Ubuntu Manpage: symlink - symbolic link handling

NAME

       symlink - symbolic link handling

DESCRIPTION

Symbolic links are files that act as pointers to other files. To understand their
behavior, you must first understand how hard links work.

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 inode number, where an inode number is an
index into the inode table, which contains metadata about all files on a filesystem. See
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).

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 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.

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 dangling link.

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.

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 proc(5) (examples include /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 ).

Because they can bypass ordinary mount_namespaces(7)-based restrictions, magic links have
been used as attack vectors in various exploits.

Symbolic link ownership, permissions, and timestamps
The owner and group of an existing symbolic link can be changed using lchown(2). The
ownership of a symbolic link matters when the link is being removed or renamed in a
directory that has the sticky bit set (see inode(7)), and when the fs.protected_symlinks
sysctl is set (see proc(5)).

The last access and last modification timestamps of a symbolic link can be changed using
utimensat(2) or lutimes(3).

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.

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.

Obtaining a file descriptor that refers to a symbolic link
Using the combination of the O_PATH and O_NOFOLLOW flags to open(2) yields a file
descriptor that can be passed as the dirfd argument in system calls such as fstatat(2),
fchownat(2), fchmodat(2), linkat(2), and readlinkat(2), in order to operate on the
symbolic link itself (rather than the file to which it refers).

By default (i.e., if the AT_SYMLINK_FOLLOW flag is not specified), if 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 O_PATH flag in a subsequent
call to open_by_handle_at(2). Again, that file descriptor can be used in the
aforementioned system calls to operate on the symbolic link itself.

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 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.)

There are three separate areas that need to be discussed. They are as follows:

• Symbolic links used as filename arguments for system calls.

• Symbolic links specified as command-line arguments to utilities that are not traversing
a file tree.

• 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).

Before describing the treatment of symbolic links by system calls and commands, we require
some terminology. Given a pathname of the form a/b/c, the part preceding the final slash
(i.e., a/b) is called the dirname component, and the part following the final slash (i.e.,
c) is called the basename component.

Treatment of symbolic links in system calls
The first area is symbolic links used as filename arguments for system calls.

The treatment of symbolic links within a pathname passed to a system call is as follows:

(1) 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
openat2(2), which provides flags that can be used to explicitly prevent following of
symbolic links in the dirname component.

(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 slink which
pointed to a file named afile, the system call open("slink" ...) would return a file
descriptor referring to the file afile.

Various system calls do not follow links in the basename component of a pathname, and
operate on the symbolic link itself. They are: lchown(2), lgetxattr(2), llistxattr(2),
lremovexattr(2), lsetxattr(2), lstat(2), readlink(2), rename(2), rmdir(2), and unlink(2).

Certain other system calls optionally follow symbolic links in the basename component of a
pathname. They are: faccessat(2), fchownat(2), fstatat(2), linkat(2),
name_to_handle_at(2), open(2), openat(2), open_by_handle_at(2), and utimensat(2); see
their manual pages for details. Because remove(3) is an alias for unlink(2), that library
function also does not follow symbolic links. When rmdir(2) is applied to a symbolic
link, it fails with the error ENOTDIR.

link(2) warrants special discussion. POSIX.1-2001 specifies that link(2) should
dereference 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.

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.

Except as noted below, commands follow symbolic links named as command-line arguments.
For example, if there were a symbolic link slink which pointed to a file named afile, the
command cat slink would display the contents of the file afile.

It is important to realize that this rule includes commands which may optionally traverse
file trees; for example, the command chown file is included in this rule, while the
command chown -R file, which performs a tree traversal, is not. (The latter is described
in the third area, below.)

If it is explicitly intended that the command operate on the symbolic link instead of
following the symbolic link—for example, it is desired that chown slink change the
ownership of the file that slink is, whether it is a symbolic link or not—then the -h
option should be used. In the above example, chown root slink would change the ownership
of the file referred to by slink, while chown -h root slink would change the ownership of
slink itself.

There are some exceptions to this rule:

• The mv(1) and 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.)

• The ls(1) command is also an exception to this rule. For compatibility with historic
systems (when ls(1) is not doing a tree walk—that is, -R option is not specified), the
ls(1) command follows symbolic links named as arguments if the -H or -L option is
specified, or if the -F, -d, or -l options are not specified. (The ls(1) command is
the only command where the -H and -L options affect its behavior even though it is not
doing a walk of a file tree.)

• The file(1) command is also an exception to this rule. The file(1) command does not
follow symbolic links named as argument by default. The file(1) command does follow
symbolic links named as argument if the -L option is specified.

Commands traversing a file tree
The following commands either optionally or always traverse file trees: chgrp(1),
chmod(1), chown(1), cp(1), du(1), find(1), ls(1), pax(1), rm(1), and tar(1).

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.

The first rule 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.

The command rm -r slink directory will remove slink, as well as any symbolic links
encountered in the tree traversal of directory, because symbolic links may be removed. In
no case will rm(1) affect the file referred to by slink.

The second rule 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).

Certain conventions are (should be) followed as consistently as possible by commands that
perform file tree walks:

• 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 -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 -H flag will be ignored if the -R flag is not also specified.)

For example, the command chown -HR user slink will traverse the file hierarchy rooted
in the file pointed to by slink. Note, the -H is not the same as the previously
discussed -h flag. The -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.

• 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 -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 -L flag will be ignored if the -R flag is
not also specified.)

For example, the command chown -LR user slink will change the owner of the file
referred to by slink. If slink refers to a directory, 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 chown traverses, they will be treated in
the same fashion as slink.

• A command can be made to provide the default behavior by specifying the -P (for
"physical") flag. This flag is intended to make the entire name space look like the
physical name space.

For commands that do not by default do file tree traversals, the -H, -L, and -P flags are
ignored if the -R flag is not also specified. In addition, you may specify the -H, -L,
and -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.

The ls(1) and rm(1) commands have exceptions to these rules:

• The rm(1) command operates on the symbolic link, and not the file it references, and
therefore never follows a symbolic link. The rm(1) command does not support the -H,
-L, or -P options.

• To maintain compatibility with historic systems, the ls(1) command acts a little
differently. If you do not specify the -F, -d, or -l options, ls(1) will follow
symbolic links specified on the command line. If the -L flag is specified, ls(1)
follows all symbolic links, regardless of their type, whether specified on the command
line or encountered in the tree walk.

NAME

DESCRIPTION

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