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

[Max Reitz <mreitz@redhat.com>]

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 <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;
>>   }
> 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.


Max