[Spice-devel] [PATCH spice-common v2] canvas_base: Avoid misaligned access decoding LZ4 data

[Christophe Fergeau]

Acked-by: Christophe Fergeau <cfergeau at redhat.com>

On Wed, Jul 04, 2018 at 04:27:11PM +0100, Frediano Ziglio wrote:
> Make code faster on platforms not supporting unaligned access by
> default.
> SPICE_UNALIGNED_CAST is just silencing possible alignment warning and,
> if enabled, produces some logs, however in this case we know that the
> pointer can be misaligned.
> Use packed structures to tell compiler to generate best code possible.
> For more details see comment on commit 74e50b57ae05116be4e2 ("Make the
> compiler work out better way to write unaligned memory").
> 
> Signed-off-by: Frediano Ziglio <fziglio at redhat.com>
> ---
>  common/canvas_base.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> Changes since v1:
> - extends commit message.
> 
> diff --git a/common/canvas_base.c b/common/canvas_base.c
> index 6bf6e5d..36546b3 100644
> --- a/common/canvas_base.c
> +++ b/common/canvas_base.c
> @@ -45,6 +45,14 @@
>  #include "mem.h"
>  #include "macros.h"
>  
> +#include <spice/start-packed.h>
> +typedef struct SPICE_ATTR_PACKED {
> +    uint32_t v;
> +} uint32_unaligned_t;
> +#include <spice/end-packed.h>
> +
> +#define read_uint32_be(ptr) ntohl(((uint32_unaligned_t *)(ptr))->v)
> +
>  #define ROUND(_x) ((int)floor((_x) + 0.5))
>  
>   static inline int fix_to_int(SPICE_FIXED28_4 fixed)
> @@ -572,7 +580,7 @@ static pixman_image_t *canvas_get_lz4(CanvasBase *canvas, SpiceImage *image)
>  
>      do {
>          // Read next compressed block
> -        enc_size = ntohl(*SPICE_UNALIGNED_CAST(uint32_t *, data));
> +        enc_size = read_uint32_be(data);
>          data += 4;
>          dec_size = LZ4_decompress_safe_continue(stream, (const char *) data,
>                                                  (char *) dest, enc_size, available);
> -- 
> 2.17.1
> 
> _______________________________________________
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> Spice-devel at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/spice-devel
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