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CentOS 5.1 |
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path_resolution(2) |
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Unix/Linux path resolution − find the file referred to by a filename
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
current 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 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 non-final 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 symlinks 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 non-directory, 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 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 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".
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
("File name 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 current 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 current process,
or is one of the supplementary group IDs of the current
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.
capabilities(7)
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path_resolution(2) | |