[PATCH v2] tracing: Fix crash in print_graph_function_event()

tracing_set_tracer() assigns the flags of the newly selected tracer
with:

	tr->current_trace_flags = t->flags ? : t->tracer->flags;

If the new tracer defines neither static flags nor default flags, like
hwlat, irqsoff or wakeup, add_tracer() has no per instance flags copy
for it, and both operands are NULL, so tr->current_trace_flags ends up
being NULL as well.

The ring buffer is not cleared when switching tracers: it is reset
inside tracer_init(), after current_trace_flags was already assigned.
If the buffer still holds function_graph entries of the previous
tracer, printing one of them dereferences the NULL pointer. This was
hit by cat trace during an ftrace stress test:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  #PF: supervisor read access in kernel mode
  RIP: 0010:print_graph_function_event+0xe/0x20
  Call Trace:
   s_show+0x4d/0x110
   seq_read_iter+0x2d4/0x490
   seq_read+0x130/0x170
   vfs_read+0xb5/0x370
   ksys_read+0x6d/0xe0
   do_syscall_64+0xbc/0x450

The snapshot buffer is not reset at all, so this can be reproduced
without a race:

  echo function_graph > /sys/kernel/tracing/current_tracer
  echo 1 > /sys/kernel/tracing/snapshot
  echo hwlat > /sys/kernel/tracing/current_tracer
  cat /sys/kernel/tracing/snapshot

Rather than teaching every user of tr->current_trace_flags about this
corner case, make sure it is never NULL: when the newly selected
tracer has no flags at all, tracing_set_tracer() falls back to a
static empty no_tracer_flags. run_tracer_selftest() can hit the same
case, so the same fallback is added there too. The existing users
already handle opts == NULL, so these tracers keep behaving as before.

Fixes: e29aa918a928 ("tracing: Have function graph tracer define options per instance")
Cc: stable@vger.kernel.org
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Hao Ge <hao.ge@linux.dev>
---
Changes in v2:
- Rework per Steve's suggestion. v1 could trip the WARN_ON_ONCE() in
  add_tracer() at boot, when an instance was created before the tracer
  registers.

v1 Link: https://lore.kernel.org/all/20260920042139.70736-1-hao.ge@linux.dev/ (local)
---
 kernel/trace/trace.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index e4a490d3d08c..fba9521d2511 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c

@@ -99,6 +99,12 @@ struct tracers {
 	struct tracer_flags	*flags;
 };
 
+/*
+ * Tracers that do not define their own flags use these empty flags,
+ * as tr->current_trace_flags must never be NULL.
+ */
+static struct tracer_flags no_tracer_flags;
+
 /*
  * To prevent the comm cache from being overwritten when no
  * tracing is active, only save the comm when a trace event

@@ -1319,6 +1325,8 @@ static int run_tracer_selftest(struct tracer *type)
 
 	tr->current_trace = type;
 	tr->current_trace_flags = type->flags ? : type->default_flags;
+	if (!tr->current_trace_flags)
+		tr->current_trace_flags = &no_tracer_flags;
 
 #ifdef CONFIG_TRACER_MAX_TRACE
 	if (tracer_uses_snapshot(type)) {

@@ -5090,6 +5098,8 @@ int tracing_set_tracer(struct trace_array *tr, const char *buf)
 	}
 
 	tr->current_trace_flags = t->flags ? : t->tracer->flags;
+	if (!tr->current_trace_flags)
+		tr->current_trace_flags = &no_tracer_flags;
 
 	if (trace->init) {
 		ret = tracer_init(trace, tr);

-- 
2.25.1