diff -rup qemu-0.9.1.orig/block.c qemu-0.9.1.new/block.c
--- qemu-0.9.1.orig/block.c	2008-02-26 18:03:00.000000000 -0500
+++ qemu-0.9.1.new/block.c	2008-02-26 18:05:26.000000000 -0500
@@ -123,6 +123,60 @@ void path_combine(char *dest, int dest_s
     }
 }
 
+static int bdrv_rd_badreq_sectors(BlockDriverState *bs,
+                                  int64_t sector_num, int nb_sectors)
+{
+    return
+        nb_sectors < 0 ||
+        sector_num < 0 ||
+        nb_sectors > bs->total_sectors ||
+        sector_num > bs->total_sectors - nb_sectors;
+}
+
+static int bdrv_rd_badreq_bytes(BlockDriverState *bs,
+                                int64_t offset, int count)
+{
+    int64_t size = bs->total_sectors << SECTOR_BITS;
+    return
+        count < 0 ||
+        size < 0 ||
+        count > size ||
+        offset > size - count;
+}
+
+static int bdrv_wr_badreq_sectors(BlockDriverState *bs,
+                                  int64_t sector_num, int nb_sectors)
+{
+    if (sector_num < 0 ||
+        nb_sectors < 0)
+        return 1;
+
+    if (sector_num > bs->total_sectors - nb_sectors) {
+        if (bs->autogrow)
+            bs->total_sectors = sector_num + nb_sectors;
+        else
+            return 1;
+    }
+    return 0;
+}
+
+static int bdrv_wr_badreq_bytes(BlockDriverState *bs,
+                                int64_t offset, int count)
+{
+    int64_t size = bs->total_sectors << SECTOR_BITS;
+    if (count < 0 ||
+        offset < 0)
+        return 1;
+
+    if (offset > size - count) {
+        if (bs->autogrow)
+            bs->total_sectors = (offset + count + SECTOR_SIZE - 1) >> SECTOR_BITS;
+        else
+            return 1;
+    }
+    return 0;
+}
+
 
 static void bdrv_register(BlockDriver *bdrv)
 {
@@ -331,6 +385,10 @@ int bdrv_open2(BlockDriverState *bs, con
     bs->read_only = 0;
     bs->is_temporary = 0;
     bs->encrypted = 0;
+    bs->autogrow = 0;
+
+    if (flags & BDRV_O_AUTOGROW)
+        bs->autogrow = 1;
 
     if (flags & BDRV_O_SNAPSHOT) {
         BlockDriverState *bs1;
@@ -375,6 +433,7 @@ int bdrv_open2(BlockDriverState *bs, con
     }
     bs->drv = drv;
     bs->opaque = qemu_mallocz(drv->instance_size);
+    bs->total_sectors = 0; /* driver will set if it does not do getlength */
     if (bs->opaque == NULL && drv->instance_size > 0)
         return -1;
     /* Note: for compatibility, we open disk image files as RDWR, and
@@ -440,6 +499,7 @@ void bdrv_close(BlockDriverState *bs)
         bs->drv = NULL;
 
         /* call the change callback */
+        bs->total_sectors = 0;
         bs->media_changed = 1;
         if (bs->change_cb)
             bs->change_cb(bs->change_opaque);
@@ -505,6 +565,8 @@ int bdrv_read(BlockDriverState *bs, int6
     if (!drv)
         return -ENOMEDIUM;
 
+    if (bdrv_rd_badreq_sectors(bs, sector_num, nb_sectors))
+        return -EDOM;
     if (sector_num == 0 && bs->boot_sector_enabled && nb_sectors > 0) {
             memcpy(buf, bs->boot_sector_data, 512);
         sector_num++;
@@ -545,6 +607,8 @@ int bdrv_write(BlockDriverState *bs, int
         return -ENOMEDIUM;
     if (bs->read_only)
         return -EACCES;
+    if (bdrv_wr_badreq_sectors(bs, sector_num, nb_sectors))
+        return -EDOM;
     if (sector_num == 0 && bs->boot_sector_enabled && nb_sectors > 0) {
         memcpy(bs->boot_sector_data, buf, 512);
     }
@@ -670,6 +734,8 @@ int bdrv_pread(BlockDriverState *bs, int
         return -ENOMEDIUM;
     if (!drv->bdrv_pread)
         return bdrv_pread_em(bs, offset, buf1, count1);
+    if (bdrv_rd_badreq_bytes(bs, offset, count1))
+        return -EDOM;
     return drv->bdrv_pread(bs, offset, buf1, count1);
 }
 
@@ -685,6 +751,8 @@ int bdrv_pwrite(BlockDriverState *bs, in
         return -ENOMEDIUM;
     if (!drv->bdrv_pwrite)
         return bdrv_pwrite_em(bs, offset, buf1, count1);
+    if (bdrv_wr_badreq_bytes(bs, offset, count1))
+        return -EDOM;
     return drv->bdrv_pwrite(bs, offset, buf1, count1);
 }
 
@@ -951,6 +1019,8 @@ int bdrv_write_compressed(BlockDriverSta
         return -ENOMEDIUM;
     if (!drv->bdrv_write_compressed)
         return -ENOTSUP;
+    if (bdrv_wr_badreq_sectors(bs, sector_num, nb_sectors))
+        return -EDOM;
     return drv->bdrv_write_compressed(bs, sector_num, buf, nb_sectors);
 }
 
@@ -1097,6 +1167,8 @@ BlockDriverAIOCB *bdrv_aio_read(BlockDri
 
     if (!drv)
         return NULL;
+    if (bdrv_rd_badreq_sectors(bs, sector_num, nb_sectors))
+        return NULL;
 
     /* XXX: we assume that nb_sectors == 0 is suppored by the async read */
     if (sector_num == 0 && bs->boot_sector_enabled && nb_sectors > 0) {
@@ -1128,6 +1200,8 @@ BlockDriverAIOCB *bdrv_aio_write(BlockDr
         return NULL;
     if (bs->read_only)
         return NULL;
+    if (bdrv_wr_badreq_sectors(bs, sector_num, nb_sectors))
+        return NULL;
     if (sector_num == 0 && bs->boot_sector_enabled && nb_sectors > 0) {
         memcpy(bs->boot_sector_data, buf, 512);
     }
diff -rup qemu-0.9.1.orig/block.h qemu-0.9.1.new/block.h
--- qemu-0.9.1.orig/block.h	2008-01-06 14:38:42.000000000 -0500
+++ qemu-0.9.1.new/block.h	2008-02-26 17:41:47.000000000 -0500
@@ -45,6 +45,7 @@ typedef struct QEMUSnapshotInfo {
                                      it (default for
                                      bdrv_file_open()) */
 #define BDRV_O_DIRECT      0x0020
+#define BDRV_O_AUTOGROW    0x0040 /* Allow backing file to extend when writing past end of file */
 
 #ifndef QEMU_IMG
 void bdrv_info(void);
diff -rup qemu-0.9.1.orig/block_int.h qemu-0.9.1.new/block_int.h
--- qemu-0.9.1.orig/block_int.h	2008-01-06 14:38:42.000000000 -0500
+++ qemu-0.9.1.new/block_int.h	2008-02-26 17:41:47.000000000 -0500
@@ -97,6 +97,7 @@ struct BlockDriverState {
     int locked;    /* if true, the media cannot temporarily be ejected */
     int encrypted; /* if true, the media is encrypted */
     int sg;        /* if true, the device is a /dev/sg* */
+    int autogrow;  /* if true, the backing store can auto-extend to allocate new extents */
     /* event callback when inserting/removing */
     void (*change_cb)(void *opaque);
     void *change_opaque;
diff -rup qemu-0.9.1.orig/block-qcow2.c qemu-0.9.1.new/block-qcow2.c
--- qemu-0.9.1.orig/block-qcow2.c	2008-01-06 14:38:42.000000000 -0500
+++ qemu-0.9.1.new/block-qcow2.c	2008-02-26 18:01:06.000000000 -0500
@@ -191,7 +191,7 @@ static int qcow_open(BlockDriverState *b
     int len, i, shift, ret;
     QCowHeader header;
 
-    ret = bdrv_file_open(&s->hd, filename, flags);
+    ret = bdrv_file_open(&s->hd, filename, flags | BDRV_O_AUTOGROW);
     if (ret < 0)
         return ret;
     if (bdrv_pread(s->hd, 0, &header, sizeof(header)) != sizeof(header))
diff -rup qemu-0.9.1.orig/block-qcow.c qemu-0.9.1.new/block-qcow.c
--- qemu-0.9.1.orig/block-qcow.c	2008-01-06 14:38:41.000000000 -0500
+++ qemu-0.9.1.new/block-qcow.c	2008-02-26 18:00:53.000000000 -0500
@@ -95,7 +95,7 @@ static int qcow_open(BlockDriverState *b
     int len, i, shift, ret;
     QCowHeader header;
 
-    ret = bdrv_file_open(&s->hd, filename, flags);
+    ret = bdrv_file_open(&s->hd, filename, flags | BDRV_O_AUTOGROW);
     if (ret < 0)
         return ret;
     if (bdrv_pread(s->hd, 0, &header, sizeof(header)) != sizeof(header))
diff -rup qemu-0.9.1.orig/block-vmdk.c qemu-0.9.1.new/block-vmdk.c
--- qemu-0.9.1.orig/block-vmdk.c	2008-01-06 14:38:42.000000000 -0500
+++ qemu-0.9.1.new/block-vmdk.c	2008-02-26 18:02:17.000000000 -0500
@@ -376,7 +376,7 @@ static int vmdk_open(BlockDriverState *b
         flags = BDRV_O_RDONLY;
     fprintf(stderr, "(VMDK) image open: flags=0x%x filename=%s\n", flags, bs->filename);
 
-    ret = bdrv_file_open(&s->hd, filename, flags);
+    ret = bdrv_file_open(&s->hd, filename, flags | BDRV_O_AUTOGROW);
     if (ret < 0)
         return ret;
     if (bdrv_pread(s->hd, 0, &magic, sizeof(magic)) != sizeof(magic))