Messages in this thread | | | Date | Wed, 9 Jul 2014 15:17:18 +0300 | Subject | Re: Procfs race condition bug | From | 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; }
| |