[Qemu-block] [PATCH 2/3] block: Fix target variable of BLKSECTGET ioctl

[Eric Farman]

Commit 6f607174 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.)

Signed-off-by: Eric Farman <address@hidden>
---
 block/file-posix.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 28b47d9..2115155 100644
--- a/block/file-posix.c
+++ b/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) {
+    short max_sectors_short = 0;
+    if (bs->sg && ioctl(fd, BLKSECTGET, &max_sectors) == 0) {
         return max_sectors;
+    } else if (!bs->sg && ioctl(fd, BLKSECTGET, &max_sectors_short) == 0) {
+        return max_sectors_short;
     } else {
         return -errno;
     }
@@ -672,7 +675,7 @@ 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);
+            int ret = hdev_get_max_transfer_length(bs, s->fd);
             if (ret > 0 && ret <= BDRV_REQUEST_MAX_SECTORS) {
                 bs->bl.max_transfer = pow2floor(ret << BDRV_SECTOR_BITS);
             }
-- 
2.8.4