Messages in this thread | | | From | David Laight <> | Subject | RE: procfs readlink after unshare() in a chroot() reports the full path | Date | Wed, 14 Sep 2022 08:05:13 +0000 |
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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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