[RFC PATCH bpf-next v2 2/2] bpf: Pass external callchain entry to get_perf_callchain

[Tao Chen <chen.dylane@linux.dev>]

As Alexei noted, get_perf_callchain() return values may be reused
if a task is preempted after the BPF program enters migrate disable
mode. Drawing on the per-cpu design of bpf_perf_callchain_entries,
stack-allocated memory of bpf_perf_callchain_entry is used here.

Signed-off-by: Tao Chen <chen.dylane@linux.dev>
---
 kernel/bpf/stackmap.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 94e46b7f340..acd72c021c0 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -31,6 +31,11 @@ struct bpf_stack_map {
 	struct stack_map_bucket *buckets[] __counted_by(n_buckets);
 };
 
+struct bpf_perf_callchain_entry {
+	u64 nr;
+	u64 ip[PERF_MAX_STACK_DEPTH];
+};
+
 static inline bool stack_map_use_build_id(struct bpf_map *map)
 {
 	return (map->map_flags & BPF_F_STACK_BUILD_ID);
@@ -305,6 +310,7 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
 	bool user = flags & BPF_F_USER_STACK;
 	struct perf_callchain_entry *trace;
 	bool kernel = !user;
+	struct bpf_perf_callchain_entry entry = { 0 };
 
 	if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
 			       BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID)))
@@ -314,12 +320,8 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
 	if (max_depth > sysctl_perf_event_max_stack)
 		max_depth = sysctl_perf_event_max_stack;
 
-	trace = get_perf_callchain(regs, NULL, kernel, user, max_depth,
-				   false, false);
-
-	if (unlikely(!trace))
-		/* couldn't fetch the stack trace */
-		return -EFAULT;
+	trace = get_perf_callchain(regs, (struct perf_callchain_entry *)&entry,
+				   kernel, user, max_depth, false, false);
 
 	return __bpf_get_stackid(map, trace, flags);
 }
@@ -412,6 +414,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
 	u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
 	bool user = flags & BPF_F_USER_STACK;
 	struct perf_callchain_entry *trace;
+	struct bpf_perf_callchain_entry entry = { 0 };
 	bool kernel = !user;
 	int err = -EINVAL;
 	u64 *ips;
@@ -451,8 +454,8 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
 	else if (kernel && task)
 		trace = get_callchain_entry_for_task(task, max_depth);
 	else
-		trace = get_perf_callchain(regs, NULL, kernel, user, max_depth,
-					   crosstask, false);
+		trace = get_perf_callchain(regs, (struct perf_callchain_entry *)&entry,
+					   kernel, user, max_depth, crosstask, false);
 
 	if (unlikely(!trace) || trace->nr < skip) {
 		if (may_fault)
-- 
2.48.1