setns vs unshare bug

Pavel Emelyanov xemul at
Fri Aug 10 15:08:15 UTC 2012

On 08/10/2012 07:00 PM, Serge Hallyn wrote:
> Hi Pavel,
> I don't believe this is a bug.  The fd is to a specific network
> namespace.  If the target task later changes his namespace, that
> doesn't change the fact that you asked for access to the old
> namespace.
> You're worried about a race?

No, it's not a race. The proc ns file doesn't reflect the actual state
of a task it belongs to, but instead has some internal state which is
not observable/controllable from the outside. Look at my proggie -- the
"else" branch does expects that setns will bring it into a new net, but
it only does so if proc dcache is empty!


> -serge
> Quoting Pavel Emelyanov (xemul at
>> Hi, Eric!
>> There's an issue with setns versus unshare syscall which I consider
>> to be worth looking at. Look -- when you open some task's namespace file,
>> e.g. /proc/<pid>/ns/net, the net namespace is cached on the proc inode.
>> If later the task with the pid <pid> unshares the namespace in question
>> (in this case -- net ns) the subsequent openings of this task's proc ns
>> file will result in old namespace obtained and the setns call will not
>> work as expected. Here's a simple proggie which demonstrates this:
>> int main(void)
>> {
>> 	int pid, fd;
>> 	char path[64];
>> 	pid = fork();
>> 	if (!pid) {
>> 		fd = open("/proc/self/ns/net", O_RDONLY);
>> 		close(fd);
>> 		unshare(CLONE_NEWNET);
>> 		printf("New net:\n");
>> 		system("ip l");
>> 		sleep(1);
>> 	} else {
>> 		sleep(1);
>> 		printf("Old net:\n");
>> 		system("ip l");
>> 		sprintf(path, "/proc/%d/ns/net", pid);
>> 		fd = open(path, O_RDONLY);
>> 		set_ns(fd, CLONE_NEWNET);
>> 		printf("New net 2:\n");
>> 		system("ip l");
>> 	}
>> 	return 0;
>> }
>> The "else" branch after set_ns expects the net it set to be the new one (and
>> contain a lo device only), but it's not so -- after the setns syscall the net
>> namespace isn't changed! If you comment out the "if" branch's open and close
>> calls (thus avoiding the ns caching) the setns works as expected.
>> I assume you're aware of this problem, so do you have plans to fix this?
>> Thanks,
>> Pavel
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