setns vs unshare bug

[Serge Hallyn]

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?

-serge

Quoting Pavel Emelyanov (xemul at parallels.com):
> 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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