[PATCH 5.15.y 2/2] bpf: Disable preemption in bpf_get_stackid

[Sasha Levin <sashal@kernel.org>]

From: Jiri Olsa <jolsa@kernel.org>

[ Upstream commit 15f1bd8574662f1b7b26aaa2e23ebf4066f0117d ]

The get_perf_callchain call needs disabled preemption plus we need
it disabled as long as we access its returned trace entries buffer.

Note the bpf_get_stackid_pe function is executed already with
preemption disabled.

Fixes: d5a3b1f69186 ("bpf: introduce BPF_MAP_TYPE_STACK_TRACE")
Reported-by: Tao Chen <chen.dylane@linux.dev>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/bpf/20260803210149.296496-6-jolsa@kernel.org

Closes: https://lore.kernel.org/bpf/20260206090653.1336687-2-chen.dylane@linux.dev/
[ replaced scoped_guard(preempt) with a nested block using guard(preempt)() for GNU89 compatibility. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/bpf/stackmap.c | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index c91e6ffa99dde..056acad851a6a 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -412,20 +412,24 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
 		return -EINVAL;
 
 	max_depth = stack_map_calculate_max_depth(map->value_size, elem_size, flags);
-	trace = get_perf_callchain(regs, kernel, user, max_depth,
-				   false, false, 0);
 
-	if (unlikely(!trace))
-		/* couldn't fetch the stack trace */
-		return -EFAULT;
+	{
+		guard(preempt)();
 
-	err = stackid_fastpath(&stackid, map, trace, flags);
-	if (err != -ENOENT)
-		return err;
+		trace = get_perf_callchain(regs, kernel, user, max_depth,
+					   false, false, 0);
+		if (unlikely(!trace))
+			/* couldn't fetch the stack trace */
+			return -EFAULT;
 
-	new_bucket = stackid_new_bucket(&stackid, map);
-	if (!new_bucket)
-		return -ENOMEM;
+		err = stackid_fastpath(&stackid, map, trace, flags);
+		if (err != -ENOENT)
+			return err;
+
+		new_bucket = stackid_new_bucket(&stackid, map);
+		if (!new_bucket)
+			return -ENOMEM;
+	}
 
 	return stackid_install(&stackid, map, new_bucket, flags);
 }
-- 
2.53.0