block: Fix target variable of BLKSECTGET ioctl

Commit 6f607174 ("raw-posix: Fetch max sectors for host block device")
introduced a routine to call the kernel BLKSECTGET ioctl, which stores the
result back to user space.  However, the size of the data returned depends
on the routine handling the ioctl.  The (compat_)blkdev_ioctl returns a
short, while sg_ioctl returns an int.  Thus, on big-endian systems, we can
find ourselves accidentally shifting the result to a much larger value.
(On s390x, a short is 16 bits while an int is 32 bits.)

Also, the two ioctl handlers return values in different scales (block
returns sectors, while sg returns bytes), so some tweaking of the outputs
is required such that hdev_get_max_transfer_length returns a value in a
consistent set of units.
Signed-off-by: NEric Farman <farman@linux.vnet.ibm.com>
Message-Id: <20170120162527.66075-3-farman@linux.vnet.ibm.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

block/file-posix.c

@@ -651,12 +651,15 @@ static void raw_reopen_abort(BDRVReopenState *state)
     state->opaque = NULL;
 }
 
-static int hdev_get_max_transfer_length(int fd)
+static int hdev_get_max_transfer_length(BlockDriverState *bs, int fd)
 {
 #ifdef BLKSECTGET
-    int max_sectors = 0;
-    if (ioctl(fd, BLKSECTGET, &max_sectors) == 0) {
-        return max_sectors;
+    int max_bytes = 0;
+    short max_sectors = 0;
+    if (bs->sg && ioctl(fd, BLKSECTGET, &max_bytes) == 0) {
+        return max_bytes;
+    } else if (!bs->sg && ioctl(fd, BLKSECTGET, &max_sectors) == 0) {
+        return max_sectors << BDRV_SECTOR_BITS;
     } else {
         return -errno;
     }
@@ -672,9 +675,9 @@ static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
 
     if (!fstat(s->fd, &st)) {
         if (S_ISBLK(st.st_mode)) {
-            int ret = hdev_get_max_transfer_length(s->fd);
-            if (ret > 0 && ret <= BDRV_REQUEST_MAX_SECTORS) {
-                bs->bl.max_transfer = pow2floor(ret << BDRV_SECTOR_BITS);
+            int ret = hdev_get_max_transfer_length(bs, s->fd);
+            if (ret > 0 && ret <= BDRV_REQUEST_MAX_BYTES) {
+                bs->bl.max_transfer = pow2floor(ret);
             }
         }
     }

include/block/block.h

@@ -116,6 +116,7 @@ typedef struct HDGeometry {
 
 #define BDRV_REQUEST_MAX_SECTORS MIN(SIZE_MAX >> BDRV_SECTOR_BITS, \
                                      INT_MAX >> BDRV_SECTOR_BITS)
+#define BDRV_REQUEST_MAX_BYTES (BDRV_REQUEST_MAX_SECTORS << BDRV_SECTOR_BITS)
 
 /*
  * Allocation status flags