Re: [Qemu-devel] [PATCH v2 01/12] block/dmg: properly detect the UDIF trailer

[Stefan Hajnoczi]

On Tue, Jan 06, 2015 at 06:48:04PM +0100, Peter Wu wrote:
> DMG files have a variable length with a UDIF trailer at the end of a
> file. This UDIF trailer is essential as it describes the contents of
> the image. At the moment however, the start of this trailer is almost
> always incorrect as bdrv_getlength() returns a multiple of the block
> size (rounded up). This results in a failure to recognize DMG files,
> resulting in Invalid argument (EINVAL) errors.
> 
> As there is no API to retrieve the real file size, look for the magic
> header in the last two sectors to find the start of this 512-byte UDIF
> trailer (the "koly" block).
> 
> The resource fork offset ("info_begin") has its offset adjusted as the
> initial value of offset does not mean "end of file" anymore, but "begin
> of UDIF trailer".
> 
> Signed-off-by: Peter Wu <address@hidden>
> Reviewed-by: John Snow <address@hidden>
> ---
>  v2: added R-b, set errp as suggested by Stefan Hajnoczi
> ---
>  block/dmg.c | 49 +++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 45 insertions(+), 4 deletions(-)
> 
> diff --git a/block/dmg.c b/block/dmg.c
> index e455886..9183459 100644
> --- a/block/dmg.c
> +++ b/block/dmg.c
> @@ -131,6 +131,48 @@ static void update_max_chunk_size(BDRVDMGState *s, 
> uint32_t chunk,
>      }
>  }
>  
> +static int64_t dmg_find_koly_offset(BlockDriverState *file_bs, Error **errp)
> +{
> +    int64_t length;
> +    int64_t offset = 0;
> +    uint8_t buffer[515];
> +    int i, ret;
> +
> +    /* bdrv_getlength returns a multiple of block size (512), rounded up. 
> Since
> +     * dmg images can have odd sizes, try to look for the "koly" magic which
> +     * marks the begin of the UDIF trailer (512 bytes). This magic can be 
> found
> +     * in the last 511 bytes of the second-last sector or the first 4 bytes 
> of
> +     * the last sector (search space: 515 bytes) */
> +    length = bdrv_getlength(file_bs);
> +    if (length < 0) {
> +        error_setg_errno(errp, -length,
> +            "Failed to get file size while reading UDIF trailer");
> +        return length;
> +    } else if (length < 512) {
> +        error_set(errp, ERROR_CLASS_GENERIC_ERROR,
> +            "dmg file must be at least 512 bytes long");
> +        return -EINVAL;

Not worth respinning but error_setg() is a shortcut for error_set(errp,
ERROR_CLASS_GENERIC_ERROR, fmt, ...).

Reviewed-by: Stefan Hajnoczi <address@hidden>

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