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Subject[PATCH AUTOSEL 5.0 056/173] tracing: Fix partial reading of trace event's id file
Date
From: Elazar Leibovich <elazar@lightbitslabs.com>

[ Upstream commit cbe08bcbbe787315c425dde284dcb715cfbf3f39 ]

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 single 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
https://github.com/golang/go/issues/29399

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.

Link: http://lkml.kernel.org/r/20181231115837.4932-1-elazar@lightbitslabs.com

Cc: Orit Wasserman <orit.was@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: stable@vger.kernel.org
Fixes: 23725aeeab10b ("ftrace: provide an id file for each event")
Signed-off-by: Elazar Leibovich <elazar@lightbitslabs.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 5b3b0c3c8a47e..d910e36c34b5c 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;

--
2.20.1
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