util/iov: make qemu_iovec_init_extended() honest

Actually, we can't extend the io vector in all cases. Handle possible
MAX_IOV and size_t overflows.

For now add assertion to callers (actually they rely on success anyway)
and fix them in the following patch.

Add also some additional good assertions to qemu_iovec_init_slice()
while being here.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20201211183934.169161-3-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

block/io.c

@@ -1680,13 +1680,17 @@ static bool bdrv_pad_request(BlockDriverState *bs,
                              int64_t *offset, unsigned int *bytes,
                              BdrvRequestPadding *pad)
 {
+    int ret;
+
     if (!bdrv_init_padding(bs, *offset, *bytes, pad)) {
         return false;
     }
 
-    qemu_iovec_init_extended(&pad->local_qiov, pad->buf, pad->head,
-                             *qiov, *qiov_offset, *bytes,
-                             pad->buf + pad->buf_len - pad->tail, pad->tail);
+    ret = qemu_iovec_init_extended(&pad->local_qiov, pad->buf, pad->head,
+                                   *qiov, *qiov_offset, *bytes,
+                                   pad->buf + pad->buf_len - pad->tail,
+                                   pad->tail);
+    assert(ret == 0);
     *bytes += pad->head + pad->tail;
     *offset -= pad->head;
     *qiov = &pad->local_qiov;

include/qemu/iov.h

@@ -222,7 +222,7 @@ static inline void *qemu_iovec_buf(QEMUIOVector *qiov)
 
 void qemu_iovec_init(QEMUIOVector *qiov, int alloc_hint);
 void qemu_iovec_init_external(QEMUIOVector *qiov, struct iovec *iov, int niov);
-void qemu_iovec_init_extended(
+int qemu_iovec_init_extended(
         QEMUIOVector *qiov,
         void *head_buf, size_t head_len,
         QEMUIOVector *mid_qiov, size_t mid_offset, size_t mid_len,

util/iov.c

@@ -415,7 +415,7 @@ int qemu_iovec_subvec_niov(QEMUIOVector *qiov, size_t offset, size_t len)
  * Compile new iovec, combining @head_buf buffer, sub-qiov of @mid_qiov,
  * and @tail_buf buffer into new qiov.
  */
-void qemu_iovec_init_extended(
+int qemu_iovec_init_extended(
         QEMUIOVector *qiov,
         void *head_buf, size_t head_len,
         QEMUIOVector *mid_qiov, size_t mid_offset, size_t mid_len,
@@ -425,12 +425,24 @@ void qemu_iovec_init_extended(
     int total_niov, mid_niov = 0;
     struct iovec *p, *mid_iov = NULL;
 
+    assert(mid_qiov->niov <= IOV_MAX);
+
+    if (SIZE_MAX - head_len < mid_len ||
+        SIZE_MAX - head_len - mid_len < tail_len)
+    {
+        return -EINVAL;
+    }
+
     if (mid_len) {
         mid_iov = qiov_slice(mid_qiov, mid_offset, mid_len,
                              &mid_head, &mid_tail, &mid_niov);
     }
 
     total_niov = !!head_len + mid_niov + !!tail_len;
+    if (total_niov > IOV_MAX) {
+        return -EINVAL;
+    }
+
     if (total_niov == 1) {
         qemu_iovec_init_buf(qiov, NULL, 0);
         p = &qiov->local_iov;
@@ -459,6 +471,8 @@ void qemu_iovec_init_extended(
         p->iov_base = tail_buf;
         p->iov_len = tail_len;
     }
+
+    return 0;
 }
 
 /*
@@ -492,7 +506,14 @@ bool qemu_iovec_is_zero(QEMUIOVector *qiov, size_t offset, size_t bytes)
 void qemu_iovec_init_slice(QEMUIOVector *qiov, QEMUIOVector *source,
                            size_t offset, size_t len)
 {
-    qemu_iovec_init_extended(qiov, NULL, 0, source, offset, len, NULL, 0);
+    int ret;
+
+    assert(source->size >= len);
+    assert(source->size - len >= offset);
+
+    /* We shrink the request, so we can't overflow neither size_t nor MAX_IOV */
+    ret = qemu_iovec_init_extended(qiov, NULL, 0, source, offset, len, NULL, 0);
+    assert(ret == 0);
 }
 
 void qemu_iovec_destroy(QEMUIOVector *qiov)