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, or may temporarily
18 use a different root directory by using openat2(2) with the
19 RESOLVE_IN_ROOT flag set.
20 A process may get an entirely private mount namespace in case it—or one
22 of its ancestors—was started by an invocation of the clone(2) system
23 call that had the CLONE_NEWNS flag set. This handles the '/' part of
24 the pathname.
25 If the pathname does not start with the '/' character, the starting
27 lookup directory of the resolution process is the current working
28 directory of the process — or in the case of openat(2)-style system
29 calls, the dfd argument (or the current working directory if AT_FDCWD
30 is passed as the dfd argument). The current working directory is
31 inherited from the parent, and can be changed by use of the chdir(2)
32 system call.)
33 Pathnames starting with a '/' character are called absolute pathnames.
35 Pathnames not starting with a '/' are called relative pathnames.
36 Step 2: walk along the path
38 Set the current lookup directory to the starting lookup directory.
39 Now, for each nonfinal component of the pathname, where a component is
40 a substring delimited by '/' characters, this component is looked up in
41 the current lookup directory.
42 If the process does not have search permission on the current lookup
44 directory, an EACCES error is returned ("Permission denied").
45 If the component is not found, an ENOENT error is returned ("No such
47 file or directory").
48 If the component is found, but is neither a directory nor a symbolic
50 link, an ENOTDIR error is returned ("Not a directory").
51 If the component is found and is a directory, we set the current lookup
53 directory to that directory, and go to the next component.
54 If the component is found and is a symbolic link (symlink), we first
56 resolve this symbolic link (with the current lookup directory as start‐
57 ing lookup directory). Upon error, that error is returned. If the
58 result is not a directory, an ENOTDIR error is returned. If the reso‐
59 lution of the symbolic link is successful and returns a directory, we
60 set the current lookup directory to that directory, and go to the next
61 component. Note that the resolution process here can involve recursion
62 if the prefix ('dirname') component of a pathname contains a filename
63 that is a symbolic link that resolves to a directory (where the prefix
64 component of that directory may contain a symbolic link, and so on).
65 In order to protect the kernel against stack overflow, and also to pro‐
66 tect against denial of service, there are limits on the maximum recur‐
67 sion depth, and on the maximum number of symbolic links followed. An
68 ELOOP error is returned when the maximum is exceeded ("Too many levels
69 of symbolic links").
70 As currently implemented on Linux, the maximum number of symbolic links
72 that will be followed while resolving a pathname is 40. In kernels
73 before 2.6.18, the limit on the recursion depth was 5. Starting with
74 Linux 2.6.18, this limit was raised to 8. In Linux 4.2, the kernel's
75 pathname-resolution code was reworked to eliminate the use of recur‐
76 sion, so that the only limit that remains is the maximum of 40 resolu‐
77 tions for the entire pathname.
78 The resolution of symbolic links during this stage can be blocked by
80 using openat2(2), with the RESOLVE_NO_SYMLINKS flag set.
81 Step 3: find the final entry
83 The lookup of the final component of the pathname goes just like that
84 of all other components, as described in the previous step, with two
85 differences: (i) the final component need not be a directory (at least
86 as far as the path resolution process is concerned—it may have to be a
87 directory, or a nondirectory, because of the requirements of the spe‐
88 cific system call), and (ii) it is not necessarily an error if the com‐
89 ponent is not found—maybe we are just creating it. The details on the
90 treatment of the final entry are described in the manual pages of the
91 specific system calls.
92 . and ..
94 By convention, every directory has the entries "." and "..", which
95 refer to the directory itself and to its parent directory, respec‐
96 tively.
97 The path resolution process will assume that these entries have their
99 conventional meanings, regardless of whether they are actually present
100 in the physical filesystem.
101 One cannot walk up past the root: "/.." is the same as "/".
103 Mount points
105 After a "mount dev path" command, the pathname "path" refers to the
106 root of the filesystem hierarchy on the device "dev", and no longer to
107 whatever it referred to earlier.
108 One can walk out of a mounted filesystem: "path/.." refers to the par‐
110 ent directory of "path", outside of the filesystem hierarchy on "dev".
111 Traversal of mount points can be blocked by using openat2(2), with the
113 RESOLVE_NO_XDEV flag set (though note that this also restricts bind
114 mount traversal).
115 Trailing slashes
117 If a pathname ends in a '/', that forces resolution of the preceding
118 component as in Step 2: it has to exist and resolve to a directory.
119 Otherwise, a trailing '/' is ignored. (Or, equivalently, a pathname
120 with a trailing '/' is equivalent to the pathname obtained by appending
121 '.' to it.)
122 Final symlink
124 If the last component of a pathname is a symbolic link, then it depends
125 on the system call whether the file referred to will be the symbolic
126 link or the result of path resolution on its contents. For example,
127 the system call lstat(2) will operate on the symlink, while stat(2)
128 operates on the file pointed to by the symlink.
129 Length limit
131 There is a maximum length for pathnames. If the pathname (or some
132 intermediate pathname obtained while resolving symbolic links) is too
133 long, an ENAMETOOLONG error is returned ("Filename too long").
134 Empty pathname
136 In the original UNIX, the empty pathname referred to the current direc‐
137 tory. Nowadays POSIX decrees that an empty pathname must not be
138 resolved successfully. Linux returns ENOENT in this case.
139 Permissions
141 The permission bits of a file consist of three groups of three bits;
142 see chmod(1) and stat(2). The first group of three is used when the
143 effective user ID of the calling process equals the owner ID of the
144 file. The second group of three is used when the group ID of the file
145 either equals the effective group ID of the calling process, or is one
146 of the supplementary group IDs of the calling process (as set by set‐
147 groups(2)). When neither holds, the third group is used.
148 Of the three bits used, the first bit determines read permission, the
150 second write permission, and the last execute permission in case of
151 ordinary files, or search permission in case of directories.
152 Linux uses the fsuid instead of the effective user ID in permission
154 checks. Ordinarily the fsuid will equal the effective user ID, but the
155 fsuid can be changed by the system call setfsuid(2).
156 (Here "fsuid" stands for something like "filesystem user ID". The con‐
158 cept was required for the implementation of a user space NFS server at
159 a time when processes could send a signal to a process with the same
160 effective user ID. It is obsolete now. Nobody should use setf‐
161 suid(2).)
162 Similarly, Linux uses the fsgid ("filesystem group ID") instead of the
164 effective group ID. See setfsgid(2).
165 Bypassing permission checks: superuser and capabilities
167 On a traditional UNIX system, the superuser (root, user ID 0) is all-
168 powerful, and bypasses all permissions restrictions when accessing
169 files.
170 On Linux, superuser privileges are divided into capabilities (see capa‐
172 bilities(7)). Two capabilities are relevant for file permissions
173 checks: CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH. (A process has these
174 capabilities if its fsuid is 0.)
175 The CAP_DAC_OVERRIDE capability overrides all permission checking, but
177 grants execute permission only when at least one of the file's three
178 execute permission bits is set.
179 The CAP_DAC_READ_SEARCH capability grants read and search permission on
181 directories, and read permission on ordinary files.
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184 readlink(2), capabilities(7), credentials(7), symlink(7)
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187 This page is part of release 5.07 of the Linux man-pages project. A
188 description of the project, information about reporting bugs, and the
189 latest version of this page, can be found at
190 https://www.kernel.org/doc/man-pages/.
191Linux 2020-04-11 PATH_RESOLUTION(7)