Re: [PATCH bpf-next 1/2] bpf: Allow sleepable programs to use tail calls

[Leon Hwang <leon.hwang@linux.dev>]

On 2026/1/30 16:12, Jiri Olsa wrote:
> Allowing sleepable programs to use tail calls.
> 
> Making sure we can't mix sleepable and non-sleepable bpf programs
> in tail call map (BPF_MAP_TYPE_PROG_ARRAY) and allowing it to be
> used in sleepable programs.
> 
> Sleepable programs can be preempted and sleep which might bring
> new source of race conditions, but both direct and indirect tail
> calls should not be affected.
> 
> Direct tail calls work by patching direct jump to callee into bpf
> caller program, so no problem there. We atomically switch from nop
> to jump instruction.
> 
> Indirect tail call reads the callee from the map and then jumps to
> it. The callee bpf program can't disappear (be released) from the
> caller, because it is executed under rcu lock (rcu_read_lock_trace).
> 
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  include/linux/bpf.h   | 1 +
>  kernel/bpf/core.c     | 4 +++-
>  kernel/bpf/verifier.c | 1 +
>  3 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 5524f9429e76..3b0ceb759075 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -287,6 +287,7 @@ struct bpf_map_owner {
>  	enum bpf_prog_type type;
>  	bool jited;
>  	bool xdp_has_frags;
> +	bool sleepable;
>  	u64 storage_cookie[MAX_BPF_CGROUP_STORAGE_TYPE];
>  	const struct btf_type *attach_func_proto;
>  	enum bpf_attach_type expected_attach_type;
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index e0b8a8a5aaa9..5ebece600aeb 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -2401,6 +2401,7 @@ static bool __bpf_prog_map_compatible(struct bpf_map *map,
>  		map->owner->type  = prog_type;
>  		map->owner->jited = fp->jited;
>  		map->owner->xdp_has_frags = aux->xdp_has_frags;
> +		map->owner->sleepable = fp->sleepable;
>  		map->owner->expected_attach_type = fp->expected_attach_type;
>  		map->owner->attach_func_proto = aux->attach_func_proto;
>  		for_each_cgroup_storage_type(i) {
> @@ -2412,7 +2413,8 @@ static bool __bpf_prog_map_compatible(struct bpf_map *map,
>  	} else {
>  		ret = map->owner->type  == prog_type &&
>  		      map->owner->jited == fp->jited &&
> -		      map->owner->xdp_has_frags == aux->xdp_has_frags;
> +		      map->owner->xdp_has_frags == aux->xdp_has_frags &&
> +		      map->owner->sleepable == fp->sleepable;
>  		if (ret &&
>  		    map->map_type == BPF_MAP_TYPE_PROG_ARRAY &&
>  		    map->owner->expected_attach_type != fp->expected_attach_type)
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index e7ff8394e0da..f185ebc6748d 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -21386,6 +21386,7 @@ static int check_map_prog_compatibility(struct bpf_verifier_env *env,
>  		case BPF_MAP_TYPE_STACK:
>  		case BPF_MAP_TYPE_ARENA:
>  		case BPF_MAP_TYPE_INSN_ARRAY:
> +		case BPF_MAP_TYPE_PROG_ARRAY:
>  			break;
>  		default:
>  			verbose(env,

This change also prevents a bad use case: a sleepable program FD could
be inserted into a prog_array map whose owner is a non-sleepable program.

Looking at __bpf_prog_map_compatible(), are there any other program
properties we should consider checking to ensure compatibility when
updating a prog_array map?

From the tail call compatibility point of view, the restriction on
mixing sleepable and non-sleepable programs looks correct to me.

Acked-by: Leon Hwang <leon.hwang@linux.dev>