privsep: Ensure we recv for real after a successful recv MSG_PEEK

Adjust the code flow so that the same errors would be caught
after the final recv.
This ensures we read what is really meant for us and not
something silly.

Should fix #555.

Author: rsmarples

src/privsep-root.c

@@ -86,6 +86,7 @@ ps_root_readerrorcb(struct psr_ctx *psr_ctx)
 		{ .iov_base = psr_error, .iov_len = sizeof(*psr_error) },
 		{ .iov_base = NULL, .iov_len = 0 },
 	};
+	struct msghdr msg = { .msg_iov = iov, .msg_iovlen = __arraycount(iov) };
 	ssize_t len;
 
 #define PSR_ERROR(e)				\
@@ -98,37 +99,40 @@ ps_root_readerrorcb(struct psr_ctx *psr_ctx)
 	if (eloop_waitfd(fd) == -1)
 		PSR_ERROR(errno);
 
-	len = recv(fd, psr_error, sizeof(*psr_error), MSG_PEEK);
+	len = recvmsg(fd, &msg, MSG_PEEK | MSG_WAITALL);
 	if (len == -1)
 		PSR_ERROR(errno);
 	else if ((size_t)len < sizeof(*psr_error))
-		PSR_ERROR(EINVAL);
+		goto recv;
 
-	if (psr_error->psr_datalen > SSIZE_MAX)
-		PSR_ERROR(ENOBUFS);
 	if (psr_ctx->psr_usemdata &&
 	    psr_error->psr_datalen > psr_ctx->psr_mdatalen)
 	{
 		void *d = realloc(psr_ctx->psr_mdata, psr_error->psr_datalen);
-		if (d == NULL)
-			PSR_ERROR(errno);
-		psr_ctx->psr_mdata = d;
-		psr_ctx->psr_mdatalen = psr_error->psr_datalen;
+
+		if (d != NULL) {
+			psr_ctx->psr_mdata = d;
+			psr_ctx->psr_mdatalen = psr_error->psr_datalen;
+		}
 	}
 	if (psr_error->psr_datalen != 0) {
-		if (psr_ctx->psr_usemdata)
+		if (psr_ctx->psr_usemdata) {
 			iov[1].iov_base = psr_ctx->psr_mdata;
-		else {
-			if (psr_error->psr_datalen > psr_ctx->psr_datalen)
-				PSR_ERROR(ENOBUFS);
+			iov[1].iov_len = psr_ctx->psr_mdatalen;
+		} else {
 			iov[1].iov_base = psr_ctx->psr_data;
+			iov[1].iov_len = psr_ctx->psr_datalen;
 		}
-		iov[1].iov_len = psr_error->psr_datalen;
 	}
 
-	len = readv(fd, iov, __arraycount(iov));
+recv:
+	len = recvmsg(fd, &msg, MSG_WAITALL);
 	if (len == -1)
 		PSR_ERROR(errno);
+	else if ((size_t)len < sizeof(*psr_error))
+		PSR_ERROR(EINVAL);
+	else if (msg.msg_flags & MSG_TRUNC)
+		PSR_ERROR(ENOBUFS);
 	else if ((size_t)len != sizeof(*psr_error) + psr_error->psr_datalen)
 		PSR_ERROR(EINVAL);
 	return len;