1PATH_RESOLUTION(7) Linux Programmer's Manual PATH_RESOLUTION(7)
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6 path_resolution - how a pathname is resolved to a file
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9 Some UNIX/Linux system calls have as parameter one or more filenames.
10 A filename (or pathname) is resolved as follows.
11 Step 1: start of the resolution process
13 If the pathname starts with the '/' character, the starting lookup
14 directory is the root directory of the calling process. (A process
15 inherits its root directory from its parent. Usually this will be the
16 root directory of the file hierarchy. A process may get a different
17 root directory by use of the chroot(2) system call. A process may get
18 an entirely private mount namespace in case it—or one of its ancestors—
19 was started by an invocation of the clone(2) system call that had the
20 CLONE_NEWNS flag set.) This handles the '/' part of the pathname.
21 If the pathname does not start with the '/' character, the starting
23 lookup directory of the resolution process is the current working
24 directory of the process. (This is also inherited from the parent. It
25 can be changed by use of the chdir(2) system call.)
26 Pathnames starting with a '/' character are called absolute pathnames.
28 Pathnames not starting with a '/' are called relative pathnames.
29 Step 2: walk along the path
31 Set the current lookup directory to the starting lookup directory.
32 Now, for each nonfinal component of the pathname, where a component is
33 a substring delimited by '/' characters, this component is looked up in
34 the current lookup directory.
35 If the process does not have search permission on the current lookup
37 directory, an EACCES error is returned ("Permission denied").
38 If the component is not found, an ENOENT error is returned ("No such
40 file or directory").
41 If the component is found, but is neither a directory nor a symbolic
43 link, an ENOTDIR error is returned ("Not a directory").
44 If the component is found and is a directory, we set the current lookup
46 directory to that directory, and go to the next component.
47 If the component is found and is a symbolic link (symlink), we first
49 resolve this symbolic link (with the current lookup directory as start‐
50 ing lookup directory). Upon error, that error is returned. If the
51 result is not a directory, an ENOTDIR error is returned. If the reso‐
52 lution of the symlink is successful and returns a directory, we set the
53 current lookup directory to that directory, and go to the next compo‐
54 nent. Note that the resolution process here can involve recursion if
55 the prefix ('dirname') component of a pathname contains a filename that
56 is a symbolic link that resolves to a directory (where the prefix com‐
57 ponent of that directory may contain a symbolic link, and so on). In
58 order to protect the kernel against stack overflow, and also to protect
59 against denial of service, there are limits on the maximum recursion
60 depth, and on the maximum number of symbolic links followed. An ELOOP
61 error is returned when the maximum is exceeded ("Too many levels of
62 symbolic links").
63 As currently implemented on Linux, the maximum number of symbolic links
65 that will be followed while resolving a pathname is 40. In kernels
66 before 2.6.18, the limit on the recursion depth was 5. Starting with
67 Linux 2.6.18, this limit was raised to 8. In Linux 4.2, the kernel's
68 pathname-resolution code was reworked to eliminate the use of recur‐
69 sion, so that the only limit that remains is the maximum of 40 resolu‐
70 tions for the entire pathname.
71 Step 3: find the final entry
73 The lookup of the final component of the pathname goes just like that
74 of all other components, as described in the previous step, with two
75 differences: (i) the final component need not be a directory (at least
76 as far as the path resolution process is concerned—it may have to be a
77 directory, or a nondirectory, because of the requirements of the spe‐
78 cific system call), and (ii) it is not necessarily an error if the com‐
79 ponent is not found—maybe we are just creating it. The details on the
80 treatment of the final entry are described in the manual pages of the
81 specific system calls.
82 . and ..
84 By convention, every directory has the entries "." and "..", which
85 refer to the directory itself and to its parent directory, respec‐
86 tively.
87 The path resolution process will assume that these entries have their
89 conventional meanings, regardless of whether they are actually present
90 in the physical filesystem.
91 One cannot walk down past the root: "/.." is the same as "/".
93 Mount points
95 After a "mount dev path" command, the pathname "path" refers to the
96 root of the filesystem hierarchy on the device "dev", and no longer to
97 whatever it referred to earlier.
98 One can walk out of a mounted filesystem: "path/.." refers to the par‐
100 ent directory of "path", outside of the filesystem hierarchy on "dev".
101 Trailing slashes
103 If a pathname ends in a '/', that forces resolution of the preceding
104 component as in Step 2: it has to exist and resolve to a directory.
105 Otherwise, a trailing '/' is ignored. (Or, equivalently, a pathname
106 with a trailing '/' is equivalent to the pathname obtained by appending
107 '.' to it.)
108 Final symlink
110 If the last component of a pathname is a symbolic link, then it depends
111 on the system call whether the file referred to will be the symbolic
112 link or the result of path resolution on its contents. For example,
113 the system call lstat(2) will operate on the symlink, while stat(2)
114 operates on the file pointed to by the symlink.
115 Length limit
117 There is a maximum length for pathnames. If the pathname (or some
118 intermediate pathname obtained while resolving symbolic links) is too
119 long, an ENAMETOOLONG error is returned ("Filename too long").
120 Empty pathname
122 In the original UNIX, the empty pathname referred to the current direc‐
123 tory. Nowadays POSIX decrees that an empty pathname must not be
124 resolved successfully. Linux returns ENOENT in this case.
125 Permissions
127 The permission bits of a file consist of three groups of three bits;
128 see chmod(1) and stat(2). The first group of three is used when the
129 effective user ID of the calling process equals the owner ID of the
130 file. The second group of three is used when the group ID of the file
131 either equals the effective group ID of the calling process, or is one
132 of the supplementary group IDs of the calling process (as set by set‐
133 groups(2)). When neither holds, the third group is used.
134 Of the three bits used, the first bit determines read permission, the
136 second write permission, and the last execute permission in case of
137 ordinary files, or search permission in case of directories.
138 Linux uses the fsuid instead of the effective user ID in permission
140 checks. Ordinarily the fsuid will equal the effective user ID, but the
141 fsuid can be changed by the system call setfsuid(2).
142 (Here "fsuid" stands for something like "filesystem user ID". The con‐
144 cept was required for the implementation of a user space NFS server at
145 a time when processes could send a signal to a process with the same
146 effective user ID. It is obsolete now. Nobody should use setf‐
147 suid(2).)
148 Similarly, Linux uses the fsgid ("filesystem group ID") instead of the
150 effective group ID. See setfsgid(2).
151 Bypassing permission checks: superuser and capabilities
153 On a traditional UNIX system, the superuser (root, user ID 0) is all-
154 powerful, and bypasses all permissions restrictions when accessing
155 files.
156 On Linux, superuser privileges are divided into capabilities (see capa‐
158 bilities(7)). Two capabilities are relevant for file permissions
159 checks: CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH. (A process has these
160 capabilities if its fsuid is 0.)
161 The CAP_DAC_OVERRIDE capability overrides all permission checking, but
163 grants execute permission only when at least one of the file's three
164 execute permission bits is set.
165 The CAP_DAC_READ_SEARCH capability grants read and search permission on
167 directories, and read permission on ordinary files.
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170 readlink(2), capabilities(7), credentials(7), symlink(7)
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173 This page is part of release 4.15 of the Linux man-pages project. A
174 description of the project, information about reporting bugs, and the
175 latest version of this page, can be found at
176 https://www.kernel.org/doc/man-pages/.
177Linux 2017-11-26 PATH_RESOLUTION(7)