[PATCH -next v2] ftrace: Fix potential warning in trace_printk_seq during ftrace_dump

[Tengda Wu]

When calling ftrace_dump_one() concurrently with reading trace_pipe,
a WARN_ON_ONCE() in trace_printk_seq() can be triggered due to a race
condition.

The issue occurs because:

CPU0 (ftrace_dump)                              CPU1 (reader)
echo z > /proc/sysrq-trigger

!trace_empty(&iter)
trace_iterator_reset(&iter) <- len = size = 0
                                                cat 
/sys/kernel/tracing/trace_pipe
trace_find_next_entry_inc(&iter)
  __find_next_entry
    ring_buffer_empty_cpu <- all empty
  return NULL

trace_printk_seq(&iter.seq)
  WARN_ON_ONCE(s->seq.len >= s->seq.size)

In the context between trace_empty() and trace_find_next_entry_inc()
during ftrace_dump, the ring buffer data was consumed by other readers.
This caused trace_find_next_entry_inc to return NULL, failing to populate
`iter.seq`. At this point, due to the prior trace_iterator_reset, both
`iter.seq.len` and `iter.seq.size` were set to 0. Since they are equal,
the WARN_ON_ONCE condition is triggered.

Add a non-NULL check on the return value of trace_find_next_entry_inc
prior to invoking trace_printk_seq, ensuring the `iter.seq` is properly
populated before subsequent operations.

Fixes: d769041f8653 ("ring_buffer: implement new locking")
Signed-off-by: Tengda Wu <wuten...@huaweicloud.com>
---
 kernel/trace/trace.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 4283ed4e8f59..b4cec22753ea 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -10617,6 +10617,7 @@ static void ftrace_dump_one(struct trace_array *tr, 
enum ftrace_dump_mode dump_m
         */
 
        while (!trace_empty(&iter)) {
+               void *ent;
 
                if (!cnt)
                        printk(KERN_TRACE 
"---------------------------------\n");
@@ -10625,17 +10626,18 @@ static void ftrace_dump_one(struct trace_array *tr, 
enum ftrace_dump_mode dump_m
 
                trace_iterator_reset(&iter);
                iter.iter_flags |= TRACE_FILE_LAT_FMT;
+               ent = trace_find_next_entry_inc(&iter);
 
-               if (trace_find_next_entry_inc(&iter) != NULL) {
+               if (ent) {
                        int ret;
 
                        ret = print_trace_line(&iter);
                        if (ret != TRACE_TYPE_NO_CONSUME)
                                trace_consume(&iter);
+
+                       trace_printk_seq(&iter.seq);
                }
                touch_nmi_watchdog();
-
-               trace_printk_seq(&iter.seq);
        }
 
        if (!cnt)
-- 
2.34.1