proc_pid_root

NAME

DESCRIPTION

/proc/pid/root/

UNIX and Linux support the idea of a pe-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 pe-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=[aq]sh1# [aq] 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=[aq]sh2# [aq] 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

Index

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SEE ALSO