[Mesa-dev] [PATCH v3 1/3] virgl: Fix a strict-aliasing violation in the encoder

[Edward O'Callaghan]

As per the C spec, it is illegal to alias pointers to different
types. This results in undefined behaviour after optimization
passes, resulting in very subtle bugs that happen only on a
full moon..

Use a memcpy() as a well defined coercion between the double
to uint64_t interpretations of the memory.

V.2: Use static_assert() instead of assert().
V.3: Use C99 compat STATIC_ASSERT() over C11 static_assert().

Signed-off-by: Edward O'Callaghan <funfunctor at folklore1984.net>
---
 src/gallium/drivers/virgl/virgl_encode.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/gallium/drivers/virgl/virgl_encode.c b/src/gallium/drivers/virgl/virgl_encode.c
index be72f70..85843b7 100644
--- a/src/gallium/drivers/virgl/virgl_encode.c
+++ b/src/gallium/drivers/virgl/virgl_encode.c
@@ -21,6 +21,8 @@
  * USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
 #include <stdint.h>
+#include <assert.h>
+#include <string.h>
 
 #include "util/u_format.h"
 #include "util/u_memory.h"
@@ -315,12 +317,16 @@ int virgl_encode_clear(struct virgl_context *ctx,
                       double depth, unsigned stencil)
 {
    int i;
+   uint64_t qword;
+
+   STATIC_ASSERT(sizeof(qword) == sizeof(depth));
+   memcpy(&qword, &depth, sizeof(qword));
 
    virgl_encoder_write_cmd_dword(ctx, VIRGL_CMD0(VIRGL_CCMD_CLEAR, 0, VIRGL_OBJ_CLEAR_SIZE));
    virgl_encoder_write_dword(ctx->cbuf, buffers);
    for (i = 0; i < 4; i++)
       virgl_encoder_write_dword(ctx->cbuf, color->ui[i]);
-   virgl_encoder_write_qword(ctx->cbuf, *(uint64_t *)&depth);
+   virgl_encoder_write_qword(ctx->cbuf, qword);
    virgl_encoder_write_dword(ctx->cbuf, stencil);
    return 0;
 }
-- 
2.9.3