by one eBPF program */
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index fed53da75025..43cbf439b9fb 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -3012,9 +3012,9 @@ __bpf_kfunc int bpf_wq_start(struct bpf_wq *wq, unsigned int flags)
>  __bpf_kfunc int bpf_wq_set_callback_impl(struct bpf_wq *wq,
>  					 int (callback_fn)(void *map, int *key, void *value),
>  					 unsigned int flags,
> -					 void *aux__ign)
> +					 void *aux__prog)
>  {
> -	struct bpf_prog_aux *aux = (struct bpf_prog_aux *)aux__ign;
> +	struct bpf_prog_aux *aux = (struct bpf_prog_aux *)aux__prog;
>  	struct bpf_async_kern *async = (struct bpf_async_kern *)wq;
> 
>  	if (flags)
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 28f5a7899bd6..f409a06099f6 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -343,6 +343,7 @@ struct bpf_kfunc_call_arg_meta {
>  		int uid;
>  	} map;
>  	u64 mem_size;
> +	u32 arg_prog;

Nit: this is a boolean flag, I'd put it after `bool arg_owning_ref`,
     as there is a 3 bytes hole there.

>  };
> 
>  struct btf *btf_vmlinux;

[...]

@@ -12906,6 +12912,17 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 		if (is_kfunc_arg_ignore(btf, &args[i]))
 			continue;
 
+		if (is_kfunc_arg_prog(btf, &args[i])) {
+			/* Used to reject repeated use of __aux. */
                                                          ^^^^^^
                                               Nit: should be __prog.

+			if (meta->arg_prog) {
+				verbose(env, "Only 1 prog->aux argument supported per-kfunc\n");
+				return -EFAULT;
+			}
+			meta->arg_prog = regno;
+			cur_aux(env)->arg_prog = regno;
+			continue;
+		}
+
 		if (btf_type_is_scalar(t)) {
 			if (reg->type != SCALAR_VALUE) {
 				verbose(env, "R%d is not a scalar\n", regno);