lguest: fix block request handling in example launcher.

virtio requests are scatter-gather-style descriptors, but no
assumptions should be made about the layout.  lguest was lazy here,
but saved by the fact that the network device hands all requests to
tun (which does it correctly) and console and random devices simply
use readv and writev.

Block devices, however, are broken: we convert to iovecs internally,
just make sure we handle the correctly.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tools/lguest/lguest.c

@@ -179,29 +179,6 @@ static struct termios orig_term;
 #define wmb() __asm__ __volatile__("" : : : "memory")
 #define mb() __asm__ __volatile__("" : : : "memory")
 
-/*
- * Convert an iovec element to the given type.
- *
- * This is a fairly ugly trick: we need to know the size of the type and
- * alignment requirement to check the pointer is kosher.  It's also nice to
- * have the name of the type in case we report failure.
- *
- * Typing those three things all the time is cumbersome and error prone, so we
- * have a macro which sets them all up and passes to the real function.
- */
-#define convert(iov, type) \
-	((type *)_convert((iov), sizeof(type), __alignof__(type), #type))
-
-static void *_convert(struct iovec *iov, size_t size, size_t align,
-		      const char *name)
-{
-	if (iov->iov_len != size)
-		errx(1, "Bad iovec size %zu for %s", iov->iov_len, name);
-	if ((unsigned long)iov->iov_base % align != 0)
-		errx(1, "Bad alignment %p for %s", iov->iov_base, name);
-	return iov->iov_base;
-}
-
 /* Wrapper for the last available index.  Makes it easier to change. */
 #define lg_last_avail(vq)	((vq)->last_avail_idx)
 
@@ -228,19 +205,25 @@ static bool iov_empty(const struct iovec iov[], unsigned int num_iov)
 }
 
 /* Take len bytes from the front of this iovec. */
-static void iov_consume(struct iovec iov[], unsigned num_iov, unsigned len)
+static void iov_consume(struct iovec iov[], unsigned num_iov,
+			void *dest, unsigned len)
 {
 	unsigned int i;
 
 	for (i = 0; i < num_iov; i++) {
 		unsigned int used;
 
 		used = iov[i].iov_len < len ? iov[i].iov_len : len;
+		if (dest) {
+			memcpy(dest, iov[i].iov_base, used);
+			dest += used;
+		}
 		iov[i].iov_base += used;
 		iov[i].iov_len -= used;
 		len -= used;
 	}
-	assert(len == 0);
+	if (len != 0)
+		errx(1, "iovec too short!");
 }
 
 /* The device virtqueue descriptors are followed by feature bitmasks. */
@@ -864,7 +847,7 @@ static void console_output(struct virtqueue *vq)
 			warn("Write to stdout gave %i (%d)", len, errno);
 			break;
 		}
-		iov_consume(iov, out, len);
+		iov_consume(iov, out, NULL, len);
 	}
 
 	/*
@@ -1591,9 +1574,9 @@ static void blk_request(struct virtqueue *vq)
 {
 	struct vblk_info *vblk = vq->dev->priv;
 	unsigned int head, out_num, in_num, wlen;
-	int ret;
+	int ret, i;
 	u8 *in;
-	struct virtio_blk_outhdr *out;
+	struct virtio_blk_outhdr out;
 	struct iovec iov[vq->vring.num];
 	off64_t off;
 
@@ -1603,43 +1586,47 @@ static void blk_request(struct virtqueue *vq)
 	 */
 	head = wait_for_vq_desc(vq, iov, &out_num, &in_num);
 
-	/*
-	 * Every block request should contain at least one output buffer
-	 * (detailing the location on disk and the type of request) and one
-	 * input buffer (to hold the result).
-	 */
-	if (out_num == 0 || in_num == 0)
-		errx(1, "Bad virtblk cmd %u out=%u in=%u",
-		     head, out_num, in_num);
+	/* Copy the output header from the front of the iov (adjusts iov) */
+	iov_consume(iov, out_num, &out, sizeof(out));
+
+	/* Find and trim end of iov input array, for our status byte. */
+	in = NULL;
+	for (i = out_num + in_num - 1; i >= out_num; i--) {
+		if (iov[i].iov_len > 0) {
+			in = iov[i].iov_base + iov[i].iov_len - 1;
+			iov[i].iov_len--;
+			break;
+		}
+	}
+	if (!in)
+		errx(1, "Bad virtblk cmd with no room for status");
 
-	out = convert(&iov[0], struct virtio_blk_outhdr);
-	in = convert(&iov[out_num+in_num-1], u8);
 	/*
 	 * For historical reasons, block operations are expressed in 512 byte
 	 * "sectors".
 	 */
-	off = out->sector * 512;
+	off = out.sector * 512;
 
 	/*
 	 * In general the virtio block driver is allowed to try SCSI commands.
 	 * It'd be nice if we supported eject, for example, but we don't.
 	 */
-	if (out->type & VIRTIO_BLK_T_SCSI_CMD) {
+	if (out.type & VIRTIO_BLK_T_SCSI_CMD) {
 		fprintf(stderr, "Scsi commands unsupported\n");
 		*in = VIRTIO_BLK_S_UNSUPP;
 		wlen = sizeof(*in);
-	} else if (out->type & VIRTIO_BLK_T_OUT) {
+	} else if (out.type & VIRTIO_BLK_T_OUT) {
 		/*
 		 * Write
 		 *
 		 * Move to the right location in the block file.  This can fail
 		 * if they try to write past end.
 		 */
 		if (lseek64(vblk->fd, off, SEEK_SET) != off)
-			err(1, "Bad seek to sector %llu", out->sector);
+			err(1, "Bad seek to sector %llu", out.sector);
 
-		ret = writev(vblk->fd, iov+1, out_num-1);
-		verbose("WRITE to sector %llu: %i\n", out->sector, ret);
+		ret = writev(vblk->fd, iov, out_num);
+		verbose("WRITE to sector %llu: %i\n", out.sector, ret);
 
 		/*
 		 * Grr... Now we know how long the descriptor they sent was, we
@@ -1655,7 +1642,7 @@ static void blk_request(struct virtqueue *vq)
 
 		wlen = sizeof(*in);
 		*in = (ret >= 0 ? VIRTIO_BLK_S_OK : VIRTIO_BLK_S_IOERR);
-	} else if (out->type & VIRTIO_BLK_T_FLUSH) {
+	} else if (out.type & VIRTIO_BLK_T_FLUSH) {
 		/* Flush */
 		ret = fdatasync(vblk->fd);
 		verbose("FLUSH fdatasync: %i\n", ret);
@@ -1669,10 +1656,9 @@ static void blk_request(struct virtqueue *vq)
 		 * if they try to read past end.
 		 */
 		if (lseek64(vblk->fd, off, SEEK_SET) != off)
-			err(1, "Bad seek to sector %llu", out->sector);
+			err(1, "Bad seek to sector %llu", out.sector);
 
-		ret = readv(vblk->fd, iov+1, in_num-1);
-		verbose("READ from sector %llu: %i\n", out->sector, ret);
+		ret = readv(vblk->fd, iov + out_num, in_num);
 		if (ret >= 0) {
 			wlen = sizeof(*in) + ret;
 			*in = VIRTIO_BLK_S_OK;
@@ -1758,7 +1744,7 @@ static void rng_input(struct virtqueue *vq)
 		len = readv(rng_info->rfd, iov, in_num);
 		if (len <= 0)
 			err(1, "Read from /dev/random gave %i", len);
-		iov_consume(iov, in_num, len);
+		iov_consume(iov, in_num, NULL, len);
 		totlen += len;
 	}