[bpf-next,1/2] bpf: Return error for missed kprobe multi bpf program execution

Commit Message

Jiri Olsa Jan. 6, 2025, 5:50 p.m. UTC

When kprobe multi bpf program can't be executed due to recursion check,
we currently return 0 (success) to fprobe layer where it's ignored for
standard kprobe multi probes.

For kprobe session the success return value will make fprobe layer to
install return probe and try to execute it as well.

But the return session probe should not get executed, because the entry
part did not run. FWIW the return probe bpf program most likely won't get
executed, because its recursion check will likely fail as well, but we
don't need to run it in the first place.. also we can make this clear
and obvious.

It also affects missed counts for kprobe session program execution, which
are now doubled (extra count for not executed return probe).

Signed-off-by: Jiri Olsa <[e-mail]>
---
 kernel/trace/bpf_trace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Song Liu Jan. 6, 2025, 10:26 p.m. UTC | #1

On Mon, Jan 6, 2025 at 9:50 AM Jiri Olsa <[e-mail]> wrote:
>
> When kprobe multi bpf program can't be executed due to recursion check,
> we currently return 0 (success) to fprobe layer where it's ignored for
> standard kprobe multi probes.
>
> For kprobe session the success return value will make fprobe layer to
> install return probe and try to execute it as well.
>
> But the return session probe should not get executed, because the entry
> part did not run. FWIW the return probe bpf program most likely won't get
> executed, because its recursion check will likely fail as well, but we
> don't need to run it in the first place.. also we can make this clear
> and obvious.
>
> It also affects missed counts for kprobe session program execution, which
> are now doubled (extra count for not executed return probe).
>
> Signed-off-by: Jiri Olsa <[e-mail]>
> ---
>  kernel/trace/bpf_trace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 48db147c6c7d..1f3d4b72a3f2 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2797,7 +2797,7 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
>
>         if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
>                 bpf_prog_inc_misses_counter(link->link.prog);
> -               err = 0;
> +               err = 1;

nit: Shall we return -EBUSY or some other error code?

>                 goto out;
>         }

>
> --
> 2.47.0
>
>

Masami Hiramatsu (Google) Jan. 6, 2025, 10:42 p.m. UTC | #2

On Mon,  6 Jan 2025 18:50:47 +0100
Jiri Olsa <[e-mail]> wrote:

> When kprobe multi bpf program can't be executed due to recursion check,
> we currently return 0 (success) to fprobe layer where it's ignored for
> standard kprobe multi probes.
> 
> For kprobe session the success return value will make fprobe layer to
> install return probe and try to execute it as well.
> 
> But the return session probe should not get executed, because the entry
> part did not run. FWIW the return probe bpf program most likely won't get
> executed, because its recursion check will likely fail as well, but we
> don't need to run it in the first place.. also we can make this clear
> and obvious.

Yeah, that's right.

> 
> It also affects missed counts for kprobe session program execution, which
> are now doubled (extra count for not executed return probe).
> 
> Signed-off-by: Jiri Olsa <[e-mail]>

Looks good to me.

Acked-by: Masami Hiramatsu (Google) <[e-mail]>

Thanks!

> ---
>  kernel/trace/bpf_trace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 48db147c6c7d..1f3d4b72a3f2 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2797,7 +2797,7 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
>  
>  	if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
>  		bpf_prog_inc_misses_counter(link->link.prog);
> -		err = 0;
> +		err = 1;
>  		goto out;
>  	}
>  
> -- 
> 2.47.0
>

Patch

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 48db147c6c7d..1f3d4b72a3f2 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -2797,7 +2797,7 @@  kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
 
 	if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
 		bpf_prog_inc_misses_counter(link->link.prog);
-		err = 0;
+		err = 1;
 		goto out;
 	}