linux-kernel - [PATCH 4.4 049/241] tracing: Fix partial reading of trace events id file

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Date:   Sun,  9 Jun 2019 18:39:51 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Orit Wasserman <orit.was@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Elazar Leibovich <elazar@...htbitslabs.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>
Subject: [PATCH 4.4 049/241] tracing: Fix partial reading of trace events id file

From: Elazar Leibovich <elazar@...htbitslabs.com>

commit cbe08bcbbe787315c425dde284dcb715cfbf3f39 upstream.

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@...il.com>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: stable@...r.kernel.org
Fixes: 23725aeeab10b ("ftrace: provide an id file for each event")
Signed-off-by: Elazar Leibovich <elazar@...htbitslabs.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/trace/trace_events.c |    3 ---
 1 file changed, 3 deletions(-)

--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -1288,9 +1288,6 @@ event_id_read(struct file *filp, char __
 	char buf[32];
 	int len;
 
-	if (*ppos)
-		return 0;
-
 	if (unlikely(!id))
 		return -ENODEV;