From: Vasily Averin vvs@virtuozzo.com
[ Upstream commit e4075e8bdffd93a9b6d6e1d52fabedceeca5a91b ]
if seq_file .next fuction does not change position index, read after some lseek can generate unexpected output.
Without patch: # dd bs=4 skip=1 if=/sys/kernel/tracing/set_ftrace_pid dd: /sys/kernel/tracing/set_ftrace_pid: cannot skip to specified offset id no pid 2+1 records in 2+1 records out 10 bytes copied, 0.000213285 s, 46.9 kB/s
Notice the "id" followed by "no pid".
With the patch: # dd bs=4 skip=1 if=/sys/kernel/tracing/set_ftrace_pid dd: /sys/kernel/tracing/set_ftrace_pid: cannot skip to specified offset id 0+1 records in 0+1 records out 3 bytes copied, 0.000202112 s, 14.8 kB/s
Notice that it only prints "id" and not the "no pid" afterward.
Link: http://lkml.kernel.org/r/4f87c6ad-f114-30bb-8506-c32274ce2992@virtuozzo.com
https://bugzilla.kernel.org/show_bug.cgi?id=206283 Signed-off-by: Vasily Averin vvs@virtuozzo.com Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org Signed-off-by: Sasha Levin sashal@kernel.org Signed-off-by: Yang Yingliang yangyingliang@huawei.com --- kernel/trace/ftrace.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c index 5379523..0c379cd4 100644 --- a/kernel/trace/ftrace.c +++ b/kernel/trace/ftrace.c @@ -6525,9 +6525,10 @@ static void *fpid_next(struct seq_file *m, void *v, loff_t *pos) struct trace_array *tr = m->private; struct trace_pid_list *pid_list = rcu_dereference_sched(tr->function_pids);
- if (v == FTRACE_NO_PIDS) + if (v == FTRACE_NO_PIDS) { + (*pos)++; return NULL; - + } return trace_pid_next(pid_list, v, pos); }