Ubuntu Manpage: execveat - execute program relative to a directory file descriptor

NAME

       execveat - execute program relative to a directory file descriptor

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

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

       int execveat(int dirfd, const char *pathname,
                    char *const _Nullable argv[],
                    char *const _Nullable envp[],
                    int flags);

DESCRIPTION

The execveat() system call executes the program referred to by the combination of dirfd
and pathname. It operates in exactly the same way as execve(2), except for the
differences described in this manual page.

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 execve(2) 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
execve(2)).

If pathname is absolute, then dirfd is ignored.

If pathname is an empty string and the AT_EMPTY_PATH flag is specified, then the file
descriptor dirfd specifies the file to be executed (i.e., dirfd refers to an executable
file, rather than a directory).

The flags argument is a bit mask that can include zero or more of the following flags:

AT_EMPTY_PATH
If pathname is an empty string, operate on the file referred to by dirfd (which may
have been obtained using the open(2) O_PATH flag).

AT_SYMLINK_NOFOLLOW
If the file identified by dirfd and a non-NULL pathname is a symbolic link, then
the call fails with the error ELOOP.

RETURN VALUE

       On success, execveat() does not return.  On error, -1 is returned, and  errno  is  set  to
       indicate the error.

ERRORS

       The  same  errors  that  occur for execve(2) can also occur for execveat().  The following
       additional errors can occur for execveat():

       pathname
              is relative but dirfd is neither AT_FDCWD nor a valid file descriptor.

       EINVAL Invalid flag specified in flags.

       ELOOP  flags includes AT_SYMLINK_NOFOLLOW and the file identified by dirfd and a  non-NULL
              pathname is a symbolic link.

       ENOENT The  program  identified  by  dirfd and pathname requires the use of an interpreter
              program (such as a script starting with "#!"), but the file  descriptor  dirfd  was
              opened  with  the  O_CLOEXEC  flag,  with  the  result  that  the  program  file is
              inaccessible to the launched interpreter.  See BUGS.

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

STANDARDS

       Linux.

HISTORY

       Linux 3.19, glibc 2.34.

NOTES

       In  addition  to  the  reasons  explained in openat(2), the execveat() system call is also
       needed to allow fexecve(3) to be implemented  on  systems  that  do  not  have  the  /proc
       filesystem mounted.

       When  asked to execute a script file, the argv[0] that is passed to the script interpreter
       is a string of the form /dev/fd/N or /dev/fd/N/P, where  N  is  the  number  of  the  file
       descriptor  passed  via  the  dirfd  argument.   A  string  of  the first form occurs when
       AT_EMPTY_PATH is employed.  A string  of  the  second  form  occurs  when  the  script  is
       specified via both dirfd and pathname; in this case, P is the value given in pathname.

       For  the  same reasons described in fexecve(3), the natural idiom when using execveat() is
       to set the close-on-exec flag on dirfd.  (But see BUGS.)

BUGS

       The ENOENT error described above means that it is not possible to  set  the  close-on-exec
       flag on the file descriptor given to a call of the form:

           execveat(fd, "", argv, envp, AT_EMPTY_PATH);

       However,  the  inability  to  set  the  close-on-exec  flag  means  that a file descriptor
       referring to the script leaks through to the script itself.  As well  as  wasting  a  file
       descriptor, this leakage can lead to file-descriptor exhaustion in scenarios where scripts
       recursively employ execveat().