Re: [PATCH RFC] proc: Don't retain negative dentries

[Ahmad Fatoum]

On 10/8/18 6:50 PM, Ahmad Fatoum wrote:
> The referenced commit 1da4d377f94 ("proc: revalidate misc dentries")
> caused following userspace code to access a stale /proc/net/dev
> after the network namespace was changed:
> 
>     system("ip netns add testns");
> 
>     printf("default:\n"); {
>         int devinfd = open("/proc/net/dev", O_RDONLY);
>         sendfile(STDOUT_FILENO, devinfd, NULL, 4096);
>         close(devinfd);
>     }
> 
>     printf("testns:\n"); {
>         int ns_fd = open("/var/run/netns/testns", O_RDONLY);
>         setns(ns_fd, 0);
> 
>         int devinfd = open("/proc/net/dev", O_RDONLY);
>         sendfile(STDOUT_FILENO, devinfd, NULL, 4096);
>         close(devinfd);
> 
>         close(ns_fd);
>     }
> 
> Despite switching the network namespace, the read access from the
> newly opened file gave back what the very first read in the default
> network namespace returned.
> 
> This doesn't occur if /proc/net/dev is opened within a new process,
> which might explain why this wasn't noticed previously.
> 
> As I don't see a reason why one would keep negative dentries for procfs
> at all, amend the code not to do this anymore.
> 
> Fixes: 1da4d377f94 ("proc: Don't retain negative dentries")

This should read

Fixes: 1da4d377f94 ("proc: revalidate misc dentries") 

instead...