Re: procfs readlink after unshare() in a chroot() reports the full path

[Alexey Dobriyan <adobriyan@gmail.com>]

On Fri, Sep 09, 2022 at 10:06:32AM +0000, David Laight wrote:
> The readlink calls in procfs (eg for /proc/self/fd/0) returns
> the full pathname if unshare() is called inside a chroot.
> 
> The program below reproduces this when run with stdin
> redirected to a file in the current directory.
> 
> This sequence is used by 'ip netns exec' so isn't actually
> that unusual.
> 
> 	David
> 
> #define _GNU_SOURCE
> #include <unistd.h>
> #include <stdio.h>
> #include <fcntl.h>
> #include <sched.h>
> 
> static void print_link(const char *where, int fd)
> {
>         char buf[256];
> 
>         printf("%s: %.*s\n", where, (int)readlinkat(fd, "", buf, sizeof buf), buf);
> }
> 
> int main(int argc, char **argv)
> {
>         int link_fd = open("/proc/self/fd/0", O_PATH | O_NOFOLLOW);
> 
>         print_link("initial", link_fd);
>         if (chroot("."))
>                 return 1;
>         print_link("after chroot", link_fd);
>         if (unshare(CLONE_NEWNS))
>                 return 2;
>         print_link("after unshare", link_fd);
>         return 0;
> }

I tested mainline and 5.19.8, both are OK:

open("/proc/self/fd/0", O_RDONLY|O_NOFOLLOW|O_PATH) = 3
readlinkat(3, "", "/dev/pts/0", 256)    = 10
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fd94753e000
write(1, "initial: /dev/pts/0\n", 20initial: /dev/pts/0
)   = 20
chroot(".")                             = 0
readlinkat(3, "", "/dev/pts/0", 256)    = 10
write(1, "after chroot: /dev/pts/0\n", 25after chroot: /dev/pts/0
) = 25
unshare(CLONE_NEWNS)                    = 0
readlinkat(3, "", "/dev/pts/0", 256)    = 10
write(1, "after unshare: /dev/pts/0\n", 26after unshare: /dev/pts/0
) = 26