[PATCH v2] perf: Add RCU read lock protection to perf_iterate_ctx()

[Breno Leitao]

The perf_iterate_ctx() function performs RCU list traversal but
currently lacks RCU read lock protection. This causes lockdep warnings
when running perf probe with unshare(1) under CONFIG_PROVE_RCU_LIST=y:

	WARNING: suspicious RCU usage
	kernel/events/core.c:8168 RCU-list traversed in non-reader section!!

	 Call Trace:
	  lockdep_rcu_suspicious
	  ? perf_event_addr_filters_apply
	  perf_iterate_ctx
	  perf_event_exec
	  begin_new_exec
	  ? load_elf_phdrs
	  load_elf_binary
	  ? lock_acquire
	  ? find_held_lock
	  ? bprm_execve
	  bprm_execve
	  do_execveat_common.isra.0
	  __x64_sys_execve
	  do_syscall_64
	  entry_SYSCALL_64_after_hwframe

This protection was previously present but was removed in commit
bd2756811766 ("perf: Rewrite core context handling"). Add back the
necessary RCU read locks around perf_iterate_ctx() call in
perf_event_exec().

CC: stable@xxxxxxxxxxxxxxx
Fixes: bd2756811766 ("perf: Rewrite core context handling")
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
Changes in v2:
- Use scoped_guard() instead of rcu_read_lock() as suggested by Peter Zijlstra.
- Link to v1: https://lore.kernel.org/r/20250117-fix_perf_rcu-v1-1-13cb9210fc6a@xxxxxxxxxx
---
 kernel/events/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index bcb09e011e9e1..7dabbcaf825a0 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8321,7 +8321,8 @@ void perf_event_exec(void)
 
 	perf_event_enable_on_exec(ctx);
 	perf_event_remove_on_exec(ctx);
-	perf_iterate_ctx(ctx, perf_event_addr_filters_exec, NULL, true);
+	scoped_guard(rcu)
+		perf_iterate_ctx(ctx, perf_event_addr_filters_exec, NULL, true);
 
 	perf_unpin_context(ctx);
 	put_ctx(ctx);

---
base-commit: d082ecbc71e9e0bf49883ee4afd435a77a5101b6
change-id: 20250117-fix_perf_rcu-2ff93190950a

Best regards,
-- 
Breno Leitao <leitao@xxxxxxxxxx>