# path_resolution

## 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, or
           may temporarily use a different root directory by using openat2(2)
           with the **RESOLVE_IN_ROOT** flag set.
    
           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 — or in the case of
           openat(2)-style system calls, the _dfd_ argument (or the current
           working directory if **AT_FDCWD** is passed as the _dfd_ argument).  The
           current working directory is inherited from the parent, and 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, 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 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 **ELOOP** error is returned when the maximum is exceeded
           ("Too many levels of symbolic links").
    
           As currently implemented on Linux, the maximum number 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 was raised to 8.  In Linux
           4.2, 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.
    
           The resolution of symbolic links during this stage can be blocked
           by using openat2(2), with the **RESOLVE_NO_SYMLINKS** flag set.
    
       **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 filesystem.
    
           One cannot walk up past the root: "/.." is the same as "/".
    
       **Mount points**
           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".
    
           Traversal of mount points can be blocked by using openat2(2), with
           the **RESOLVE_NO_XDEV** flag set (though note that this also restricts
           bind mount traversal).
    
       **Trailing slashes**
           If a pathname ends in a '/', 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 '/' is ignored.
    
       **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 lstat(2) will operate on
           the symbolic link, while stat(2) operates on the file pointed to
           by the symbolic link.
    
       **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; see 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).
    
       **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 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.
    


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