PATH_RESOLUTION(7) manual page
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path_resolution - how a pathname is resolved
to a file
Some UNIX/Linux system calls have as parameter one
or more filenames. A filename (or pathname) is resolved as follows.
If the pathname starts with the aq/aq
character, the starting lookup directory is the root directory of the calling
process. (A process inherits its root directory from its parent. Usually
this will be the root directory of the file hierarchy. A process may get
a different root directory by use of the chroot(2)
system call. A process
may get an entirely private mount namespace in case it--or one of its ancestors--was
started by an invocation of the clone(2)
system call that had the CLONE_NEWNS
flag set.) This handles the aq/aq part of the pathname.
If the pathname
does not start with the aq/aq character, the starting lookup directory
of the resolution process is the current working directory of the process.
(This is also inherited from the parent. It can be changed by use of the
chdir(2)
system call.)
Pathnames starting with a aq/aq character are called
absolute pathnames. Pathnames not starting with a aq/aq are called relative
pathnames.
Set the current lookup directory to
the starting lookup directory. Now, for each nonfinal component of the pathname,
where a component is a substring delimited by aq/aq characters, this component
is looked up in the current lookup directory.
If the process does not have
search permission on the current lookup directory, an EACCES error is returned
("Permission denied").
If the component is not found, an ENOENT error is
returned ("No such file or directory").
If the component is found, but
is neither a directory nor a symbolic link, an ENOTDIR error is returned
("Not a directory").
If the component is found and is a directory, we set
the current lookup directory to that directory, and go to the next component.
If the component is found and is a symbolic link (symlink), we first resolve
this symbolic link (with the current lookup directory as starting lookup
directory). Upon error, that error is returned. If the result is not a directory,
an ENOTDIR error is returned. If the resolution of the symlink is successful
and returns a directory, we set the current lookup directory to that directory,
and go to the next component. Note that the resolution process here involves
recursion. In order to protect the kernel against stack overflow, and also
to protect against denial of service, there are limits on the maximum recursion
depth, and on the maximum number of symbolic links followed. An ELOOP error
is returned when the maximum is exceeded ("Too many levels of symbolic
links").
The lookup of the final component
of the pathname goes just like that of all other components, as described
in the previous step, with two differences: (i) the final component need
not be a directory (at least as far as the path resolution process is concerned--it
may have to be a directory, or a nondirectory, because of the requirements
of the specific system call), and (ii) it is not necessarily an error if
the component is not found--maybe we are just creating it. The details on
the treatment of the final entry are described in the manual pages of the
specific system calls.
By convention, every directory has the entries
"." and "..", which refer to the directory itself and to its parent directory,
respectively.
The path resolution process will assume that these entries
have their conventional meanings, regardless of whether they are actually
present in the physical filesystem.
One cannot walk down past the root:
"/.." is the same as "/".
After a "mount dev path" command, the
pathname "path" refers to the root of the filesystem hierarchy on the device
"dev", and no longer to whatever it referred to earlier.
One can walk out
of a mounted filesystem: "path/.." refers to the parent directory of "path",
outside of the filesystem hierarchy on "dev".
If a pathname
ends in a aq/aq, that forces resolution of the preceding component as in
Step 2: it has to exist and resolve to a directory. Otherwise, a trailing
aq/aq is ignored. (Or, equivalently, a pathname with a trailing aq/aq is
equivalent to the pathname obtained by appending aq.aq to it.)
If
the last component of a pathname is a symbolic link, then it depends on
the system call whether the file referred to will be the symbolic link
or the result of path resolution on its contents. For example, the system
call lstat(2)
will operate on the symlink, while stat(2)
operates on the
file pointed to by the symlink.
There is a maximum length for
pathnames. If the pathname (or some intermediate pathname obtained while
resolving symbolic links) is too long, an ENAMETOOLONG error is returned
("Filename too long").
In the original UNIX, the empty pathname
referred to the current directory. Nowadays POSIX decrees that an empty
pathname must not be resolved successfully. Linux returns ENOENT in this
case.
The permission bits of a file consist of three groups of
three bits, cf. chmod(1)
and stat(2)
. The first group of three is used when
the effective user ID of the calling process equals the owner ID of the
file. The second group of three is used when the group ID of the file either
equals the effective group ID of the calling process, or is one of the
supplementary group IDs of the calling process (as set by setgroups(2)
).
When neither holds, the third group is used.
Of the three bits used, the
first bit determines read permission, the second write permission, and
the last execute permission in case of ordinary files, or search permission
in case of directories.
Linux uses the fsuid instead of the effective user
ID in permission checks. Ordinarily the fsuid will equal the effective user
ID, but the fsuid can be changed by the system call setfsuid(2)
.
(Here
"fsuid" stands for something like "filesystem user ID". The concept was
required for the implementation of a user space NFS server at a time when
processes could send a signal to a process with the same effective user
ID. It is obsolete now. Nobody should use setfsuid(2)
.)
Similarly, Linux
uses the fsgid ("filesystem group ID") instead of the effective group ID.
See setfsgid(2)
.
On
a traditional UNIX system, the superuser (root, user ID 0) is all-powerful,
and bypasses all permissions restrictions when accessing files.
On
Linux, superuser privileges are divided into capabilities (see capabilities(7)
).
Two capabilities are relevant for file permissions checks: CAP_DAC_OVERRIDE
and CAP_DAC_READ_SEARCH. (A process has these capabilities if its fsuid
is 0.)
The CAP_DAC_OVERRIDE capability overrides all permission checking,
but grants execute permission only when at least one of the file’s three
execute permission bits is set.
The CAP_DAC_READ_SEARCH capability grants
read and search permission on directories, and read permission on ordinary
files.
readlink(2)
, capabilities(7)
, credentials(7)
, symlink(7)
This page is part of release 3.78 of the Linux man-pages project.
A description of the project, information about reporting bugs, and the
latest version of this page, can be found at http://www.kernel.org/doc/man-pages/.
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