[Qemu-devel] [PATCH] block: fix backing file segfault

[Peter Feiner <peter@gridcentric.ca>]

When a backing file is opened such that (1) a protocol is directly
used as the block driver and (2) the block driver has bdrv_file_open,
bdrv_open_backing_file segfaults. The problem arises because
bdrv_open_common returns without setting bd->backing_hd->file.

To effect (1), you seem to have to use the -F flag in qemu-img. There
are several block drivers that satisfy (2), such as "file" and "nbd".
Here are some concrete examples:

    #!/bin/bash

    echo Test file format
    ./qemu-img create -f file base.file 1m
    ./qemu-img create -f qcow2 -F file -o backing_file=base.file\
        file-overlay.qcow2
    ./qemu-img convert -O raw file-overlay.qcow2 file-convert.raw

    echo Test nbd format
    SOCK=$PWD/nbd.sock
    ./qemu-img create -f raw base.raw 1m
    ./qemu-nbd -t -k $SOCK base.raw &
    trap "kill $!" EXIT
    while ! test -e $SOCK; do sleep 1; done
    ./qemu-img create -f qcow2 -F nbd -o backing_file=nbd:unix:$SOCK\
        nbd-overlay.qcow2
    ./qemu-img convert -O raw nbd-overlay.qcow2 nbd-convert.raw

Without this patch, the two qemu-img convert commands segfault.

This is a regression that was introduced in v1.7 by
dbecebddfa4932d1c83915bcb9b5ba5984eb91be.

Signed-off-by: Peter Feiner <peter@gridcentric.ca>
---
 block.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/block.c b/block.c
index 64e7d22..a4a172d 100644
--- a/block.c
+++ b/block.c
@@ -1016,8 +1016,9 @@ int bdrv_open_backing_file(BlockDriverState *bs, QDict *options, Error **errp)
         error_free(local_err);
         return ret;
     }
-    pstrcpy(bs->backing_file, sizeof(bs->backing_file),
-            bs->backing_hd->file->filename);
+    if (bs->backing_hd->file)
+        pstrcpy(bs->backing_file, sizeof(bs->backing_file),
+                bs->backing_hd->file->filename);
     return 0;
 }
 
-- 
1.7.9.5