1PATH_RESOLUTION(7)         Linux Programmer's Manual        PATH_RESOLUTION(7)
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NAME

6       path_resolution - how a pathname is resolved to a file
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DESCRIPTION

9       Some  Unix/Linux  system calls have as parameter one or more filenames.
10       A filename (or pathname) is resolved as follows.
11
12   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 ances‐
19       tors — was started by an invocation of the clone(2)  system  call  that
20       had  the CLONE_NEWNS flag set.)  This handles the '/' part of the path‐
21       name.
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23       If the pathname does not start with the  '/'  character,  the  starting
24       lookup  directory  of  the  resolution  process  is the current working
25       directory of the process.  (This is also inherited from the parent.  It
26       can be changed by use of the chdir(2) system call.)
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28       Pathnames  starting with a '/' character are called absolute pathnames.
29       Pathnames not starting with a '/' are called relative pathnames.
30
31   Step 2: Walk along the path
32       Set the current lookup directory  to  the  starting  lookup  directory.
33       Now,  for each nonfinal component of the pathname, where a component is
34       a substring delimited by '/' characters, this component is looked up in
35       the current lookup directory.
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37       If  the  process  does not have search permission on the current lookup
38       directory, an EACCES error is returned ("Permission denied").
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40       If the component is not found, an ENOENT error is  returned  ("No  such
41       file or directory").
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43       If  the  component  is found, but is neither a directory nor a symbolic
44       link, an ENOTDIR error is returned ("Not a directory").
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46       If the component is found and is a directory, we set the current lookup
47       directory to that directory, and go to the next component.
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49       If  the  component  is found and is a symbolic link (symlink), we first
50       resolve this symbolic link (with the current lookup directory as start‐
51       ing  lookup  directory).   Upon  error, that error is returned.  If the
52       result is not a directory, an ENOTDIR error is returned.  If the  reso‐
53       lution of the symlink is successful and returns a directory, we set the
54       current lookup directory to that directory, and go to the  next  compo‐
55       nent.   Note  that  the resolution process here involves recursion.  In
56       order to protect the kernel against stack overflow, and also to protect
57       against  denial  of  service, there are limits on the maximum recursion
58       depth, and on the maximum number of symbolic links followed.  An  ELOOP
59       error  is  returned  when  the maximum is exceeded ("Too many levels of
60       symbolic links").
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62   Step 3: Find the final entry
63       The lookup of the final component of the pathname goes just  like  that
64       of  all  other  components, as described in the previous step, with two
65       differences: (i) the final component need not be a directory (at  least
66       as  far as the path resolution process is concerned — it may have to be
67       a directory, or a nondirectory, because of the requirements of the spe‐
68       cific system call), and (ii) it is not necessarily an error if the com‐
69       ponent is not found — maybe we are just creating it.   The  details  on
70       the  treatment  of the final entry are described in the manual pages of
71       the specific system calls.
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73   . and ..
74       By convention, every directory has the  entries  "."  and  "..",  which
75       refer  to  the  directory  itself  and to its parent directory, respec‐
76       tively.
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78       The path resolution process will assume that these entries  have  their
79       conventional  meanings, regardless of whether they are actually present
80       in the physical file system.
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82       One cannot walk down past the root: "/.." is the same as "/".
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84   Mount points
85       After a "mount dev path" command, the pathname  "path"  refers  to  the
86       root of the file system hierarchy on the device "dev", and no longer to
87       whatever it referred to earlier.
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89       One can walk out of a mounted file system: "path/.." refers to the par‐
90       ent directory of "path", outside of the file system hierarchy on "dev".
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92   Trailing slashes
93       If  a  pathname  ends in a '/', that forces resolution of the preceding
94       component as in Step 2: it has to exist and  resolve  to  a  directory.
95       Otherwise  a  trailing  '/'  is ignored.  (Or, equivalently, a pathname
96       with a trailing '/' is equivalent to the pathname obtained by appending
97       '.' to it.)
98
99   Final symlink
100       If the last component of a pathname is a symbolic link, then it depends
101       on the system call whether the file referred to will  be  the  symbolic
102       link  or  the  result of path resolution on its contents.  For example,
103       the system call lstat(2) will operate on  the  symlink,  while  stat(2)
104       operates on the file pointed to by the symlink.
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106   Length limit
107       There  is  a  maximum  length  for pathnames.  If the pathname (or some
108       intermediate pathname obtained while resolving symbolic links)  is  too
109       long, an ENAMETOOLONG error is returned ("File name too long").
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111   Empty pathname
112       In the original Unix, the empty pathname referred to the current direc‐
113       tory.  Nowadays POSIX decrees  that  an  empty  pathname  must  not  be
114       resolved successfully.  Linux returns ENOENT in this case.
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116   Permissions
117       The  permission  bits  of a file consist of three groups of three bits,
118       cf. chmod(1) and stat(2).  The first group of three is  used  when  the
119       effective  user  ID  of  the calling process equals the owner ID of the
120       file.  The second group of three is used when the group ID of the  file
121       either  equals the effective group ID of the calling process, or is one
122       of the supplementary group IDs of the calling process (as set  by  set‐
123       groups(2)).  When neither holds, the third group is used.
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125       Of  the  three bits used, the first bit determines read permission, the
126       second write permission, and the last execute  permission  in  case  of
127       ordinary files, or search permission in case of directories.
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129       Linux  uses  the  fsuid  instead of the effective user ID in permission
130       checks.  Ordinarily the fsuid will equal the effective user ID, but the
131       fsuid can be changed by the system call setfsuid(2).
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133       (Here  "fsuid"  stands  for  something like "file system user ID".  The
134       concept was required for the implementation of a user space NFS  server
135       at a time when processes could send a signal to a process with the same
136       effective user ID.  It  is  obsolete  now.   Nobody  should  use  setf‐
137       suid(2).)
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139       Similarly, Linux uses the fsgid ("file system group ID") instead of the
140       effective group ID.  See setfsgid(2).
141
142   Bypassing permission checks: superuser and capabilities
143       On a traditional Unix system, the superuser (root, user ID 0)  is  all-
144       powerful,  and  bypasses  all  permissions  restrictions when accessing
145       files.
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147       On Linux, superuser privileges are divided into capabilities (see capa‐
148       bilities(7)).   Two  capabilities  are  relevant  for  file permissions
149       checks: CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH.  (A process has these
150       capabilities if its fsuid is 0.)
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152       The  CAP_DAC_OVERRIDE capability overrides all permission checking, but
153       only grants execute permission when at least one of  the  file's  three
154       execute permission bits is set.
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156       The CAP_DAC_READ_SEARCH capability grants read and search permission on
157       directories, and read permission on ordinary files.
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SEE ALSO

160       readlink(2), capabilities(7), credentials(7), symlink(7)
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COLOPHON

163       This page is part of release 3.25 of the Linux  man-pages  project.   A
164       description  of  the project, and information about reporting bugs, can
165       be found at http://www.kernel.org/doc/man-pages/.
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169Linux                             2009-12-05                PATH_RESOLUTION(7)
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