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 <[email protected]>
---
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
