[Spice-devel] [PATCH spice-server v2] glz-encoder: Avoid double byte swap sending image magic

[Frediano Ziglio]

encode_32 already deals with endian, don't swap twice.
Tested with a ppc64 server machine and a x64 client.

This looks the reverse of a previous patch (59c6c82) supposed to fix big
endian machine. encode_32 has been always:

static inline void encode_32(Encoder *encoder, unsigned int word)
{
    encode(encoder, (uint8_t)(word >> 24));
    encode(encoder, (uint8_t)(word >> 16) & 0x0000ff);
    encode(encoder, (uint8_t)(word >> 8) & 0x0000ff);
    encode(encoder, (uint8_t)(word & 0x0000ff));
}

while encode basically is similar to a putc on a FILE stream so is writing
numbers from host endian to big endian order.
The "main" endian (the one more tested since ever) is host/guest being
little endian. So if you call encode_32 with a 0x01020304 you get 4 bytes
in the order 4, 3, 2, 1.
Before and after 59c6c82 LZ_MAGIC was defined as:
  #define LZ_MAGIC (*(uint32_t *)"LZ  ")
so on little endian this was 0x4c, 0x5a, 0x20, 0x20 that is 0x20205a4c
which written through encode_32 become 0x20, 0x20, 0x5a, 0x4c so we can say
that at the end on the network we must have 0x20, 0x20, 0x5a, 0x4c.
On big endian however LZ_MAGIC got the value 0x4c5a2020 which written
through encode_32 get 0x4c, 0x5a, 0x20, 0x20 which is the opposite
expected. So patch 59c6c82 reverted the order having again 0x20, 0x20,
0x5a, 0x4c on the network.
However commit 5a7e587 (spice-common), in an attempt to avoid double
swapping on LZ, changed LZ_MAGIC to
  #define LZ_MAGIC 0x20205a4c
breaking endianness again for GLZ code.

Signed-off-by: Frediano Ziglio <fziglio at redhat.com>
---
 server/glz-encoder.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Changes since v1:
- extend commit message

diff --git a/server/glz-encoder.c b/server/glz-encoder.c
index dba2cd12..3ad2b97c 100644
--- a/server/glz-encoder.c
+++ b/server/glz-encoder.c
@@ -262,7 +262,7 @@ int glz_encode(GlzEncoderContext *opaque_encoder,
     encoder->cur_image.id = dict_image->id;
     encoder->cur_image.first_win_seg = dict_image->first_seg;
 
-    encode_32(encoder, GUINT32_TO_LE(LZ_MAGIC));
+    encode_32(encoder, LZ_MAGIC);
     encode_32(encoder, LZ_VERSION);
     if (top_down) {
         encode(encoder, (type & LZ_IMAGE_TYPE_MASK) | (1 << LZ_IMAGE_TYPE_LOG));
-- 
2.17.1