HardenedBSD/lib/libsys/access.2

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.Dd May 21, 2024
.Dt ACCESS 2
.Os
.Sh NAME
.Nm access ,
.Nm eaccess ,
.Nm faccessat
.Nd check accessibility of a file
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn access "const char *path" "int mode"
.Ft int
.Fn eaccess "const char *path" "int mode"
.Ft int
.Fn faccessat "int fd" "const char *path" "int mode" "int flag"
.Sh DESCRIPTION
The
.Fn access ,
.Fn eaccess
and
.Fn faccessat
system calls report whether an attempt to access the file designated
by their
.Fa path
in the manner described by their
.Fa mode
argument is likely to succeed.
The value of
.Fa mode
is either the bitwise-inclusive OR of the desired permissions
.Po
.Dv R_OK
for read permission,
.Dv W_OK
for write permission, and
.Dv X_OK
for execute / search permission
.Pc
or
.Dv F_OK
to simply check whether the file exists.
.Pp
For a number of reasons, these system calls cannot be relied upon to
give a correct and definitive answer.
They can at best provide an early indication of the expected outcome,
to be confirmed by actually attempting the operation.
For existence checks, either
.Xr stat 2
or
.Xr lstat 2
should be used instead.
See also
.Sx SECURITY CONSIDERATIONS
below.
.Pp
The
.Fn eaccess
system call uses
the effective user ID and the group access list
to authorize the request;
the
.Fn access
system call uses
the real user ID in place of the effective user ID,
the real group ID in place of the effective group ID,
and the rest of the group access list.
.Pp
See the
.Sx DEFINITIONS
section of
.Xr intro 2
for additional information on file access permissions and real
vs. effective user and group IDs.
.Pp
The
.Fn faccessat
system call is equivalent to
.Fn access
except in the case where
.Fa path
specifies a relative path.
In this case the file whose accessibility is to be determined is
located relative to the directory associated with the file descriptor
.Fa fd
instead of the current working directory.
If
.Fn faccessat
is passed the special value
.Dv AT_FDCWD
in the
.Fa fd
parameter, the current working directory is used and the behavior is
identical to a call to
.Fn access .
Values for
.Fa flag
are constructed by a bitwise-inclusive OR of flags from the following
list, defined in
.In fcntl.h :
.Bl -tag -width indent
.It Dv AT_EACCESS
The checks are performed using the effective user and group IDs,
like
.Fn eaccess ,
instead of the real user and group ID, like
.Fn access .
.It Dv AT_RESOLVE_BENEATH
Only walk paths below the directory specified by the
.Ar fd
descriptor.
See the description of the
.Dv O_RESOLVE_BENEATH
flag in the
.Xr open 2
manual page.
.It Dv AT_EMPTY_PATH
If the
.Fa path
argument is an empty string, operate on the file or directory
referenced by the descriptor
.Fa fd .
If
.Fa fd
is equal to
.Dv AT_FDCWD ,
operate on the current working directory.
.El
.Pp
Even if a process's real or effective user has appropriate privileges
and indicates success for
.Dv X_OK ,
the file may not actually have execute permission bits set.
Likewise for
.Dv R_OK
and
.Dv W_OK .
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
The
.Fn access ,
.Fn eaccess ,
and
.Fn faccessat
system calls may fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The value of the
.Fa mode
argument is invalid.
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded 255 characters,
or an entire path name exceeded 1023 characters.
.It Bq Er ENOENT
The named file does not exist.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EROFS
Write access is requested for a file on a read-only file system.
.It Bq Er ETXTBSY
Write access is requested for a pure procedure (shared text)
file presently being executed.
.It Bq Er EACCES
Permission bits of the file mode do not permit the requested
access, or search permission is denied on a component of the
path prefix.
.It Bq Er EFAULT
The
.Fa path
argument
points outside the process's allocated address space.
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.It Bq Er EINTEGRITY
Corrupted data was detected while reading from the file system.
.El
.Pp
Also, the
.Fn faccessat
system call may fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa path
argument does not specify an absolute path and the
.Fa fd
argument is
neither
.Dv AT_FDCWD
nor a valid file descriptor.
.It Bq Er EINVAL
The value of the
.Fa flag
argument is not valid.
.It Bq Er ENOTDIR
The
.Fa path
argument is not an absolute path and
.Fa fd
is neither
.Dv AT_FDCWD
nor a file descriptor associated with a directory.
.It Bq Er ENOTCAPABLE
.Fa path
is an absolute path,
or contained a ".." component leading to a
directory outside of the directory hierarchy specified by
.Fa fd ,
and the process is in capability mode.
.El
.Sh SEE ALSO
.Xr chmod 2 ,
.Xr intro 2 ,
.Xr stat 2
.Sh STANDARDS
The
.Fn access
system call is expected to conform to
.St -p1003.1-90 .
The
.Fn faccessat
system call follows The Open Group Extended API Set 2 specification.
.Sh HISTORY
The
.Fn access
function appeared in
.At v7 .
The
.Fn faccessat
system call appeared in
.Fx 8.0 .
.Sh SECURITY CONSIDERATIONS
The
.Fn access ,
.Fn eaccess ,
and
.Fn faccessat
system calls are subject to time-of-check-to-time-of-use races and
should not be relied upon for file permission enforcement purposes.
Instead, applications should perform the desired action using the
requesting user's credentials.