linux-kernel - RE: procfs readlink after unshare() in a chroot() reports the full path

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Date:   Wed, 14 Sep 2022 08:05:13 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alexey Dobriyan' <adobriyan@...il.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: procfs readlink after unshare() in a chroot() reports the full
 path

From: Alexey Dobriyan
> Sent: 13 September 2022 19:44
> 
> 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

You need the path to be inside the chroot.
In some sense "/dev/pts/0" is actually invalid and probably
ought to be tagged as such or an error returned.
So rerun and redirect stdin to a file inside the chroot.

In my original case everything was inside a chroot.

	David

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