Re: [PATCH v4 09/12] unix-socket: disallow chdir() when creating unix domain sockets

From: Jeff King <peff@peff.net>
Date: Mar 5, 2021, 09:25 UTC
Message-ID: <YEH5AUxgFxWTxb6u@coredump.intra.peff.net>
In-Reply-To: <YEHzmIOYgRtI1Ak1@coredump.intra.peff.net>

On Fri, Mar 05, 2021 at 04:02:16AM -0500, Jeff King wrote:

Show 8 quoted lines

> I don't know offhand if this trick actually works. I can imagine it
> does, but it hinges on the subtlety between an integer descriptor and
> the underlying "file description" (the term used in POSIX). Does binding
> a socket operate on the former (like close() does not close the parent's
> descriptor) or the latter (like lseek() impacts other descriptors).
> 
> I'd guess the latter, but I wasn't sure if you were suggesting this from
> experience or if you just invented the technique. ;)

I was curious, but this does indeed work:

-- >8 -- #include <sys/types.h> #include <sys/socket.h> #include <netdb.h> #include <sys/wait.h> #include <unistd.h> #include <stdlib.h>

int main(void)
{
	int listen_fd, client_fd;
	struct addrinfo *ai;
	pid_t pid;

	getaddrinfo("127.0.0.1", "1234", NULL, &ai);
	listen_fd = socket(ai->ai_family, ai->ai_socktype, ai->ai_protocol);
	pid = fork();
	if (!pid) {
		bind(listen_fd, ai->ai_addr, ai->ai_addrlen);
		return 0;
	}
	waitpid(pid, NULL, 0);

	listen(listen_fd, 5);
	client_fd = accept(listen_fd, NULL, NULL);
	write(client_fd, "foo\n", 4);
	return 0;
}
-- >8 --

-Peff