bpf: Factor callchain_finalize function from __bpf_get_stack

The new callchain_finalize function calls the build-id retrieval
(if needed) and zeroes the buffer. This makes things easier for
preemption fix in following change.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260803210149.296496-8-jolsa@kernel.org

1 files changed, 17 insertions, 8 deletions

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index ee9905d67b8e..cdeb6c2e50da 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -764,16 +764,31 @@ static u32 callchain_store(struct perf_callchain_entry *trace, void *buf,
 	return trace_nr;
 }
 
+static long callchain_finalize(void *buf, u32 size, u32 trace_nr, u32 elem_size,
+			       u64 flags, bool may_fault)
+{
+	bool user_build_id = flags & BPF_F_USER_BUILD_ID;
+	bool user = flags & BPF_F_USER_STACK;
+	u32 copy_len = trace_nr * elem_size;
+
+	if (user_build_id)
+		stack_map_get_build_id_offset(buf, trace_nr, user, may_fault);
+
+	if (size > copy_len)
+		memset(buf + copy_len, 0, size - copy_len);
+	return copy_len;
+}
+
 static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
 			    struct perf_callchain_entry *trace_in,
 			    void *buf, u32 size, u64 flags, bool may_fault)
 {
-	u32 trace_nr, copy_len, elem_size, max_depth;
 	bool user_build_id = flags & BPF_F_USER_BUILD_ID;
 	bool crosstask = task && task != current;
 	u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
 	bool user = flags & BPF_F_USER_STACK;
 	struct perf_callchain_entry *trace;
+	u32 trace_nr, elem_size, max_depth;
 	bool kernel = !user;
 	int err = -EINVAL;
 
@@ -821,18 +836,12 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
 	}
 
 	trace_nr = callchain_store(trace, buf, elem_size, flags);
-	copy_len = trace_nr * elem_size;
 
 	/* trace should not be dereferenced after this point */
 	if (may_fault)
 		rcu_read_unlock();
 
-	if (user_build_id)
-		stack_map_get_build_id_offset(buf, trace_nr, user, may_fault);
-
-	if (size > copy_len)
-		memset(buf + copy_len, 0, size - copy_len);
-	return copy_len;
+	return callchain_finalize(buf, size, trace_nr, elem_size, flags, may_fault);
 
 err_fault:
 	err = -EFAULT;