[libvirt] [PATCH 12/30] storagefile: Fix backing format \0 check

[Michal Privoznik]

On 10/7/19 11:49 PM, Cole Robinson wrote:
>>From qemu.git docs/interop/qcow2.txt
> 
>    == String header extensions ==
> 
>    Some header extensions (such as the backing file format name and
>    the external data file name) are just a single string. In this case,
>    the header extension length is the string length and the string is
>    not '\0' terminated. (The header extension padding can make it look
>    like a string is '\0' terminated, but neither is padding always
>    necessary nor is there a guarantee that zero bytes are used
>    for padding.)
> 
> So we shouldn't be checking for a \0 byte at the end of the backing
> format section. I think in practice there always is a \0 but we
> shouldn't depend on that.
> 
> Signed-off-by: Cole Robinson <crobinso at redhat.com>
> ---
>   src/util/virstoragefile.c | 13 +++++++++----
>   1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/src/util/virstoragefile.c b/src/util/virstoragefile.c
> index 8cd576a463..5a4e4b24ae 100644
> --- a/src/util/virstoragefile.c
> +++ b/src/util/virstoragefile.c
> @@ -506,13 +506,18 @@ qcow2GetExtensions(const char *buf,
>           case QCOW2_HDR_EXTENSION_END:
>               goto done;
>   
> -        case QCOW2_HDR_EXTENSION_BACKING_FORMAT:
> -            if (buf[offset+len] != '\0')
> -                break;
> -            *backingFormat = virStorageFileFormatTypeFromString(buf+offset);
> +        case QCOW2_HDR_EXTENSION_BACKING_FORMAT: {
> +            VIR_AUTOFREE(char *) tmp = NULL;
> +            if (VIR_ALLOC_N(tmp, len + 1) < 0)
> +                return -1;
> +            memcpy(tmp, buf + offset, len);
> +            tmp[len] = '\0';
> +
> +            *backingFormat = virStorageFileFormatTypeFromString(tmp);
>               if (*backingFormat <= VIR_STORAGE_FILE_NONE)
>                   return -1;
>           }
> +        }
>   

Pre-existing, but there's missing 'break;' here. Since you're touching 
these lines, might be worth putting it here.

>           offset += len;
>       }
> 

Michal