Re: [PATCH spice-gtk v2 1/4] Avoid clang warnings on casts with stricter alignment requirements

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



Hey,

On Thu, May 11, 2017 at 12:47:05PM +0200, Christophe de Dinechin 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) is the same as (type) value, but
>   silences the warning (using an intermediate cast to (void *)) and
>   adds a runtime check to verify that the pointer value is actually
>   aligned as expected. If that expectation is violated, a warning
>   is issued.
> 
> - SPICE_UNALIGNED_CAST(type, value) works like SPICE_ALIGNED_CAST,
>   but asssumes that the value is not known to be aligned. If there
>   is an alignment error, no warning is issued. It is still possible
>   to know if misalignment occurs by defining environment variable
>   SPICE_DEBUG_ALIGNMENT, in which case a debug message is issued for
>   every misaligned pointer.

For some reason, after reading the log, it's still not 100% obvious to
me that SPICE_ALIGNED_CAST means "I checked, the cast which is being
done is legit as the pointer has the right aligment", while
SPICE_UNALIGNED_CAST means "pointer does not have the right alignment,
fixing might need to be done".

> 
> Signed-off-by: Christophe de Dinechin <dinechin@xxxxxxxxxx>
> ---
>  spice-common                |  2 +-
>  src/channel-cursor.c        |  6 +++---
>  src/channel-display-mjpeg.c |  2 +-
>  src/decode-glz-tmpl.c       |  2 +-
>  src/spice-channel.c         | 14 +++++++++-----
>  5 files changed, 15 insertions(+), 11 deletions(-)
> 
> diff --git a/spice-common b/spice-common
> index af682b1..1239c82 160000
> --- a/spice-common
> +++ b/spice-common
> @@ -1 +1 @@
> -Subproject commit af682b1b06dea55007d9aa7c37cd443e4349e43f
> +Subproject commit 1239c82c54dee0244cca262cca0aa21071b23e24
> diff --git a/src/channel-cursor.c b/src/channel-cursor.c
> index 609243b..50de5ce 100644
> --- a/src/channel-cursor.c
> +++ b/src/channel-cursor.c
> @@ -340,7 +340,7 @@ 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);
> @@ -350,7 +350,7 @@ 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 {
> @@ -364,7 +364,7 @@ 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 {
> diff --git a/src/channel-display-mjpeg.c b/src/channel-display-mjpeg.c
> index 3ae9d21..17c0f4f 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/decode-glz-tmpl.c b/src/decode-glz-tmpl.c
> index b337a8b..7695a28 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..88d0567 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 *p32;
>      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
> +    p32 = SPICE_UNALIGNED_CAST(uint32_t *,p);

I don't think the comment is necessary.
p32 could be named current_cap, caps, or something like this instead

>      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);
> +        *p32++ = GUINT32_TO_LE(g_array_index(c->common_caps, uint32_t, i));

Assignment followed by increment is more readable imo.

>      }
>      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);
> +        *p32++ = GUINT32_TO_LE(g_array_index(c->caps, uint32_t, i));
>      }
> +    p = (uint8_t *) p32;
>      CHANNEL_DEBUG(channel, "channel type %d id %d num common caps %u num caps %u",
>                    c->channel_type,
>                    c->channel_id,
> @@ -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);

Nit: Sometimes you use a ", " inside SPICE_*_CAST, sometimes just ",".

Overall looks fine to me,

Acked-by: Christophe Fergeau <cfergeau@xxxxxxxxxx>

Christophe

Attachment: signature.asc
Description: PGP signature

_______________________________________________
Spice-devel mailing list
Spice-devel@xxxxxxxxxxxxxxxxxxxxx
https://lists.freedesktop.org/mailman/listinfo/spice-devel

[Index of Archives]     [Linux ARM Kernel]     [Linux ARM]     [Linux Omap]     [Fedora ARM]     [IETF Annouce]     [Security]     [Bugtraq]     [Linux]     [Linux OMAP]     [Linux MIPS]     [ECOS]     [Asterisk Internet PBX]     [Linux API]     [Monitors]