execveat(2) System Calls Manual execveat(2)
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 com-
bination 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 sym-
bolic 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 de-
scriptor.
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 argu-
ment. 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 us-
ing 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-de-
scriptor exhaustion in scenarios where scripts recursively employ ex-
ecveat().
SEE ALSO
execve(2), openat(2), fexecve(3)
Linux man-pages 6.9.1 2024-05-02 execveat(2)
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