PATH_RESOLUTION
PATH_RESOLUTION
NAMEDESCRIPTION
SEE ALSO
COLOPHON
NAME
path_resolution − how a pathname is resolved to a file
DESCRIPTION
Some UNIX/Linux system calls have as parameter one or more filenames. A filename (or pathname) is resolved as follows.
Step 1:
Start of the resolution process
If the pathname starts with the '/' 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 '/' part of the pathname.
If the pathname does not start with the '/' 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 '/' character are called absolute pathnames. Pathnames not starting with a '/' are called relative pathnames.
Step 2: Walk
along the path
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 '/'
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").
Step 3: Find
the final entry
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.
. and ..
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 file system.
One cannot walk down past the root: "/.." is the same as "/".
Mount
points
After a "mount dev path" command, the pathname
"path" refers to the root of the file system
hierarchy on the device "dev", and no longer to
whatever it referred to earlier.
One can walk out of a mounted file system: "path/.." refers to the parent directory of "path", outside of the file system hierarchy on "dev".
Trailing
slashes
If a pathname ends in a '/', that forces resolution of the
preceding component as in Step 2: it has to exist and
resolve to a directory. Otherwise a trailing '/' is ignored.
(Or, equivalently, a pathname with a trailing '/' is
equivalent to the pathname obtained by appending '.' to
it.)
Final
symlink
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.
Length
limit
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").
Empty
pathname
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.
Permissions
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 "file system 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 ("file system group ID") instead of the effective group ID. See setfsgid(2).
Bypassing
permission checks: superuser and capabilities
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 only grants execute permission 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.
SEE ALSO
readlink(2), capabilities(7), credentials(7), symlink(7)
COLOPHON
This page is part of release 3.35 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://man7.org/linux/man-pages/.
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
