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FEXECVE(3) Linux Programmer's Manual FEXECVE(3)
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, 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.
ENOSYS 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, 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.)
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)
This page is part of release 4.12 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 2016-03-15 FEXECVE(3)
Pages that refer to this page: execve(2), execveat(2), exec(3), posix_spawn(3), signal-safety(7)