[PATCH v2] perf: get_perf_callchain return NULL for crosstask

[Jordan Rome]

Return NULL instead of returning 1 incorrect frame, which
currently happens when trying to walk the user stack for
any task that isn't current. Returning NULL is a better
indicator that this behavior is not supported.

This issue was found using bpf_get_task_stack inside a BPF
iterator ("iter/task"), which iterates over all tasks. The
single address/frame in the buffer when getting user stacks
for tasks that aren't current could not be symbolized (testing
multiple symbolizers).

Signed-off-by: Jordan Rome <linux@xxxxxxxxxxxxxx>
---

Changes in v2:
* move user and crosstask check before get_callchain_entry

v1:
https://lore.kernel.org/linux-perf-users/CAEf4BzaWtOeTBb_+b7Td3NHaKjZU+OohuBJje_nvw9kd6xPA3g@xxxxxxxxxxxxxx/T/#t

 kernel/events/callchain.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 1273be84392c..104ea2975a57 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -184,6 +184,9 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
 	struct perf_callchain_entry_ctx ctx;
 	int rctx;
 
+	if (user && crosstask)
+		return NULL;
+
 	entry = get_callchain_entry(&rctx);
 	if (!entry)
 		return NULL;
@@ -209,9 +212,6 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
 		}
 
 		if (regs) {
-			if (crosstask)
-				goto exit_put;
-
 			if (add_mark)
 				perf_callchain_store_context(&ctx, PERF_CONTEXT_USER);
 
@@ -219,7 +219,6 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
 		}
 	}
 
-exit_put:
 	put_callchain_entry(rctx);
 
 	return entry;
-- 
2.39.3