[PATCH bpf 1/2] bpf: Do not release trampoline image in case off unregister error

[PATCH bpf 1/2] bpf: Do not release trampoline image in case off unregister error

[Jiri Olsa]

If unregister_fentry fails we still have trampoline image attached
to a function, so releasing it could trigger crash. Releasing the
trampoline image only when the unregister succeeds.

Cc: stable@vger.kernel.org
Fixes: e21aa341785c ("bpf: Fix fexit trampoline.")
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 kernel/bpf/trampoline.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index f02254a21585..01082ecc5c4f 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -618,8 +618,10 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mut
 
 	if (total == 0) {
 		err = unregister_fentry(tr, orig_flags, tr->cur_image->image);
-		bpf_tramp_image_put(tr->cur_image);
-		tr->cur_image = NULL;
+		if (!err) {
+			bpf_tramp_image_put(tr->cur_image);
+			tr->cur_image = NULL;
+		}
 		goto out;
 	}
 
-- 
2.53.0

[PATCH bpf 2/2] bpf: Remove obsolete WARN_ON call

[Jiri Olsa]

The WARN_ON call in bpf_trampoline_update could never hit, because we
direct the code path with (total == 0) to out label, which effectively
skips the WARN_ON call.

The WARN_ON made sense back then when it checked tr->selector, but now
with total being set just inside the function it's useless.

Cc: stable@vger.kernel.org
Fixes: 47e79cbeea4b ("bpf: Remove bpf trampoline selector")
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 kernel/bpf/trampoline.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 01082ecc5c4f..c09d64b83fa4 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -687,7 +687,6 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mut
 	if (err)
 		goto out_free;
 
-	WARN_ON(tr->cur_image && total == 0);
 	if (tr->cur_image)
 		/* progs already running at this address */
 		err = modify_fentry(tr, orig_flags, tr->cur_image->image,
-- 
2.53.0

Re: [PATCH bpf 2/2] bpf: Remove obsolete WARN_ON call

[Song Liu]

On Fri, Apr 24, 2026 at 8:39 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> The WARN_ON call in bpf_trampoline_update could never hit, because we
> direct the code path with (total == 0) to out label, which effectively
> skips the WARN_ON call.
>
> The WARN_ON made sense back then when it checked tr->selector, but now
> with total being set just inside the function it's useless.
>
> Cc: stable@vger.kernel.org
> Fixes: 47e79cbeea4b ("bpf: Remove bpf trampoline selector")
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>

Acked-by: Song Liu <song@kernel.org>

Re: [PATCH bpf 1/2] bpf: Do not release trampoline image in case off unregister error

[Song Liu]

On Fri, Apr 24, 2026 at 8:39 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> If unregister_fentry fails we still have trampoline image attached
> to a function, so releasing it could trigger crash. Releasing the
> trampoline image only when the unregister succeeds.
>
> Cc: stable@vger.kernel.org
> Fixes: e21aa341785c ("bpf: Fix fexit trampoline.")
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>

The fix looks good.

Acked-by: Song Liu <song@kernel.org>

Can we add a test case that triggers this crash without the fix?

Thanks,
Song

Re: [PATCH bpf 1/2] bpf: Do not release trampoline image in case off unregister error

[Jiri Olsa]

On Fri, Apr 24, 2026 at 08:50:51AM -0700, Song Liu wrote:
> On Fri, Apr 24, 2026 at 8:39 AM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > If unregister_fentry fails we still have trampoline image attached
> > to a function, so releasing it could trigger crash. Releasing the
> > trampoline image only when the unregister succeeds.
> >
> > Cc: stable@vger.kernel.org
> > Fixes: e21aa341785c ("bpf: Fix fexit trampoline.")
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> 
> The fix looks good.
> 
> Acked-by: Song Liu <song@kernel.org>
> 
> Can we add a test case that triggers this crash without the fix?

I don't about valid scenario where unregister_fentry would fail now,
that'd be bug that we'd need to fix. We have WARN_ON_ONCE on tampoline
unlink fail.

The fix is meant for when this happens let's go with un-released
trampoline image rather than kernel crash.

jirka

Re: [PATCH bpf 1/2] bpf: Do not release trampoline image in case off unregister error

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 400 bytes --]

> bpf: Do not release trampoline image in case off unregister error

This isn't a bug, but there's a typo in the subject line: "case off"
should be "case of".


---
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