On 09.01.2014 11:59, Kevin Wolf wrote:
[ 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.
bs->backing_hd->filename would be set by the bdrv_open_common() in
bdrv_open(), the filename is read from file->filename (if file !=
NULL; in this case, that would be bs->backing_hd->file->filename) or
from the configuration option "filename".
The latter configuration option is not used by
bdrv_open_backing_file(), as far as I can see. However,
bs->backing_hd->file->filename is exactly the field the old code
uses, therefore, using bs->backing_hd->filename directly should not
break anything.
However, the patch does something different: If file is NULL, it
leaves bs->backing_file unchanged; whereas using
bs->backing_hd->filename would in this case result in the value of
the "filename" option. I think leaving bs->backing_file unchanged is
probably better, unless it is "" and the "filename" option is set.
If we want bs->backing_hd->filename to always point to a valid
filename, we'd probably have to copy to contents of bs->backing_file
there at some point in time, if it is not valid. But this is exactly
a point in code where bs->backing_file is updated, so there'd be no
gain if we instead updated bs->backing_hd->filename if necessary and
then copied that to bs->backing_file, as long as there is no other
place in the code where bs->backing_hd->filename always has to be a
valid filename.
Thus, I think the patch is okay, but I'd probably prefer "if
(bs->backing_hd->filename[0]) pstrcpy(...,
bs->backing_hd->filename);" - although that should not differ from
the given patch, unless the "filename" option is set for the
backing_hd.