FEXECVE(3) Linux Programmer's Manual FEXECVE(3)
NAME
fexecve - execute program specified via file descriptor
SYNOPSIS
#include <unistd.h>
int fexecve(int fd, char *const argv[], char *const envp[]);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fexecve():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
DESCRIPTION
fexecve() performs the same task as execve(2), with the difference that the file to be executed is specified via a file descriptor, fd,
rather than via a pathname. The file descriptor fd must be opened read-only (O_RDONLY) or with the O_PATH flag and the caller must have
permission to execute the file that it refers to.
RETURN VALUE
A successful call to fexecve() never returns. On error, the function does return, with a result value of -1, and errno is set appropri-
ately.
ERRORS
Errors are as for execve(2), with the following additions:
EINVAL fd is not a valid file descriptor, or argv is NULL, or envp is NULL.
ENOSYS The /proc filesystem could not be accessed.
VERSIONS
fexecve() is implemented since glibc 2.3.2.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
+----------+---------------+---------+
|Interface | Attribute | Value |
+----------+---------------+---------+
|fexecve() | Thread safety | MT-Safe |
+----------+---------------+---------+
CONFORMING TO
POSIX.1-2008. This function is not specified in POSIX.1-2001, and is not widely available on other systems. It is specified in
POSIX.1-2008.
NOTES
On Linux with glibc versions 2.26 and earlier, fexecve() is implemented using the proc(5) filesystem, so /proc needs to be mounted and
available at the time of the call. Since glibc 2.27, if the underlying kernel supports the execveat(2) system call, then fexecve() is
implemented using that system call, with the benefit that /proc does not need to be mounted.
The idea behind fexecve() is to allow the caller to verify (checksum) the contents of an executable before executing it. Simply opening
the file, checksumming the contents, and then doing an execve(2) would not suffice, since, between the two steps, the filename, or a direc-
tory prefix of the pathname, could have been exchanged (by, for example, modifying the target of a symbolic link). fexecve() does not mit-
igate the problem that the contents of a file could be changed between the checksumming and the call to fexecve(); for that, the solution
is to ensure that the permissions on the file prevent it from being modified by malicious users.
The natural idiom when using fexecve() is to set the close-on-exec flag on fd, so that the file descriptor does not leak through to the
program that is executed. This approach is natural for two reasons. First, it prevents file descriptors being consumed unnecessarily.
(The executed program normally has no need of a file descriptor that refers to the program itself.) Second, if fexecve() is used recur-
sively, employing the close-on-exec flag prevents the file descriptor exhaustion that would result from the fact that each step in the
recursion would cause one more file descriptor to be passed to the new program. (But see BUGS.)
BUGS
If fd refers to a script (i.e., it is an executable text file that names a script interpreter with a first line that begins with the char-
acters #!) and the close-on-exec flag has been set for fd, then fexecve() fails with the error ENOENT. This error occurs because, by the
time the script interpreter is executed, fd has already been closed because of the close-on-exec flag. Thus, the close-on-exec flag can't
be set on fd if it refers to a script, leading to the problems described in NOTES.
SEE ALSO
execve(2), execveat(2)
COLOPHON
This page is part of release 4.15 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 2017-09-15 FEXECVE(3)