FEXECVE
Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
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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,
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 appropriately.
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,
fexecve()
is implemented using the
proc(5)
filesystem, so
/proc
needs to be mounted and available at the time of the call.
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.)
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.
SEE ALSO
execve(2),
execveat(2)
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- ATTRIBUTES
-
- CONFORMING TO
-
- NOTES
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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