ACCESS(2)                 (2020-12-21)                  ACCESS(2)

     NAME
          access, faccessat, faccessat2 - check user's permissions for
          a file

     SYNOPSIS
          #include <unistd.h>

          int access(const char *pathname, int mode);

          #include <fcntl.h>           /* Definition of AT_* constants */
          #include <unistd.h>

          int faccessat(int dirfd, const char *pathname, int mode
                          /* But see C library/kernel differences, below */

          int faccessat2(int dirfd, const char *pathname, int mode

     Feature Test Macro Requirements for glibc (see
     feature_test_macros(7)):

          faccessat():
              Since glibc 2.10:
                  _POSIX_C_SOURCE >= 200809L
              Before glibc 2.10:
                  _ATFILE_SOURCE

     DESCRIPTION
          access() checks whether the calling process can  access  the
          file  pathname. If pathname is a symbolic link, it is deref-
          erenced.

          The mode specifies the accessibility  check(s)  to  be  per-
          formed,  and  is either the value F_OK, or a mask consisting
          of the bitwise OR of one or more of R_OK,  W_OK,  and  X_OK.
          F_OK  tests  for the existence of the file.  R_OK, W_OK, and
          X_OK test whether the file exists and  grants  read,  write,
          and execute permissions, respectively.

          The check is done using the calling process's real  UID  and
          GID,  rather than the effective IDs as is done when actually
          attempting an operation (e.g., open(2)) on the file.   Simi-
          larly,  for the root user, the check uses the set of permit-
          ted capabilities rather than the set of effective  capabili-
          ties; and for non-root users, the check uses an empty set of
          capabilities.

          This allows set-user-ID programs and capability-endowed pro-
          grams to easily determine the invoking user's authority.  In
          other  words,  access()  does  not   answer   the   "can   I
          read/write/execute  this  file?"  question.   It  answers  a

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          slightly different question: "(assuming I'm a setuid binary)
          can  the user who invoked me read/write/execute this file?",
          which gives set-user-ID programs the possibility to  prevent
          malicious  users from causing them to read files which users
          shouldn't be able to read.

          If the calling process is privileged (i.e., its real UID  is
          zero),  then  an X_OK check is successful for a regular file
          if execute permission is enabled for any of the file  owner,
          group, or other.

        faccessat()
          faccessat() operates in exactly the same  way  as  access(),
          except for the differences described here.

          If the pathname given in pathname is relative,  then  it  is
          interpreted  relative  to  the  directory referred to by the
          file descriptor dirfd (rather than relative to  the  current
          working  directory  of  the  calling  process, as is done by
          access() for a relative pathname).

          If pathname is relative  and  dirfd  is  the  special  value
          AT_FDCWD,  then pathname is interpreted relative to the cur-
          rent  working  directory  of  the  calling   process   (like
          access()).

          If pathname is absolute, then dirfd is ignored.

          flags is constructed by ORing together zero or more  of  the
          following values:

          AT_EACCESS
               Perform access checks  using  the  effective  user  and
               group  IDs.   By default, faccessat() uses the real IDs
               (like access()).

          AT_SYMLINK_NOFOLLOW
               If pathname is a symbolic link, do not dereference  it:
               instead return information about the link itself.

          See  openat(2)  for  an  explanation   of   the   need   for
          faccessat().

        faccessat2()
          The description of faccessat() given  above  corresponds  to
          POSIX.1  and  to the implementation provided by glibc.  How-
          ever, the glibc implementation was  an  imperfect  emulation
          (see  BUGS)  that  papered  over the fact that the raw Linux
          faccessat() system call does not have a flags argument.   To
          allow  for  a  proper  implementation,  Linux  5.8 added the
          faccessat2() system call, which supports the flags  argument
          and  allows  a  correct  implementation  of  the faccessat()

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          wrapper function.

     RETURN VALUE
          On success (all requested permissions granted,  or  mode  is
          F_OK  and  the file exists), zero is returned.  On error (at
          least one bit in mode asked for a permission that is denied,
          or  mode  is F_OK and the file does not exist, or some other
          error occurred), -1 is returned, and errno is set  appropri-
          ately.

     ERRORS
          access() and faccessat() shall fail if:

          EACCES
               The requested access would be denied to  the  file,  or
               search  permission is denied for one of the directories
               in   the   path   prefix   of   pathname.   (See   also
               path_resolution(7).)

          ELOOP
               Too many symbolic links were encountered  in  resolving
               pathname.

          ENAMETOOLONG
               pathname is too long.

          ENOENT
               A component of pathname does not exist or is a dangling
               symbolic link.

          ENOTDIR
               A component used as a directory in pathname is not,  in
               fact, a directory.

          EROFS
               Write permission was requested for a file  on  a  read-
               only filesystem.

          access() and faccessat() may fail if:

          EFAULT
               pathname points outside your accessible address space.

          EINVAL
               mode was incorrectly specified.

          EIO  An I/O error occurred.

          ENOMEM
               Insufficient kernel memory was available.

          ETXTBSY

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               Write access was requested to an  executable  which  is
               being executed.

          The following additional errors can occur for faccessat():

          EBADF
               dirfd is not a valid file descriptor.

          EINVAL
               Invalid flag specified in flags.

          ENOTDIR
               pathname is relative and dirfd  is  a  file  descriptor
               referring to a file other than a directory.

     VERSIONS
          faccessat() was added to Linux  in  kernel  2.6.16;  library
          support was added to glibc in version 2.4.

          faccessat2() was added to Linux in version 5.8.

     CONFORMING TO
          access(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

          faccessat(): POSIX.1-2008.

          faccessat2(): Linux-specific.

     NOTES
          Warning: Using these calls to check if a user is  authorized
          to,  for example, open a file before actually doing so using
          open(2) creates a security  hole,  because  the  user  might
          exploit the short time interval between checking and opening
          the file to manipulate it.  For this reason, the use of this
          system  call  should  be  avoided.   (In  the  example  just
          described, a  safer  alternative  would  be  to  temporarily
          switch  the  process's  effective user ID to the real ID and
          then call open(2).)

          access() always dereferences symbolic links.  If you need to
          check  the  permissions  on a symbolic link, use faccessat()
          with the flag AT_SYMLINK_NOFOLLOW.

          These calls return an error if any of the  access  types  in
          mode  is  denied,  even if some of the other access types in
          mode are permitted.

          If the calling process has appropriate privileges (i.e.,  is
          superuser),  POSIX.1-2001 permits an implementation to indi-
          cate success for an X_OK check even if none of  the  execute
          file permission bits are set.  Linux does not do this.

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          A file is accessible only if the permissions on each of  the
          directories  in  the  path  prefix  of pathname grant search
          (i.e., execute) access.  If any directory  is  inaccessible,
          then  the access() call fails, regardless of the permissions
          on the file itself.

          Only access bits are checked, not the file type or contents.
          Therefore, if a directory is found to be writable, it proba-
          bly means that files can be created in  the  directory,  and
          not that the directory can be written as a file.  Similarly,
          a DOS  file  may  be  found  to  be  "executable,"  but  the
          execve(2) call will still fail.

          These calls may not work correctly on NFSv2 filesystems with
          UID  mapping  enabled,  because  UID  mapping is done on the
          server and hidden from the client, which checks permissions.
          (NFS versions 3 and higher perform the check on the server.)
          Similar problems can occur to FUSE mounts.

        C library/kernel differences
          The raw faccessat() system call takes only the  first  three
          arguments.  The AT_EACCESS and AT_SYMLINK_NOFOLLOW flags are
          actually implemented within the glibc wrapper  function  for
          faccessat().   If  either  of these flags is specified, then
          the wrapper function employs fstatat(2) to determine  access
          permissions, but see BUGS.

        Glibc notes
          On older kernels where faccessat() is unavailable (and  when
          the  AT_EACCESS and AT_SYMLINK_NOFOLLOW flags are not speci-
          fied), the glibc wrapper function falls back to the  use  of
          access().   When pathname is a relative pathname, glibc con-
          structs  a  pathname  based  on   the   symbolic   link   in
          /proc/self/fd that corresponds to the dirfd argument.

     BUGS
          Because the Linux kernel's faccessat() system call does  not
          support  a  flags  argument,  the  glibc faccessat() wrapper
          function provided in glibc 2.32  and  earlier  emulates  the
          required   functionality   using   a   combination   of  the
          faccessat() system call and fstatat(2).  However, this  emu-
          lation does not take ACLs into account.  Starting with glibc
          2.33, the wrapper function avoids this bug by making use  of
          the  faccessat2()  system  call  where it is provided by the
          underlying kernel.

          In kernel 2.4 (and earlier) there is some strangeness in the
          handling  of X_OK tests for superuser.  If all categories of
          execute permission are disabled  for  a  nondirectory  file,
          then  the only access() test that returns -1 is when mode is
          specified as just X_OK; if R_OK or W_OK is also specified in
          mode,  then  access()  returns  0 for such files.  Early 2.6

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          kernels (up to and including 2.6.3) also behaved in the same
          way as kernel 2.4.

          In kernels before 2.6.20, these calls ignored the effect  of
          the MS_NOEXEC flag if it was used to mount(2) the underlying
          filesystem.  Since kernel 2.6.20, the MS_NOEXEC flag is hon-
          ored.

     SEE ALSO
          chmod(2), chown(2), open(2), setgid(2), setuid(2),  stat(2),
          euidaccess(3),      credentials(7),      path_resolution(7),
          symlink(7)

     COLOPHON
          This page is part of release 5.10  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/.

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