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Date: Mon, 10 Jul 2006 14:54:35 -0400
From: Jeff Dike <jdike@addtoit.com>
To: Joshua Hudson <joshudson@gmail.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [OT] 'volatile' in userspace
Message-ID: <20060710185435.GA5445@ccure.user-mode-linux.org>
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On Mon, Jul 10, 2006 at 10:00:37AM -0700, Joshua Hudson wrote:
> Yes, I use vfork. So far, the only way I have found for the parent to
> know whether or not the child's exec() failed is this way:

What I do in UML is make a pipe between the parent and child, set it
CLOEXEC, and write a byte down it if the exec fails.

The parent reads it - if it gets no bytes, the exec succeeded, if it
gets one byte, it failed.

child -
	execvp(argv[0], argv);
	errval = errno;
	write(data->fd, &errval, sizeof(errval));

parent -
	socketpair(AF_UNIX, SOCK_STREAM, 0, fds);
	fcntl(fds[1], F_SETFD, flag);
	pid = clone(child, NULL, SIGCHLD, NULL);
	if(pid < 0){
		...
	}

	close(fds[1]);

	/* Read the errno value from the child, if the exec failed, or get 0 if
	 * the exec succeeded because the pipe fd was set as close-on-exec.
	 */
	n = read(fds[0], &ret, sizeof(ret));
	if (n < 0) {
	} else if(n != 0){
		/* exec failed */
	} else {
		/* exec succeeded */
	}

				Jeff