[ CCing Max, who was recently active in this area, for another opinion ]
Am 08.01.2014 um 20:43 hat Peter Feiner geschrieben:
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 <address@hidden>
---
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;
}
I think if there is no bs->backing_hd->file, we should get the filename
from bs->backing_hd->filename instead of leaving it empty.
In fact, can we always do that or does bs->backing_hd normally lack the
filename? If so, perhaps that is what we need to fix, so we can always
directly use bs->backing_hd->filename here.