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

[Fam Zheng]

Hi Peter,

Thank your for catching this.

On 2014年01月09日 03:43, Peter Feiner wrote:
> 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

It would be nice if you can add this as a test case under 
tests/qemu-iotests.

> 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);

Braces are mandatory for if statements (scripts/checkpatch.pl can check 
the coding style for you).

Thanks,
Fam