Procfs race condition bug

Procfs race condition bug

[Mike Cardwell]

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I originally posted this two years ago (*) but received no response. I
just had a look and the problem still exists on the 3.14 kernel I am
currently running.

I *think* I've uncovered a race condition bug in procfs. If I attempt to
open a file in /proc/net, eg "/proc/net/tcp" it works fine, but if I
spawn a POSIX thread and attempt to do it from there, it *usually* fails
with a "No such file or directory", but some times succeeds. If I do a
system call inside the thread to look up the thread ID and then open
"/proc/THREADID/net/tcp" instead, it works fine.

There are more details and some example code so you can replicate the
problem on a stack overflow question I asked previously here:
http://stackoverflow.com/questions/11580020/opening-proc-net-tcp-in-c-from-a-posix-thread-fails-most-of-the-time

(*) https://lkml.org/lkml/2012/7/20/331

-- 
Mike Cardwell  https://grepular.com https://emailprivacytester.com
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/proc/net/ symlink interacts badly with threads (Re: Procfs race condition bug)

[One Thousand Gnomes]

On Fri, 4 Jul 2014 11:13:11 +0100
Mike Cardwell <linux@lists.grepular.com> wrote:

> I originally posted this two years ago (*) but received no response. I
> just had a look and the problem still exists on the 3.14 kernel I am
> currently running.
> 
> I *think* I've uncovered a race condition bug in procfs. If I attempt to

I don't think you have found a race condition.

/proc/net/tcp is a symbolic link to /proc/self/net/tcp

Your thread does an open of /proc/net/tcp, which is a symlink
to /proc/self/net/tcp. If your thread id matches your process id then all
will be as you expect, if not then all will be pear shaped.

As far as I can see the code is doing precisely what it should do. It's
just that the use of the symlink when namespaces were added produces some
rather odd effects. 

It's certainly a bug. It used to work, it's sensible it should work.

Added netdev

Alan

Re: Procfs race condition bug

[Alexey Dobriyan]

> I originally posted this two years ago (*) but received no response.
> I just had a look and the problem still exists on the 3.14 kernel
> I am currently running.
>
> I *think* I've uncovered a race condition bug in procfs.
> If I attempt to open a file in /proc/net, eg "/proc/net/tcp"
> it works fine, but if I spawn a POSIX thread and attempt to do it
> from there, it *usually* fails with a "No such file or directory",
> but some times succeeds. If I do a system call inside the thread
> to look up the thread ID and then open "/proc/THREADID/net/tcp"
> instead, it works fine.
>
> There are more details and some example code
> so you can replicate the problem on a stack overflow question
> I asked previously here:
> http://stackoverflow.com/questions/11580020/opening-proc-net-tcp-in-c-from-a-posix-thread-fails-most-of-the-time
>
> (*) https://lkml.org/lkml/2012/7/20/331

Mike,

as was correctly notes on SO, what's happening is that original thread exits
before spawned thread does open().

->lookup
proc_tgid_net_lookup
get_proc_task_net
nsproxy = NULL          <== thread is dead
ENOENT

This was probably broken when /proc/net became symlink:

commit e9720acd728a46cb40daa52c99a979f7c4ff195c
Author: Pavel Emelyanov <xemul@openvz.org>
Date:   Fri Mar 7 11:08:40 2008 -0800

    [NET]: Make /proc/net a symlink on /proc/self/net (v3)


So, userspace has two solutions:
1) original thread doesn't exit too early
2) spawned thread uses /proc/$TID


So,
we definitely broke /proc/net/tcp somewhere after netns concept was introduced.

But,
you'd have very same problem with other /proc files (anything under
/proc/$PID/).

    Alexey


#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>

void *f(void *_)
{
int fd;

sleep(1);

fd = open("/proc/net/tcp", O_RDONLY);
if (fd == -1) {
fprintf(stderr, "FAIL\n");
return NULL;
}
fprintf(stderr, "OK\n");
return NULL;
}

int main(void)
{
pthread_t thread;

pthread_create(&thread, NULL, f, NULL);
pthread_exit(0);
return 0;
}

Re: Procfs race condition bug

[Alexey Dobriyan]

[broken email]

On Wed, Jul 9, 2014 at 3:17 PM, Alexey Dobriyan <adobriyan@gmail.com> wrote:
>> I originally posted this two years ago (*) but received no response.
>> I just had a look and the problem still exists on the 3.14 kernel
>> I am currently running.
>>
>> I *think* I've uncovered a race condition bug in procfs.
>> If I attempt to open a file in /proc/net, eg "/proc/net/tcp"
>> it works fine, but if I spawn a POSIX thread and attempt to do it
>> from there, it *usually* fails with a "No such file or directory",
>> but some times succeeds. If I do a system call inside the thread
>> to look up the thread ID and then open "/proc/THREADID/net/tcp"
>> instead, it works fine.
>>
>> There are more details and some example code
>> so you can replicate the problem on a stack overflow question
>> I asked previously here:
>> http://stackoverflow.com/questions/11580020/opening-proc-net-tcp-in-c-from-a-posix-thread-fails-most-of-the-time
>>
>> (*) https://lkml.org/lkml/2012/7/20/331
>
> Mike,
>
> as was correctly notes on SO, what's happening is that original thread exits
> before spawned thread does open().
>
> ->lookup
> proc_tgid_net_lookup
> get_proc_task_net
> nsproxy = NULL          <== thread is dead
> ENOENT
>
> This was probably broken when /proc/net became symlink:
>
> commit e9720acd728a46cb40daa52c99a979f7c4ff195c
> Author: Pavel Emelyanov <xemul@openvz.org>
> Date:   Fri Mar 7 11:08:40 2008 -0800
>
>     [NET]: Make /proc/net a symlink on /proc/self/net (v3)
>
>
> So, userspace has two solutions:
> 1) original thread doesn't exit too early
> 2) spawned thread uses /proc/$TID
>
>
> So,
> we definitely broke /proc/net/tcp somewhere after netns concept was introduced.
>
> But,
> you'd have very same problem with other /proc files (anything under
> /proc/$PID/).
>
>     Alexey
>
>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <pthread.h>
> #include <unistd.h>
>
> void *f(void *_)
> {
> int fd;
>
> sleep(1);
>
> fd = open("/proc/net/tcp", O_RDONLY);
> if (fd == -1) {
> fprintf(stderr, "FAIL\n");
> return NULL;
> }
> fprintf(stderr, "OK\n");
> return NULL;
> }
>
> int main(void)
> {
> pthread_t thread;
>
> pthread_create(&thread, NULL, f, NULL);
> pthread_exit(0);
> return 0;
> }

Re: Procfs race condition bug

[Eric W. Biederman]

Alexey Dobriyan <adobriyan@gmail.com> writes:

> [broken email]
>
> On Wed, Jul 9, 2014 at 3:17 PM, Alexey Dobriyan <adobriyan@gmail.com> wrote:
>>> I originally posted this two years ago (*) but received no response.
>>> I just had a look and the problem still exists on the 3.14 kernel
>>> I am currently running.
>>>
>>> I *think* I've uncovered a race condition bug in procfs.
>>> If I attempt to open a file in /proc/net, eg "/proc/net/tcp"
>>> it works fine, but if I spawn a POSIX thread and attempt to do it
>>> from there, it *usually* fails with a "No such file or directory",
>>> but some times succeeds. If I do a system call inside the thread
>>> to look up the thread ID and then open "/proc/THREADID/net/tcp"
>>> instead, it works fine.
>>>
>>> There are more details and some example code
>>> so you can replicate the problem on a stack overflow question
>>> I asked previously here:
>>> http://stackoverflow.com/questions/11580020/opening-proc-net-tcp-in-c-from-a-posix-thread-fails-most-of-the-time
>>>
>>> (*) https://lkml.org/lkml/2012/7/20/331
>>
>> Mike,
>>
>> as was correctly notes on SO, what's happening is that original thread exits
>> before spawned thread does open().
>>
>> ->lookup
>> proc_tgid_net_lookup
>> get_proc_task_net
>> nsproxy = NULL          <== thread is dead
>> ENOENT
>>
>> This was probably broken when /proc/net became symlink:
>>
>> commit e9720acd728a46cb40daa52c99a979f7c4ff195c
>> Author: Pavel Emelyanov <xemul@openvz.org>
>> Date:   Fri Mar 7 11:08:40 2008 -0800
>>
>>     [NET]: Make /proc/net a symlink on /proc/self/net (v3)
>>
>>
>> So, userspace has two solutions:
>> 1) original thread doesn't exit too early
>> 2) spawned thread uses /proc/$TID
>>
>>
>> So,
>> we definitely broke /proc/net/tcp somewhere after netns concept was introduced.
>>
>> But,
>> you'd have very same problem with other /proc files (anything under
>> /proc/$PID/).


Agreed it is a /proc/$TGID vs /proc/$TID issue.

In principle this is fixable by creating a /proc/current symlink that
always points to the proc directory for the current thread and then
pointing /proc/net and /proc/mounts at it.

This is one of those weird cases it so that /proc/net or /proc/mounts
resolves may actually break an existing userspace application, because
different threads can point at different values.  (I very much dislike
what the linux pthread support did to /proc/self).

I tilted at that windmill once and ran out of steam.  While I can write
the patch I don't have the energy to test and see if there are any
pthread programs that will break if /proc/net points to
/proc/current/net instead of /proc/self/net.

Frankly new applications should be using netlink and not /proc/net so I
personally don't think this is worth fixing for the /proc/net case.  Are
there real world applications that are broken by the kernel change in
behavior?  The stackoverflow discussion sounds like it was just an
investigation into weird kernel behavior.

Eric

Procfs race condition bug

[Mike Cardwell]

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I *think* I've uncovered a race condition bug in procfs. If I attempt to
open a file in /proc/net, eg "/proc/net/tcp" it works fine, but if I
spawn a POSIX thread and attempt to do it from there, it *usually* fails
with a "No such file or directory", but some times succeeds. If I do a
system call inside the thread to look up the thread ID and then open
"/proc/THREADID/net/tcp" instead, it works fine.

There are more details and  some example code so you can replicate the
problem on a stack overflow question I asked earlier today here:
http://stackoverflow.com/questions/11580020/opening-proc-net-tcp-in-c-from-a-posix-thread-fails-most-of-the-time

This is the first time I have attempted to report a (suspected) Linux
kernel bug, so I apologise if I have made any mistakes. I am not
subscribed to the list, so please Cc me in on any responses.

Regards,

-- 
Mike Cardwell  https://grepular.com/     http://cardwellit.com/
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XMPP OTR Key   8924 B06A 7917 AAF3 DBB1  BF1B 295C 3C78 3EF1 46B4


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