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man:path_resolution

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

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.  A process may  get
     an  entirely  private  mount namespace in case it--or one of its ances-
     tors--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 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 (symlink), we first
     resolve this symbolic link (with the current lookup directory as start-
     ing  lookup  directory).   Upon  error, that error is returned.  If the
     result is not a directory, an ENOTDIR error is returned.  If the  reso-
     lution of the symlink is successful and returns a directory, we set the
     current lookup directory to that directory, and go to the  next  compo-
     nent.   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 com-
     ponent 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.   In  kernels
     before  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 recur-
     sion, so that the only limit that remains is the maximum of 40  resolu-
     tions for the entire pathname.
 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 spe-
     cific system call), and (ii) it is not necessarily an error if the com-
     ponent 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,  respec-
     tively.
     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 par-
     ent 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 ("Filename too long").
 Empty pathname
     In the original UNIX, the empty pathname referred to the current direc-
     tory.  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  set-
     groups(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 con-
     cept 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  setf-
     suid(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 capa-
     bilities(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.

SEE ALSO

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

COLOPHON

     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at
     https://www.kernel.org/doc/man-pages/.

Linux 2017-11-26 PATH_RESOLUTION(7)

/data/webs/external/dokuwiki/data/pages/man/path_resolution.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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