[for-next][PATCH 4/7] ftrace: Update func_pos in t_start() when all functions are enabled

From: Steven Rostedt
Date: Wed Apr 05 2017 - 12:23:29 EST


From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>

If all functions are enabled, there's a comment displayed in the file to
denote that:

# cd /sys/kernel/debug/tracing
# cat set_ftrace_filter
#### all functions enabled ####

If a function trigger is set, those are displayed as well:

# echo schedule:traceoff >> /debug/tracing/set_ftrace_filter
# cat set_ftrace_filter
#### all functions enabled ####
schedule:traceoff:unlimited

But if you read that file with dd, the output can change:

# dd if=/debug/tracing/set_ftrace_filter bs=1
#### all functions enabled ####
32+0 records in
32+0 records out
32 bytes copied, 7.0237e-05 s, 456 kB/s

This is because the "pos" variable is updated for the comment, but func_pos
is not. "func_pos" is used by the triggers (or hashes) to know how many
functions were printed and it bases its index from the pos - func_pos.
func_pos should be 1 to count for the comment printed. But since it is not,
t_hash_start() thinks that one trigger was already printed.

The cat gets to t_hash_start() via t_next() and not t_start() which updates
both pos and func_pos.

Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
---
kernel/trace/ftrace.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 421530831ddd..d4b18ce9ba88 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -3230,6 +3230,7 @@ static void *t_start(struct seq_file *m, loff_t *pos)
*/
if ((iter->flags & (FTRACE_ITER_FILTER | FTRACE_ITER_NOTRACE)) &&
ftrace_hash_empty(iter->hash)) {
+ iter->func_pos = 1; /* Account for the message */
if (*pos > 0)
return t_hash_start(m, pos);
iter->flags |= FTRACE_ITER_PRINTALL;
--
2.10.2