fexecve - execute program specified via file descriptor
#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
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.
A successful call to
fexecve() never returns. On error, the function does
return, with a result value of -1, and
errno is set appropriately.
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.
- ENOENT
- The close-on-exec flag is set on fd, and fd refers to a
script. See BUGS.
- ENOSYS
- The kernel does not provide the execveat(2) system call, and the
/proc filesystem could not be accessed.
fexecve() is implemented since glibc 2.3.2.
For an explanation of the terms used in this section, see
attributes(7).
| Interface |
Attribute |
Value |
| fexecve () |
Thread safety |
MT-Safe |
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.
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 directory prefix of
the pathname, could have been exchanged (by, for example, modifying the target
of a symbolic link).
fexecve() does not mitigate 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 recursively, 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.)
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 characters
#!) 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.
execve(2),
execveat(2)