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-.\" Copyright (C) 2003 Andries Brouwer (aeb@cwi.nl)
-.\"
-.\" SPDX-License-Identifier: Linux-man-pages-copyleft
-.\"
-.TH path_resolution 7 (date) "Linux man-pages (unreleased)"
-.SH NAME
-path_resolution \- how a pathname is resolved to a file
-.SH DESCRIPTION
-Some UNIX/Linux system calls have as parameter one or more filenames.
-A filename (or pathname) is resolved as follows.
-.SS Step 1: start of the resolution process
-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
-.BR chroot (2)
-system call, or may temporarily use a different root directory by using
-.BR openat2 (2)
-with the
-.B RESOLVE_IN_ROOT
-flag set.
-.PP
-A process may get an entirely private mount namespace in case
-it\[em]or one of its ancestors\[em]was started by an invocation of the
-.BR clone (2)
-system call that had the
-.B CLONE_NEWNS
-flag set.
-This handles the \[aq]/\[aq] part of the pathname.
-.PP
-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 \[em] or in the case of
-.BR openat (2)-style
-system calls, the
-.I dfd
-argument (or the current working directory if
-.B AT_FDCWD
-is passed as the
-.I dfd
-argument).
-The current working directory is inherited from the parent, and can
-be changed by use of the
-.BR chdir (2)
-system call.
-.PP
-Pathnames starting with a \[aq]/\[aq] character are called absolute pathnames.
-Pathnames not starting with a \[aq]/\[aq] are called relative pathnames.
-.SS 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 \[aq]/\[aq] characters, this component is looked up
-in the current lookup directory.
-.PP
-If the process does not have search permission on
-the current lookup directory,
-an
-.B EACCES
-error is returned ("Permission denied").
-.PP
-If the component is not found, an
-.B ENOENT
-error is returned
-("No such file or directory").
-.PP
-If the component is found, but is neither a directory nor a symbolic link,
-an
-.B ENOTDIR
-error is returned ("Not a directory").
-.PP
-If the component is found and is a directory, we set the
-current lookup directory to that directory, and go to the
-next component.
-.PP
-If the component is found and is a symbolic link,
-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
-.B ENOTDIR
-error is returned.
-If the resolution of the symbolic link 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 can involve recursion if the
-prefix ('dirname') component of a pathname contains a filename
-that is a symbolic link that resolves to a directory (where the
-prefix component of that directory may contain a symbolic link, and so on).
-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
-.B ELOOP
-error is returned when the maximum is
-exceeded ("Too many levels of symbolic links").
-.PP
-.\"
-.\" presently: max recursion depth during symlink resolution: 5
-.\" max total number of symbolic links followed: 40
-.\" _POSIX_SYMLOOP_MAX is 8
-As currently implemented on Linux, the maximum number
-.\" MAXSYMLINKS is 40
-of symbolic links that will be followed while resolving a pathname is 40.
-Before Linux 2.6.18, the limit on the recursion depth was 5.
-Starting with Linux 2.6.18, this limit
-.\" MAX_NESTED_LINKS
-was raised to 8.
-In Linux 4.2,
-.\" commit 894bc8c4662ba9daceafe943a5ba0dd407da5cd3
-the kernel's pathname-resolution code
-was reworked to eliminate the use of recursion,
-so that the only limit that remains is the maximum of 40
-resolutions for the entire pathname.
-.PP
-The resolution of symbolic links during this stage can be blocked by using
-.BR openat2 (2),
-with the
-.B RESOLVE_NO_SYMLINKS
-flag set.
-.SS 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\[em]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\[em]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.
-.SS . and ..
-By convention, every directory has the entries "." and "..",
-which refer to the directory itself and to its parent directory,
-respectively.
-.PP
-The path resolution process will assume that these entries have
-their conventional meanings, regardless of whether they are
-actually present in the physical filesystem.
-.PP
-One cannot walk up past the root: "/.." is the same as "/".
-.SS Mount points
-After a
-.I 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.
-.PP
-One can walk out of a mounted filesystem: "path/.." refers to
-the parent directory of "path",
-outside of the filesystem hierarchy on "dev".
-.PP
-Traversal of mount points can be blocked by using
-.BR openat2 (2),
-with the
-.B RESOLVE_NO_XDEV
-flag set (though note that this also restricts bind mount traversal).
-.SS Trailing slashes
-If a pathname ends in a \[aq]/\[aq], that forces resolution of the preceding
-component as in Step 2:
-the component preceding the slash either exists and resolves to a directory
-or it names a directory that is to be created
-immediately after the pathname is resolved.
-Otherwise, a trailing \[aq]/\[aq] is ignored.
-.SS Final symbolic link
-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
-.BR lstat (2)
-will operate on the symbolic link,
-while
-.BR stat (2)
-operates on the file pointed to by the symbolic link.
-.SS 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
-.B ENAMETOOLONG
-error is returned ("Filename too long").
-.SS 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
-.B ENOENT
-in this case.
-.SS Permissions
-The permission bits of a file consist of three groups of three bits; see
-.BR chmod (1)
-and
-.BR 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
-.BR setgroups (2)).
-When neither holds, the third group is used.
-.PP
-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.
-.PP
-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
-.BR setfsuid (2).
-.PP
-(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
-.BR setfsuid (2).)
-.PP
-Similarly, Linux uses the fsgid ("filesystem group ID")
-instead of the effective group ID.
-See
-.BR setfsgid (2).
-.\" FIXME . say something about filesystem mounted read-only ?
-.SS Bypassing permission checks: superuser and capabilities
-On a traditional UNIX system, the superuser
-.RI ( root ,
-user ID 0) is all-powerful, and bypasses all permissions restrictions
-when accessing files.
-.\" (but for exec at least one x bit must be set) -- AEB
-.\" but there is variation across systems on this point: for
-.\" example, HP-UX and Tru64 are as described by AEB.  However,
-.\" on some implementations (e.g., Solaris, FreeBSD),
-.\" access(X_OK) by superuser will report success, regardless
-.\" of the file's execute permission bits. -- MTK (Oct 05)
-.PP
-On Linux, superuser privileges are divided into capabilities (see
-.BR capabilities (7)).
-Two capabilities are relevant for file permissions checks:
-.B CAP_DAC_OVERRIDE
-and
-.BR CAP_DAC_READ_SEARCH .
-(A process has these capabilities if its fsuid is 0.)
-.PP
-The
-.B 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.
-.PP
-The
-.B CAP_DAC_READ_SEARCH
-capability grants read and search permission
-on directories, and read permission on ordinary files.
-.\" FIXME . say something about immutable files
-.\" FIXME . say something about ACLs
-.SH SEE ALSO
-.BR readlink (2),
-.BR capabilities (7),
-.BR credentials (7),
-.BR symlink (7)