proc_pid_root - Man Page

/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).

proc(5)