block: Guarantee that *file is set on bdrv_get_block_status()

We document that *file is valid if the return is not an error and
includes BDRV_BLOCK_OFFSET_VALID, but forgot to obey this contract
when a driver (such as blkdebug) lacks a callback.  Messed up in
commit 67a0fd2 (v2.6), when we added the file parameter.

Enhance qemu-iotest 177 to cover this, using a sequence that would
print garbage or even SEGV, because it was dererefencing through
uninitialized memory.  [The resulting test output shows that we
have less-than-ideal block status from the blkdebug driver, but
that's a separate fix coming up soon.]

Setting *file on all paths that return BDRV_BLOCK_OFFSET_VALID is
enough to fix the crash, but we can go one step further: always
setting *file, even on error, means that a broken caller that
blindly dereferences file without checking for error is now more
likely to get a reliable SEGV instead of randomly acting on garbage,
making it easier to diagnose such buggy callers.  Adding an
assertion that file is set where expected doesn't hurt either.

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/block/io.c b/block/io.c
index 2de7c77983..5c146b5a10 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1734,6 +1734,7 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
     int64_t n;
     int64_t ret, ret2;
 
+    *file = NULL;
     total_sectors = bdrv_nb_sectors(bs);
     if (total_sectors < 0) {
         return total_sectors;
@@ -1757,11 +1758,11 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
         }
         if (bs->drv->protocol_name) {
             ret |= BDRV_BLOCK_OFFSET_VALID | (sector_num * BDRV_SECTOR_SIZE);
+            *file = bs;
         }
         return ret;
     }
 
-    *file = NULL;
     bdrv_inc_in_flight(bs);
     ret = bs->drv->bdrv_co_get_block_status(bs, sector_num, nb_sectors, pnum,
                                             file);
@@ -1771,7 +1772,7 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
     }
 
     if (ret & BDRV_BLOCK_RAW) {
-        assert(ret & BDRV_BLOCK_OFFSET_VALID);
+        assert(ret & BDRV_BLOCK_OFFSET_VALID && *file);
         ret = bdrv_co_get_block_status(*file, ret >> BDRV_SECTOR_BITS,
                                        *pnum, pnum, file);
         goto out;