proc.5: Remove duplicated /proc/[pid]/gid_map entry

[man-pages.git] / man7 / symlink.7

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.\"     @(#)symlink.7   8.3 (Berkeley) 3/31/94
.\" $FreeBSD: src/bin/ln/symlink.7,v 1.30 2005/02/13 22:25:09 ru Exp $
.\"
.\" 2008-06-11, mtk, Taken from FreeBSD 6.2 and heavily edited for
.\"     specific Linux details, improved readability, and man-pages style.
.\"
.TH SYMLINK 7 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
symlink \- symbolic link handling
.SH DESCRIPTION
Symbolic links are files that act as pointers to other files.
To understand their behavior, you must first understand how hard links
work.
.PP
A hard link to a file is indistinguishable from the original file because
it is a reference to the object underlying the original filename.
(To be precise: each of the hard links to a file is a reference to
the same
.IR "inode number" ,
where an inode number is an index into the inode table,
which contains metadata about all files on a filesystem.
See
.BR stat (2).)
Changes to a file are independent of the name used to reference the file.
Hard links may not refer to directories
(to prevent the possibility of loops within the filesystem tree,
which would confuse many programs)
and may not refer to files on different filesystems
(because inode numbers are not unique across filesystems).
.PP
A symbolic link is a special type of file whose contents are a string
that is the pathname of another file, the file to which the link refers.
(The contents of a symbolic link can be read using
.BR readlink (2).)
In other words, a symbolic link is a pointer to another name,
and not to an underlying object.
For this reason, symbolic links may refer to directories and may cross
filesystem boundaries.
.PP
There is no requirement that the pathname referred to by a symbolic link
should exist.
A symbolic link that refers to a pathname that does not exist is said
to be a
.IR "dangling link" .
.PP
Because a symbolic link and its referenced object coexist in the filesystem
name space, confusion can arise in distinguishing between the link itself
and the referenced object.
On historical systems,
commands and system calls adopted their own link-following
conventions in a somewhat ad-hoc fashion.
Rules for a more uniform approach,
as they are implemented on Linux and other systems,
are outlined here.
It is important that site-local applications also conform to these rules,
so that the user interface can be as consistent as possible.
.\"
.SS Magic links
There is a special class of symbolic-link-like objects
known as "magic links", which
can be found in certain pseudofilesystems such as
.BR proc (5)
(examples include
.IR /proc/[pid]/exe " and " /proc/[pid]/fd/* ).
Unlike normal symbolic links, magic links are not resolved through
pathname-expansion, but instead act as direct references to the kernel's own
representation of a file handle.
As such, these magic links allow users to
access files which cannot be referenced with normal paths (such as unlinked
files still referenced by a running program ).
.PP
Because they can bypass ordinary
.BR mount_namespaces (7)-based
restrictions,
magic links have been used as attack vectors in various exploits.
.\"
.SS Symbolic link ownership, permissions, and timestamps
The owner and group of an existing symbolic link can be changed
using
.BR lchown (2).
The only time that the ownership of a symbolic link matters is
when the link is being removed or renamed in a directory that
has the sticky bit set (see
.BR stat (2)).
.PP
The last access and last modification timestamps
of a symbolic link can be changed using
.BR utimensat (2)
or
.BR lutimes (3).
.PP
.\" Linux does not currently implement an lchmod(2).
On Linux, the permissions of an ordinary symbolic link are not used in any
operations; the permissions are always 0777 (read, write, and execute for all
user categories), and can't be changed.
.PP
However, magic links do not follow this rule.
They can have a non-0777 mode,
though this mode is not currently used in any permission checks.
.\" .PP
.\" The
.\" 4.4BSD
.\" system differs from historical
.\" 4BSD
.\" systems in that the system call
.\" .BR chown (2)
.\" has been changed to follow symbolic links.
.\" The
.\" .BR lchown (2)
.\" system call was added later when the limitations of the new
.\" .BR chown (2)
.\" became apparent.
.SS Obtaining a file descriptor that refers to a symbolic link
Using the combination of the
.B O_PATH
and
.BR O_NOFOLLOW
flags to
.BR open (2)
yields a file descriptor that can be passed as the
.IR dirfd
argument in system calls such as
.BR fstatat (2),
.BR fchownat (2),
.BR fchmodat (2),
.BR linkat (2),
and
.BR readlinkat (2),
in order to operate on the symbolic link itself
(rather than the file to which it refers).
.PP
By default
(i.e., if the
.BR AT_SYMLINK_FOLLOW
flag is not specified), if
.BR name_to_handle_at (2)
is applied to a symbolic link, it yields a handle for the symbolic link
(rather than the file to which it refers).
One can then obtain a file descriptor for the symbolic link
(rather than the file to which it refers)
by specifying the
.B O_PATH
flag in a subsequent call to
.BR open_by_handle_at (2).
Again, that file descriptor can be used in the
aforementioned system calls to operate on the symbolic link itself.
.SS Handling of symbolic links by system calls and commands
Symbolic links are handled either by operating on the link itself,
or by operating on the object referred to by the link.
In the latter case,
an application or system call is said to
.I follow
the link.
Symbolic links may refer to other symbolic links,
in which case the links are dereferenced until an object that is
not a symbolic link is found,
a symbolic link that refers to a file which does not exist is found,
or a loop is detected.
(Loop detection is done by placing an upper limit on the number of
links that may be followed, and an error results if this limit is
exceeded.)
.PP
There are three separate areas that need to be discussed.
They are as follows:
.IP 1. 3
Symbolic links used as filename arguments for system calls.
.IP 2.
Symbolic links specified as command-line arguments to utilities that
are not traversing a file tree.
.IP 3.
Symbolic links encountered by utilities that are traversing a file tree
(either specified on the command line or encountered as part of the
file hierarchy walk).
.PP
Before describing the treatment of symbolic links by system calls and commands,
we require some terminology.
Given a pathname of the form
.IR a/b/c ,
the part preceding the final slash (i.e.,
.IR a/b )
is called the
.I dirname
component, and the part following the final slash (i.e.,
.IR c )
is called the
.IR basename
component.
.\"
.SS Treatment of symbolic links in system calls
The first area is symbolic links used as filename arguments for
system calls.
.PP
The treatment of symbolic links within a pathname passed to
a system call is as follows:
.IP 1. 3
Within the dirname component of a pathname,
symbolic links are always followed in nearly every system call.
(This is also true for commands.)
The one exception is
.BR openat2 (2),
which provides flags that can be used to explicitly
prevent following of symbolic links in the dirname component.
.IP 2.
Except as noted below,
all system calls follow symbolic links
in the basename component of a pathname.
For example, if there were a symbolic link
.I slink
which pointed to a file named
.IR afile ,
the system call
.I "open(""slink"" ...\&)"
would return a file descriptor referring to the file
.IR afile .
.PP
Various system calls do not follow links in
the basename component of a pathname,
and operate on the symbolic link itself.
They are:
.BR lchown (2),
.BR lgetxattr (2),
.BR llistxattr (2),
.BR lremovexattr (2),
.BR lsetxattr (2),
.BR lstat (2),
.BR readlink (2),
.BR rename (2),
.BR rmdir (2),
and
.BR unlink (2).
.PP
Certain other system calls optionally follow symbolic links
in the basename component of a pathname.
They are:
.BR faccessat (2),
.\" Maybe one day: .BR fchownat (2)
.BR fchownat (2),
.BR fstatat (2),
.BR linkat (2),
.BR name_to_handle_at (2),
.BR open (2),
.BR openat (2),
.BR open_by_handle_at (2),
and
.BR utimensat (2);
see their manual pages for details.
Because
.BR remove (3)
is an alias for
.BR unlink (2),
that library function also does not follow symbolic links.
When
.BR rmdir (2)
is applied to a symbolic link, it fails with the error
.BR ENOTDIR .
.PP
.BR link (2)
warrants special discussion.
POSIX.1-2001 specifies that
.BR link (2)
should dereference
.I oldpath
if it is a symbolic link.
However, Linux does not do this.
(By default, Solaris is the same,
but the POSIX.1-2001 specified behavior can be obtained with
suitable compiler options.)
POSIX.1-2008 changed the specification to allow
either behavior in an implementation.
.SS Commands not traversing a file tree
The second area is symbolic links, specified as command-line
filename arguments, to commands which are not traversing a file tree.
.PP
Except as noted below, commands follow symbolic links named as
command-line arguments.
For example, if there were a symbolic link
.I slink
which pointed to a file named
.IR afile ,
the command
.I "cat slink"
would display the contents of the file
.IR afile .
.PP
It is important to realize that this rule includes commands which may
optionally traverse file trees; for example, the command
.I "chown file"
is included in this rule, while the command
.IR "chown\ \-R file" ,
which performs a tree traversal, is not.
(The latter is described in the third area, below.)
.PP
If it is explicitly intended that the command operate on the symbolic
link instead of following the symbolic link\(emfor example, it is desired that
.I "chown slink"
change the ownership of the file that
.I slink
is, whether it is a symbolic link or not\(emthen the
.I \-h
option should be used.
In the above example,
.I "chown root slink"
would change the ownership of the file referred to by
.IR slink ,
while
.I "chown\ \-h root slink"
would change the ownership of
.I slink
itself.
.PP
There are some exceptions to this rule:
.IP * 2
The
.BR mv (1)
and
.BR rm (1)
commands do not follow symbolic links named as arguments,
but respectively attempt to rename and delete them.
(Note, if the symbolic link references a file via a relative path,
moving it to another directory may very well cause it to stop working,
since the path may no longer be correct.)
.IP *
The
.BR ls (1)
command is also an exception to this rule.
For compatibility with historic systems (when
.BR ls (1)
is not doing a tree walk\(emthat is,
.I \-R
option is not specified),
the
.BR ls (1)
command follows symbolic links named as arguments if the
.I \-H
or
.I \-L
option is specified,
or if the
.IR \-F ,
.IR \-d ,
or
.I \-l
options are not specified.
(The
.BR ls (1)
command is the only command where the
.I \-H
and
.I \-L
options affect its behavior even though it is not doing a walk of
a file tree.)
.IP *
The
.BR file (1)
command is also an exception to this rule.
The
.BR file (1)
command does not follow symbolic links named as argument by default.
The
.BR file (1)
command does follow symbolic links named as argument if the
.I \-L
option is specified.
.\"
.\"The 4.4BSD system differs from historical 4BSD systems in that the
.\".BR chown (1)
.\"and
.\".BR chgrp (1)
.\"commands follow symbolic links specified on the command line.
.SS Commands traversing a file tree
The following commands either optionally or always traverse file trees:
.BR chgrp (1),
.BR chmod (1),
.BR chown (1),
.BR cp (1),
.BR du (1),
.BR find (1),
.BR ls (1),
.BR pax (1),
.BR rm (1),
and
.BR tar (1).
.PP
It is important to realize that the following rules apply equally to
symbolic links encountered during the file tree traversal and symbolic
links listed as command-line arguments.
.PP
The \fIfirst rule\fP applies to symbolic links that reference files other
than directories.
Operations that apply to symbolic links are performed on the links
themselves, but otherwise the links are ignored.
.PP
The command
.I "rm\ \-r slink directory"
will remove
.IR slink ,
as well as any symbolic links encountered in the tree traversal of
.IR directory ,
because symbolic links may be removed.
In no case will
.BR rm (1)
affect the file referred to by
.IR slink .
.PP
The \fIsecond rule\fP applies to symbolic links that refer to directories.
Symbolic links that refer to directories are never followed by default.
This is often referred to as a "physical" walk, as opposed to a "logical"
walk (where symbolic links that refer to directories are followed).
.PP
Certain conventions are (should be) followed as consistently as
possible by commands that perform file tree walks:
.IP * 2
A command can be made to follow
any symbolic links named on the command line,
regardless of the type of file they reference, by specifying the
.I \-H
(for "half-logical") flag.
This flag is intended to make the command-line name space look
like the logical name space.
(Note, for commands that do not always do file tree traversals, the
.I \-H
flag will be ignored if the
.I \-R
flag is not also specified.)
.IP
For example, the command
.I "chown\ \-HR user slink"
will traverse the file hierarchy rooted in the file pointed to by
.IR slink .
Note, the
.I \-H
is not the same as the previously discussed
.I \-h
flag.
The
.I \-H
flag causes symbolic links specified on the command line to be
dereferenced for the purposes of both the action to be performed
and the tree walk, and it is as if the user had specified the
name of the file to which the symbolic link pointed.
.IP *
A command can be made to
follow any symbolic links named on the command line,
as well as any symbolic links encountered during the traversal,
regardless of the type of file they reference, by specifying the
.I \-L
(for "logical") flag.
This flag is intended to make the entire name space look like
the logical name space.
(Note, for commands that do not always do file tree traversals, the
.I \-L
flag will be ignored if the
.I \-R
flag is not also specified.)
.IP
For example, the command
.I "chown\ \-LR user slink"
will change the owner of the file referred to by
.IR slink .
If
.I slink
refers to a directory,
.B chown
will traverse the file hierarchy rooted in the directory that it
references.
In addition, if any symbolic links are encountered in any file tree that
.B chown
traverses, they will be treated in the same fashion as
.IR slink .
.IP *
A command can be made to
provide the default behavior by specifying the
.I \-P
(for "physical") flag.
This flag is intended to make the entire name space look like the
physical name space.
.PP
For commands that do not by default do file tree traversals, the
.IR \-H ,
.IR \-L ,
and
.I \-P
flags are ignored if the
.I \-R
flag is not also specified.
In addition, you may specify the
.IR \-H ,
.IR \-L ,
and
.I \-P
options more than once;
the last one specified determines the command's behavior.
This is intended to permit you to alias commands to behave one way
or the other, and then override that behavior on the command line.
.PP
The
.BR ls (1)
and
.BR rm (1)
commands have exceptions to these rules:
.IP * 2
The
.BR rm (1)
command operates on the symbolic link, and not the file it references,
and therefore never follows a symbolic link.
The
.BR rm (1)
command does not support the
.IR \-H ,
.IR \-L ,
or
.I \-P
options.
.IP *
To maintain compatibility with historic systems,
the
.BR ls (1)
command acts a little differently.
If you do not specify the
.IR \-F ,
.IR \-d ,
or
.I \-l
options,
.BR ls (1)
will follow symbolic links specified on the command line.
If the
.I \-L
flag is specified,
.BR ls (1)
follows all symbolic links,
regardless of their type,
whether specified on the command line or encountered in the tree walk.
.SH SEE ALSO
.BR chgrp (1),
.BR chmod (1),
.BR find (1),
.BR ln (1),
.BR ls (1),
.BR mv (1),
.BR namei (1),
.BR rm (1),
.BR lchown (2),
.BR link (2),
.BR lstat (2),
.BR readlink (2),
.BR rename (2),
.BR symlink (2),
.BR unlink (2),
.BR utimensat (2),
.BR lutimes (3),
.BR path_resolution (7)