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Date:	Sun, 28 Dec 2014 18:13:32 -0500
From:	Eric Paris <eparis@...hat.com>
To:	Heinrich Schuchardt <xypron.glpk@....de>
Cc:	ivo.welch@...il.com, linux-kernel@...r.kernel.org
Subject: Re: fanotify bug on gdb -- hard crash

Why are you setting FAN_OPEN_PERM and then not responding to perm
requests? Of course the system is going to appear locked, until you
start responding to open events, remove that mark, or close the fanotify
fd...

-Eric

On Fri, 2014-12-26 at 19:40 +0100, Heinrich Schuchardt wrote:
> Hello Ivo,
> 
> On 26.12.2014 15:45, ivo welch wrote:
> > 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 was not able to reproduce your problem according to your description 
> with Ubuntu 14.10.
> 
> I ran a Ubuntu 14.04 amd64 LiveImage in a VM and compiled your example with
> gcc -g -o test test.c
> 
> The gdb version in Ubuntu 14.10 is 7.4 and not 7.8.1. The kernel version 
> is 3.13.
> 
> root@...ntu:/home/ubuntu/temp# gdb ./test
> GNU gdb (Ubuntu/Linaro 7.4-2012.04-0ubuntu2.1) 7.4-2012.04
> Copyright (C) 2012 Free Software Foundation, Inc.
> License GPLv3+: GNU GPL version 3 or later 
> <http://gnu.org/licenses/gpl.html>
> This is free software: you are free to change and redistribute it.
> There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
> and "show warranty" for details.
> This GDB was configured as "x86_64-linux-gnu".
> For bug reporting instructions, please see:
> <http://bugs.launchpad.net/gdb-linaro/>...
> Reading symbols from /home/ubuntu/temp/test...done.
> (gdb) break main
> Breakpoint 1 at 0x400693: file test.c, line 10.
> (gdb) run
> Starting program: /home/ubuntu/temp/test
> warning: no loadable sections found in added symbol-file system-supplied 
> DSO at 0x7ffff7ffa000
> 
> Breakpoint 1, main (argc=1, argv=0x7fffffffe638) at test.c:10
> 10        fd = fanotify_init(FAN_CLOEXEC | FAN_CLASS_CONTENT | FAN_NONBLOCK,
> (gdb) n
> 12        if (fd == -1) exit(1);
> (gdb) n
> 13        fprintf(stderr, "calling fanotify_mark: fd=%d\n", fd);
> (gdb) n
> calling fanotify_mark: fd=7
> 14        if (fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_MOUNT, 
> FAN_OPEN_PERM |
> (gdb) n
> 16        fprintf(stderr, "in gdb step through with 'n' for repeat.\n");
> (gdb) n
> in gdb step through with 'n' for repeat.
> 17        fprintf(stderr, "  (and sometimes otherwise), a ^C works, but 
> a ^Z and then ^C does not.\n");
> (gdb) n
>    (and sometimes otherwise), a ^C works, but a ^Z and then ^C does not.
> 18      }
> (gdb) n
> 0x00007ffff7a3b78d in __libc_start_main () from 
> /lib/x86_64-linux-gnu/libc.so.6
> (gdb) n
> Single stepping until exit from function __libc_start_main,
> which has no line number information.
> [Inferior 1 (process 4423) exited with code 0110]
> (gdb) n
> The program is not being run.
> (gdb) q
> root@...ntu:/home/ubuntu/temp#
> 
> >
> > I don't know who else to tell this.  I hope this report is useful, if
> > someone competent can confirm it.  /iaw
> 
> Bug reports for the Linux kernel should be adressed to the maintainer. 
> You can find him in the MAINTAINERS file of the linux source.
> 
> See
> https://www.kernel.org/pub/linux/docs/lkml/reporting-bugs.html
> https://bugzilla.kernel.org/
> 
> Before reporting a bug it is worthwhile to check if the problem also 
> occurs with the current kernel version (as of today 3.18.1 or 3.19-rc1).
> 
>  > PS: Is there an alternative to fanotify to avoid this?  I want to
>  > learn of all file-open requests on a ro device.
>  > ----
> 
> The fanotify API is the right choice. Inotify is an alternative but 
> requires marking all directories.
> 
> For your task you can use the code provided at
> https://launchpad.net/fatrace
> 
> Best regards
> 
> Heinrich Schuchardt


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