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 ex-
ample 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 6.9.1 2024-05-02 proc_pid_root(5)
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