linux-kernel - fanotify bug on gdb -- hard crash

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Date:	Fri, 26 Dec 2014 22:45:38 +0800
From:	ivo welch <ivo.welch@...erson.ucla.edu>
To:	linux-kernel@...r.kernel.org
Subject: fanotify bug on gdb -- hard crash

I am not a kernel developer, so forgive the intrusion.

I suspect I have found either a bug in gdb (less likely) or a bug in
fanotify (more likely).  it is replicable, and the code is almost
unchanged from the example in the fanotify man page.  to trigger it,
all an su needs to do is to step through the program below with gdb
7.8.1 'n' command, and linux locks up the system pretty hard (reboot
required).  I have confirmed the replicability of this issue on a
clean arch 2014.12.01 3.17.6-1 system and on a clean ubuntu 14.10
system, both VMs created just to check it.  /iaw

#define _GNU_SOURCE     /* Needed to get O_LARGEFILE definition */
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <poll.h>
#include <sys/fanotify.h>

int main(int argc, char *argv[]) {
  int fd;
  fd = fanotify_init(FAN_CLOEXEC | FAN_CLASS_CONTENT | FAN_NONBLOCK,
O_RDONLY | O_LARGEFILE);
  if (fd == -1) exit(1);
  fprintf(stderr, "calling fanotify_mark: fd=%d\n", fd);
  if (fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_MOUNT, FAN_OPEN_PERM |
FAN_CLOSE_WRITE, -1, "/") == -1) exit(2);
  fprintf(stderr, "in gdb step through with 'n' for repeat.\n");
  fprintf(stderr, "  (and sometimes otherwise), a ^C works, but a ^Z
and then ^C does not.\n");
}

I don't know who else to tell this.  I hope this report is useful, if
someone competent can confirm it.  /iaw

PS: Is there an alternative to fanotify to avoid this?  I want to
learn of all file-open requests on a ro device.
----
Ivo Welch (ivo.welch@...il.com)
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