[ 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 <pe...@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; > }
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. Kevin
