> On 30 May 2017, at 15:49, Frediano Ziglio <fziglio@xxxxxxxxxx> wrote:
>
>>
>> From: Christophe de Dinechin <dinechin@xxxxxxxxxx>
>>
>> For example, something like this:
>> uint8_t *p8;
>> uint32_t *p32 = (uint32_t *) p8;
>>
>> generates a warning like this:
>> spice-channel.c:1350:10: error: cast from 'uint8_t *' (aka 'unsigned char
>> *') to
>> 'uint32_t *' (aka 'unsigned int *') increases required alignment from 1
>> to
>> 4 [-Werror,-Wcast-align]
>>
>> The warning indicates that we end up with a pointer to data that
>> should be 4-byte aligned, but its value may be misaligned. On x86,
>> this does not make much of a difference, except a relatively minor
>> performance penalty. However, on platforms such as ARM, misaligned
>> accesses are emulated by the kernel, and support for them is optional.
>> So we may end up with a fault.
>>
>> The intent of the fix here is to make it easy to identify and rework
>> places where actual mis-alignment occurs. Wherever casts raise the warning,
>> they are replaced with a macro:
>>
>> - SPICE_ALIGNED_CAST(type, value) casts value to type, and indicates that
>> we believe the resulting pointer is aligned. If it is not, a warning will
>> be issued.
>>
>> - SPICE_UNALIGNED_CAST(type, value) casts value to type, and indicates that
>> we believe the resulting pointer is not always aligned
>>
>> Any code using SPICE_UNALIGNED_CAST may need to be revisited in order
>> to improve performance, e.g. by using memcpy.
>>
>> There are normally no warnings for SPICE_UNALIGNED_CAST, but it is possible
>> to emit debug messages for mis-alignment in SPICE_UNALIGNED_CAST.
>> Set environment variable SPICE_DEBUG_ALIGNMENT to activate this check.
>>
>> Signed-off-by: Christophe de Dinechin <dinechin@xxxxxxxxxx>
>> ---
>> spice-common | 2 +-
>> src/channel-cursor.c | 6 +++---
>> src/channel-display-mjpeg.c | 2 +-
>> src/continuation.h | 6 ++++--
>> src/decode-glz-tmpl.c | 2 +-
>> src/spice-channel.c | 14 +++++++++-----
>> 6 files changed, 19 insertions(+), 13 deletions(-)
>>
>> diff --git a/spice-common b/spice-common
>> index af682b1..692d303 160000
>> --- a/spice-common
>> +++ b/spice-common
>> @@ -1 +1 @@
>> -Subproject commit af682b1b06dea55007d9aa7c37cd443e4349e43f
>> +Subproject commit 692d303c8b1e2da7a47da713ad499f57ed5e6b5e
>> diff --git a/src/channel-cursor.c b/src/channel-cursor.c
>> index cddba03..85160eb 100644
>> --- a/src/channel-cursor.c
>> +++ b/src/channel-cursor.c
>> @@ -422,7 +422,7 @@ static display_cursor *set_cursor(SpiceChannel *channel,
>> SpiceCursor *scursor)
>> memcpy(cursor->data, data, size);
>> for (i = 0; i < hdr->width * hdr->height; i++) {
>> pix_mask = get_pix_mask(data, size, i);
>> - if (pix_mask && *((guint32*)data + i) == 0xffffff) {
>> + if (pix_mask && *(SPICE_ALIGNED_CAST(guint32 *, data) + i) ==
>> 0xffffff) {
>> cursor->data[i] = get_pix_hack(i, hdr->width);
>> } else {
>> cursor->data[i] |= (pix_mask ? 0 : 0xff000000);
>
> Here cursor->data[i] can be used which is aligned.
Yes.
>
>> @@ -432,7 +432,7 @@ static display_cursor *set_cursor(SpiceChannel *channel,
>> SpiceCursor *scursor)
>> case SPICE_CURSOR_TYPE_COLOR16:
>> for (i = 0; i < hdr->width * hdr->height; i++) {
>> pix_mask = get_pix_mask(data, size, i);
>> - pix = *((guint16*)data + i);
>> + pix = *(SPICE_ALIGNED_CAST(guint16 *, data) + i);
>> if (pix_mask && pix == 0x7fff) {
>> cursor->data[i] = get_pix_hack(i, hdr->width);
>> } else {
>
> actually this is unaligned, the data came from network buffer.
Changed to unaligned.
>
>> @@ -446,7 +446,7 @@ static display_cursor *set_cursor(SpiceChannel *channel,
>> SpiceCursor *scursor)
>> for (i = 0; i < hdr->width * hdr->height; i++) {
>> pix_mask = get_pix_mask(data, size + (sizeof(uint32_t) << 4),
>> i);
>> int idx = (i & 1) ? (data[i >> 1] & 0x0f) : ((data[i >> 1] &
>> 0xf0) >> 4);
>> - pix = *((uint32_t*)(data + size) + idx);
>> + pix = *(SPICE_UNALIGNED_CAST(uint32_t *, (data + size)) + idx);
>> if (pix_mask && pix == 0xffffff) {
>> cursor->data[i] = get_pix_hack(i, hdr->width);
>> } else {
>
> Maybe the palette can be copied with a memcpy?
Good idea.
> Looks like this code is also little endian dependent.
So is the code above, no?
>
>> diff --git a/src/channel-display-mjpeg.c b/src/channel-display-mjpeg.c
>> index 3ae9d21..ee33b01 100644
>> --- a/src/channel-display-mjpeg.c
>> +++ b/src/channel-display-mjpeg.c
>> @@ -151,7 +151,7 @@ static gboolean mjpeg_decoder_decode_frame(gpointer
>> video_decoder)
>> #ifndef JCS_EXTENSIONS
>> {
>> uint8_t *s = lines[0];
>> - uint32_t *d = (uint32_t *)s;
>> + uint32_t *d = SPICE_ALIGNED_CAST(uint32_t *, s);
>>
>> if (back_compat) {
>> for (unsigned int j = lines_read * width; j > 0; ) {
>> diff --git a/src/continuation.h b/src/continuation.h
>> index 675a257..d1fd137 100644
>> --- a/src/continuation.h
>> +++ b/src/continuation.h
>> @@ -21,6 +21,7 @@
>> #ifndef _CONTINUATION_H_
>> #define _CONTINUATION_H_
>>
>> +#include "spice-common.h"
>> #include <stddef.h>
>> #include <ucontext.h>
>> #include <setjmp.h>
>> @@ -48,8 +49,9 @@ int cc_release(struct continuation *cc);
>> int cc_swap(struct continuation *from, struct continuation *to);
>>
>> #define offset_of(type, member) ((unsigned long)(&((type *)0)->member))
>> -#define container_of(obj, type, member) \
>> - (type *)(((char *)obj) - offset_of(type, member))
>> +#define container_of(obj, type, member) \
>> + SPICE_ALIGNED_CAST(type *, \
>> + (((char *)obj) - offset_of(type, member)))
>>
>> #endif
>> /*
>
> No reason to check, just add the void* cast.
There’s no check anymore unless you configure —enable-alignment-checks.
>
>> diff --git a/src/decode-glz-tmpl.c b/src/decode-glz-tmpl.c
>> index b337a8b..76d832c 100644
>> --- a/src/decode-glz-tmpl.c
>> +++ b/src/decode-glz-tmpl.c
>> @@ -178,7 +178,7 @@ static size_t FNAME(decode)(SpiceGlzDecoderWindow
>> *window,
>> uint64_t image_id, SpicePalette *plt)
>> {
>> uint8_t *ip = in_buf;
>> - OUT_PIXEL *out_pix_buf = (OUT_PIXEL *)out_buf;
>> + OUT_PIXEL *out_pix_buf = SPICE_ALIGNED_CAST(OUT_PIXEL *, out_buf);
>> OUT_PIXEL *op = out_pix_buf;
>> OUT_PIXEL *op_limit = out_pix_buf + size;
>>
>> diff --git a/src/spice-channel.c b/src/spice-channel.c
>> index 3b6231e..6c52547 100644
>> --- a/src/spice-channel.c
>> +++ b/src/spice-channel.c
>> @@ -1312,6 +1312,7 @@ static void spice_channel_send_link(SpiceChannel
>> *channel)
>> {
>> SpiceChannelPrivate *c = channel->priv;
>> uint8_t *buffer, *p;
>> + uint32_t *caps;
>> int protocol, i;
>>
>> c->link_hdr.magic = SPICE_MAGIC;
>> @@ -1356,14 +1357,15 @@ static void spice_channel_send_link(SpiceChannel
>> *channel)
>> memcpy(p, &c->link_hdr, sizeof(c->link_hdr)); p += sizeof(c->link_hdr);
>> memcpy(p, &c->link_msg, sizeof(c->link_msg)); p += sizeof(c->link_msg);
>>
>> + // Need this to avoid error with -Wcast-align
>> + caps = SPICE_UNALIGNED_CAST(uint32_t *,p);
>> for (i = 0; i < c->common_caps->len; i++) {
>> - *(uint32_t *)p = GUINT32_TO_LE(g_array_index(c->common_caps,
>> uint32_t, i));
>> - p += sizeof(uint32_t);
>> + *caps++ = GUINT32_TO_LE(g_array_index(c->common_caps, uint32_t, i));
>> }
>> for (i = 0; i < c->caps->len; i++) {
>> - *(uint32_t *)p = GUINT32_TO_LE(g_array_index(c->caps, uint32_t, i));
>> - p += sizeof(uint32_t);
>> + *caps++ = GUINT32_TO_LE(g_array_index(c->caps, uint32_t, i));
>> }
>> + p = (uint8_t *) caps;
>> CHANNEL_DEBUG(channel, "channel type %d id %d num common caps %u num
>> caps %u",
>> c->channel_type,
>> c->channel_id,
>
> another way is to align caps_offset.
Not worth it IMO.
>
>> @@ -1887,6 +1889,7 @@ static gboolean
>> spice_channel_recv_link_msg(SpiceChannel *channel)
>> SpiceChannelPrivate *c;
>> int rc, num_caps, i;
>> uint32_t *caps, num_channel_caps, num_common_caps;
>> + uint8_t *caps_src;
>> SpiceChannelEvent event = SPICE_CHANNEL_ERROR_LINK;
>>
>> g_return_val_if_fail(channel != NULL, FALSE);
>> @@ -1926,7 +1929,8 @@ static gboolean
>> spice_channel_recv_link_msg(SpiceChannel *channel)
>> /* see original spice/client code: */
>> /* g_return_if_fail(c->peer_msg + c->peer_msg->caps_offset *
>> sizeof(uint32_t) > c->peer_msg + c->peer_hdr.size); */
>>
>> - caps = (uint32_t *)((uint8_t *)c->peer_msg +
>> GUINT32_FROM_LE(c->peer_msg->caps_offset));
>> + caps_src = (uint8_t *)c->peer_msg +
>> GUINT32_FROM_LE(c->peer_msg->caps_offset);
>> + caps = SPICE_UNALIGNED_CAST(uint32_t *, caps_src);
>>
>> g_array_set_size(c->remote_common_caps, num_common_caps);
>> for (i = 0; i < num_common_caps; i++, caps++) {
>
> A memcpy before the conversion would solve any cases.
Yes. But I think there is a bug in the existing code. It sets the size to num_common_caps, I think it should be num_common_caps * sizeof(uint32_t).
Christophe
>
> Frediano
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