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

[Christophe Fergeau]

On Tue, Jul 03, 2018 at 11:28:41AM -0400, Frediano Ziglio wrote:
> > 
> > On Thu, Jun 28, 2018 at 07:40:19AM +0100, Frediano Ziglio wrote:
> > > Make code faster on platforms not supporting unaligned access
> > > by default.
> > 
> > How does this work exactly? Rather than silencing the warning with
> > SPICE_UNALIGNED_CAST, and then potentially dereferencing a non-aligned
> > 32 bit value, and getting a trap from the kernel/cpu, we instead give a
> > lot of hints to the compiler to let it know the value is not going to be
> > aligned, and we expect it's going to emit the right code?
> > 
> > Christophe
> > 
> 
> Same "trick" used in the marshaller. Yes, compiler will generate the
> right code. The SPICE_UNALIGNED_CAST marks that the code is doing
> a misaligned access (or helps discovering it), does not solve the problem.
> This solves the problem :-)

Imo this belongs in the commit log.

Christophe

> 
> OT: yes, strangely this is encoded as big endian!
> 
> Frediano
> 
> > > 
> > > Signed-off-by: Frediano Ziglio <fziglio at redhat.com>
> > > ---
> > >  common/canvas_base.c | 10 +++++++++-
> > >  1 file changed, 9 insertions(+), 1 deletion(-)
> > > 
> > > 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);
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