projects / thirdparty / man-pages.git / blame
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1 | .\" Copyright (C) 1994, 1995, Daniel Quinlan <quinlan@yggdrasil.com> |
| 2 | .\" Copyright (C) 2002-2008, 2017, Michael Kerrisk <mtk.manpages@gmail.com> | |
| 3 | .\" Copyright (C) 2023, Alejandro Colomar <alx@kernel.org> | |
| 4 | .\" | |
| 5 | .\" SPDX-License-Identifier: GPL-3.0-or-later | |
| 6 | .\" | |
| 7 | .TH proc_pid_root 5 (date) "Linux man-pages (unreleased)" | |
| 8 | .SH NAME | |
| 9 | /proc/pid/root/ \- symbolic link to root directory | |
| 10 | .SH DESCRIPTION | |
| 11 | .TP | |
| 12 | .IR /proc/ pid /root/ | |
| 13 | UNIX and Linux support the idea of a per-process root of the | |
| 14 | filesystem, set by the | |
| 15 | .BR chroot (2) | |
| 16 | system call. | |
| 17 | This file is a symbolic link that points to the process's | |
| 18 | root directory, and behaves in the same way as | |
| 19 | .IR exe , | |
| 20 | and | |
| 21 | .IR fd/* . | |
| 22 | .IP | |
| 23 | Note however that this file is not merely a symbolic link. | |
| 24 | It provides the same view of the filesystem (including namespaces and the | |
| 25 | set of per-process mounts) as the process itself. | |
| 26 | An example illustrates this point. | |
| 27 | In one terminal, we start a shell in new user and mount namespaces, | |
| 28 | and in that shell we create some new mounts: | |
| 29 | .IP | |
| 30 | .in +4n | |
| 31 | .EX | |
| 32 | $ \fBPS1=\[aq]sh1# \[aq] unshare \-Urnm\fP | |
| 33 | sh1# \fBmount \-t tmpfs tmpfs /etc\fP # Mount empty tmpfs at /etc | |
| 34 | sh1# \fBmount \-\-bind /usr /dev\fP # Mount /usr at /dev | |
| 35 | sh1# \fBecho $$\fP | |
| 36 | 27123 | |
| 37 | .EE | |
| 38 | .in | |
| 39 | .IP | |
| 40 | In a second terminal window, in the initial mount namespace, | |
| 41 | we look at the contents of the corresponding mounts in | |
| 42 | the initial and new namespaces: | |
| 43 | .IP | |
| 44 | .in +4n | |
| 45 | .EX | |
| 46 | $ \fBPS1=\[aq]sh2# \[aq] sudo sh\fP | |
| 47 | sh2# \fBls /etc | wc \-l\fP # In initial NS | |
| 48 | 309 | |
| 49 | sh2# \fBls /proc/27123/root/etc | wc \-l\fP # /etc in other NS | |
| 50 | 0 # The empty tmpfs dir | |
| 51 | sh2# \fBls /dev | wc \-l\fP # In initial NS | |
| 52 | 205 | |
| 53 | sh2# \fBls /proc/27123/root/dev | wc \-l\fP # /dev in other NS | |
| 54 | 11 # Actually bind | |
| 55 | # mounted to /usr | |
| 56 | sh2# \fBls /usr | wc \-l\fP # /usr in initial NS | |
| 57 | 11 | |
| 58 | .EE | |
| 59 | .in | |
| 60 | .IP | |
| 61 | .\" The following was still true as at kernel 2.6.13 | |
| 62 | In a multithreaded process, the contents of the | |
| 63 | .IR /proc/ pid /root | |
| 64 | symbolic link are not available if the main thread has already terminated | |
| 65 | (typically by calling | |
| 66 | .BR pthread_exit (3)). | |
| 67 | .IP | |
| 68 | Permission to dereference or read | |
| 69 | .RB ( readlink (2)) | |
| 70 | this symbolic link is governed by a ptrace access mode | |
| 71 | .B PTRACE_MODE_READ_FSCREDS | |
| 72 | check; see | |
| 73 | .BR ptrace (2). | |
| 74 | .SH SEE ALSO | |
| 75 | .BR proc (5) |