[PATCH] tracefs: fix partial reading of tracefs id file

From: Elazar Leibovich
Date: Mon Dec 31 2018 - 06:59:10 EST

When reading only part of the id file, the ppos isn't tracked correctly.
This is taken care by simple_read_from_buffer.

Reading a sinlge byte, and then the next byte would result EOF.

While this seems like not a big deal, this breaks abstractions that
reads information from files unbuffered. See for example

This code was mentioned as problematic in
commit cd458ba9d5a5
("tracing: Do not (ab)use trace_seq in event_id_read()")

An example C code that show this bug is:

#include <stdio.h>
#include <stdint.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char **argv) {
if (argc < 2)
return 1;
int fd = open(argv[1], O_RDONLY);
char c;
read(fd, &c, 1);
printf("First %c\n", c);
read(fd, &c, 1);
printf("Second %c\n", c);

Then run with, e.g.

sudo ./a.out /sys/kernel/debug/tracing/events/tcp/tcp_set_state/id

You'll notice you're getting the first character twice, instead of the
first two characters in the id file.

Cc: Orit Wasserman <orit.was@xxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Elazar Leibovich <elazar@xxxxxxxxxxxxxxxxx>
kernel/trace/trace_events.c | 3 ---
1 file changed, 3 deletions(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index f94be0c2827b..7345f5f8f3fe 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -1318,9 +1318,6 @@ event_id_read(struct file *filp, char __user *ubuf, size_t cnt, loff_t *ppos)
char buf[32];
int len;

- if (*ppos)
- return 0;
if (unlikely(!id))
return -ENODEV;