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

[Tengda Wu <wutengda@huaweicloud.com>]

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.

Furthermore, per the seq_buf specification, the condition len == size
indicates a full buffer, which constitutes a valid state. Consequently,
the equality check and size - 1 adjustment in WARN_ON_ONCE() are redundant
and should be eliminated.

Fixes: d769041f8653 ("ring_buffer: implement new locking")
Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>
---
 kernel/trace/trace.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 4283ed4e8f59..61c5d389dbd3 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -10521,8 +10521,8 @@ trace_printk_seq(struct trace_seq *s)
 	 * PAGE_SIZE, and TRACE_MAX_PRINT is 1000, this is just
 	 * an extra layer of protection.
 	 */
-	if (WARN_ON_ONCE(s->seq.len >= s->seq.size))
-		s->seq.len = s->seq.size - 1;
+	if (WARN_ON_ONCE(s->seq.len > s->seq.size))
+		s->seq.len = s->seq.size;
 
 	/* should be zero ended, but we are paranoid. */
 	s->buffer[s->seq.len] = 0;
@@ -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