man > proc_pid_root(5)

proc_pid_root(5)                         File Formats Manual                        proc_pid_root(5)

NAME
       /proc/pid/root/ - symbolic link to root directory

DESCRIPTION
       /proc/pid/root/
              UNIX  and  Linux  support the idea of a per-process root of the filesystem, set by the
              chroot(2) system call.  This file is a symbolic link that points to the process's root
              directory, and behaves in the same way as exe, and fd/*.

              Note however that this file is not merely a symbolic link.  It provides the same  view
              of  the  filesystem  (including  namespaces  and the set of per-process mounts) as the
              process itself.  An example illustrates this point.  In one terminal, we start a shell
              in new user and mount namespaces, and in that shell we create some new mounts:

                  $ PS1='sh1# ' unshare -Urnm
                  sh1# mount -t tmpfs tmpfs /etc  # Mount empty tmpfs at /etc
                  sh1# mount --bind /usr /dev     # Mount /usr at /dev
                  sh1# echo $$
                  27123

              In a second terminal window, in the initial mount namespace, we look at  the  contents
              of the corresponding mounts in the initial and new namespaces:

                  $ PS1='sh2# ' sudo sh
                  sh2# ls /etc | wc -l                  # In initial NS
                  309
                  sh2# ls /proc/27123/root/etc | wc -l  # /etc in other NS
                  0                                     # The empty tmpfs dir
                  sh2# ls /dev | wc -l                  # In initial NS
                  205
                  sh2# ls /proc/27123/root/dev | wc -l  # /dev in other NS
                  11                                    # Actually bind
                                                        # mounted to /usr
                  sh2# ls /usr | wc -l                  # /usr in initial NS
                  11

              In  a  multithreaded process, the contents of the /proc/pid/root symbolic link are not
              available  if  the  main  thread  has  already  terminated   (typically   by   calling
              pthread_exit(3)).

              Permission  to  dereference  or read (readlink(2)) this symbolic link is governed by a
              ptrace access mode PTRACE_MODE_READ_FSCREDS check; see ptrace(2).

SEE ALSO
       proc(5)

Linux man-pages 6.7                          2023-08-15                             proc_pid_root(5)