Messages in this thread | | | Date | Tue, 13 Sep 2022 21:44:06 +0300 | From | Alexey Dobriyan <> | Subject | Re: procfs readlink after unshare() in a chroot() reports the full path |
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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
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