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

[David Laight <David.Laight@ACULAB.COM>]

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;
}

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