tracing: Fix partial reading of trace event's id file

BugLink: https://bugs.launchpad.net/bugs/1832661

commit cbe08bcb 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 cd458ba9
("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: 23725aee ("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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

kernel/trace/trace_events.c

@@ -1288,9 +1288,6 @@ 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;