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man:faccessat

ACCESS(2) Linux Programmer's Manual ACCESS(2)

NAME

     access, faccessat - 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, int flags);
 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 path-
     name.  If pathname is a symbolic link, it is dereferenced.
     The mode specifies the accessibility check(s) to be performed,  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.  Similarly, for the root user,  the  check
     uses the set of permitted capabilities rather than the set of effective
     capabilities; and for non-root users, the check uses an  empty  set  of
     capabilities.
     This  allows  set-user-ID  programs  and capability-endowed programs 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 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()
     The faccessat() system  call  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 current 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().

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 appropriately.

ERRORS

     access() and faccessat() shall fail if:
     EACCES The requested access would be denied to the file, or search per-
            mission 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
            Write  access was requested to an executable which is being exe-
            cuted.
     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.

CONFORMING TO

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

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 rea-
     son, 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_SYM-
     LINK_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 indicate success for an  X_OK
     check  even if none of the execute file permission bits are set.  Linux
     does not do this.
     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.  There-
     fore, if a directory is found to be writable, it  probably  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  "exe-
     cutable," but the execve(2) call will still fail.
     These  calls  may not work correctly on NFSv2 filesystems with UID map-
     ping enabled, because UID mapping is done on the server and hidden from
     the  client, which checks permissions.  (NFS versions 3 and higher per-
     form 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.
 Glibc notes
     On older kernels where faccessat() is unavailable (and when the AT_EAC-
     CESS and AT_SYMLINK_NOFOLLOW flags are not specified), the glibc  wrap-
     per  function  falls  back  to the use of access().  When pathname is a
     relative pathname, glibc constructs a pathname based  on  the  symbolic
     link in /proc/self/fd that corresponds to the dirfd argument.

BUGS

     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 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 honored.

SEE ALSO

     chmod(2),  chown(2),  open(2),  setgid(2),  setuid(2), stat(2), euidac-
     cess(3), credentials(7), path_resolution(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 2016-03-15 ACCESS(2)

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