Re: [Qemu-devel] [PATCH] qxl: fix -Waddress-of-packed-member

[Philippe Mathieu-Daudé]

On 4/8/19 10:12 PM, Marc-André Lureau wrote:
> The GCC9 compiler complains about QXL code that takes the address of
> members of the 'struct QXLReleaseRing' which is marked packed:
> 
>   CC      hw/display/qxl.o
> /home/elmarco/src/qemu/hw/display/qxl.c: In function ‘init_qxl_ram’:
> /home/elmarco/src/qemu/hw/display/qxl.c:50:19: warning: taking address of 
> packed member of ‘struct QXLReleaseRing_ring_el’ may result in an unaligned 
> pointer value [-Waddress-of-packed-member]
>    50 |             ret = &(r)->items[prod].el;                               
>   \
>       |                   ^~~~~~~~~~~~~~~~~~~~
> /home/elmarco/src/qemu/hw/display/qxl.c:429:5: note: in expansion of macro 
> ‘SPICE_RING_PROD_ITEM’
>   429 |     SPICE_RING_PROD_ITEM(d, &d->ram->release_ring, item);
>       |     ^~~~~~~~~~~~~~~~~~~~
> /home/elmarco/src/qemu/hw/display/qxl.c: In function ‘qxl_push_free_res’:
> /home/elmarco/src/qemu/hw/display/qxl.c:50:19: warning: taking address of 
> packed member of ‘struct QXLReleaseRing_ring_el’ may result in an unaligned 
> pointer value [-Waddress-of-packed-member]
>    50 |             ret = &(r)->items[prod].el;                               
>   \
>       |                   ^~~~~~~~~~~~~~~~~~~~
> /home/elmarco/src/qemu/hw/display/qxl.c:762:5: note: in expansion of macro 
> ‘SPICE_RING_PROD_ITEM’
>   762 |     SPICE_RING_PROD_ITEM(d, ring, item);
>       |     ^~~~~~~~~~~~~~~~~~~~
> /home/elmarco/src/qemu/hw/display/qxl.c: In function 
> ‘interface_release_resource’:
> /home/elmarco/src/qemu/hw/display/qxl.c:50:19: warning: taking address of 
> packed member of ‘struct QXLReleaseRing_ring_el’ may result in an unaligned 
> pointer value [-Waddress-of-packed-member]
>    50 |             ret = &(r)->items[prod].el;                               
>   \
>       |                   ^~~~~~~~~~~~~~~~~~~~
> /home/elmarco/src/qemu/hw/display/qxl.c:795:5: note: in expansion of macro 
> ‘SPICE_RING_PROD_ITEM’
>   795 |     SPICE_RING_PROD_ITEM(qxl, ring, item);
>       |     ^~~~~~~~~~~~~~~~~~~~
> 
> Replace pointer usage by direct structure/array access instead.
> 
> Signed-off-by: Marc-André Lureau <address@hidden>

Tested-by: Philippe Mathieu-Daudé <address@hidden>

> ---
>  hw/display/qxl.c | 83 +++++++++++++++++++++++++++++-------------------
>  1 file changed, 50 insertions(+), 33 deletions(-)
> 
> diff --git a/hw/display/qxl.c b/hw/display/qxl.c
> index c8ce5781e0..12d83dd6f1 100644
> --- a/hw/display/qxl.c
> +++ b/hw/display/qxl.c
> @@ -39,29 +39,49 @@
>   * abort we just qxl_set_guest_bug and set the return to NULL. Still
>   * it may happen as a result of emulator bug as well.
>   */
> -#undef SPICE_RING_PROD_ITEM
> -#define SPICE_RING_PROD_ITEM(qxl, r, ret) {                             \
> -        uint32_t prod = (r)->prod & SPICE_RING_INDEX_MASK(r);           \
> -        if (prod >= ARRAY_SIZE((r)->items)) {                           \
> -            qxl_set_guest_bug(qxl, "SPICE_RING_PROD_ITEM indices mismatch " \
> -                          "%u >= %zu", prod, ARRAY_SIZE((r)->items));   \
> -            ret = NULL;                                                 \
> -        } else {                                                        \
> -            ret = &(r)->items[prod].el;                                 \
> -        }                                                               \
> +#define SPICE_RING_GET_CHECK(qxl, r, field) ({                          \
> +    field = (r)->field & SPICE_RING_INDEX_MASK(r);                      \
> +    bool mismatch = field >= ARRAY_SIZE((r)->items);                    \
> +    if (mismatch) {                                                     \
> +        qxl_set_guest_bug(qxl, "SPICE_RING_GET %s indices mismatch "    \
> +                          "%u >= %zu", stringify(field), field,         \
> +                          ARRAY_SIZE((r)->items));                      \
> +    }                                                                   \
> +    !mismatch;                                                          \
> +})
> +
> +static inline uint64_t
> +qxl_release_ring_get_prod(PCIQXLDevice *qxl)
> +{
> +    struct QXLReleaseRing *ring = &qxl->ram->release_ring;
> +    uint32_t prod;
> +    bool ok = SPICE_RING_GET_CHECK(qxl, ring, prod);
> +    assert(ok);
> +
> +    return ring->items[prod].el;
> +}
> +
> +static inline bool
> +qxl_release_ring_set_prod(PCIQXLDevice *qxl, uint64_t val)
> +{
> +    struct QXLReleaseRing *ring = &qxl->ram->release_ring;
> +    uint32_t prod;
> +    bool ok = SPICE_RING_GET_CHECK(qxl, ring, prod);
> +    if (ok) {
> +        ring->items[prod].el = val;
>      }
> +    return ok;
> +}
>  
>  #undef SPICE_RING_CONS_ITEM
> -#define SPICE_RING_CONS_ITEM(qxl, r, ret) {                             \
> -        uint32_t cons = (r)->cons & SPICE_RING_INDEX_MASK(r);           \
> -        if (cons >= ARRAY_SIZE((r)->items)) {                           \
> -            qxl_set_guest_bug(qxl, "SPICE_RING_CONS_ITEM indices mismatch " \
> -                          "%u >= %zu", cons, ARRAY_SIZE((r)->items));   \
> -            ret = NULL;                                                 \
> -        } else {                                                        \
> -            ret = &(r)->items[cons].el;                                 \
> -        }                                                               \
> -    }
> +#define SPICE_RING_CONS_ITEM(qxl, r, ret) {     \
> +    uint32_t cons;                              \
> +    if (!SPICE_RING_GET_CHECK(qxl, r, cons)) {  \
> +        ret = NULL;                             \
> +    } else {                                    \
> +        ret = &(r)->items[cons].el;             \
> +    }                                           \
> +}
>  
>  #undef ALIGN
>  #define ALIGN(a, b) (((a) + ((b) - 1)) & ~((b) - 1))
> @@ -414,7 +434,6 @@ static void init_qxl_rom(PCIQXLDevice *d)
>  static void init_qxl_ram(PCIQXLDevice *d)
>  {
>      uint8_t *buf;
> -    uint64_t *item;
>  
>      buf = d->vga.vram_ptr;
>      d->ram = (QXLRam *)(buf + le32_to_cpu(d->shadow_rom.ram_header_offset));
> @@ -426,9 +445,9 @@ static void init_qxl_ram(PCIQXLDevice *d)
>      SPICE_RING_INIT(&d->ram->cmd_ring);
>      SPICE_RING_INIT(&d->ram->cursor_ring);
>      SPICE_RING_INIT(&d->ram->release_ring);
> -    SPICE_RING_PROD_ITEM(d, &d->ram->release_ring, item);
> -    assert(item);
> -    *item = 0;
> +    if (!qxl_release_ring_set_prod(d, 0)) {
> +        g_assert_not_reached();
> +    }
>      qxl_ring_set_dirty(d);
>  }
>  
> @@ -732,7 +751,6 @@ static int interface_req_cmd_notification(QXLInstance 
> *sin)
>  static inline void qxl_push_free_res(PCIQXLDevice *d, int flush)
>  {
>      QXLReleaseRing *ring = &d->ram->release_ring;
> -    uint64_t *item;
>      int notify;
>  
>  #define QXL_FREE_BUNCH_SIZE 32
> @@ -759,11 +777,9 @@ static inline void qxl_push_free_res(PCIQXLDevice *d, 
> int flush)
>      if (notify) {
>          qxl_send_events(d, QXL_INTERRUPT_DISPLAY);
>      }
> -    SPICE_RING_PROD_ITEM(d, ring, item);
> -    if (!item) {
> +    if (!qxl_release_ring_set_prod(d, 0)) {
>          return;
>      }
> -    *item = 0;
>      d->num_free_res = 0;
>      d->last_release = NULL;
>      qxl_ring_set_dirty(d);
> @@ -775,7 +791,8 @@ static void interface_release_resource(QXLInstance *sin,
>  {
>      PCIQXLDevice *qxl = container_of(sin, PCIQXLDevice, ssd.qxl);
>      QXLReleaseRing *ring;
> -    uint64_t *item, id;
> +    uint32_t prod;
> +    uint64_t id;
>  
>      if (ext.group_id == MEMSLOT_GROUP_HOST) {
>          /* host group -> vga mode update request */
> @@ -792,16 +809,16 @@ static void interface_release_resource(QXLInstance *sin,
>       * pci bar 0, $command.release_info
>       */
>      ring = &qxl->ram->release_ring;
> -    SPICE_RING_PROD_ITEM(qxl, ring, item);
> -    if (!item) {
> +
> +    if (!SPICE_RING_GET_CHECK(qxl, ring, prod)) {
>          return;
>      }
> -    if (*item == 0) {
> +    if (qxl_release_ring_get_prod(qxl) == 0) {
>          /* stick head into the ring */
>          id = ext.info->id;
>          ext.info->next = 0;
>          qxl_ram_set_dirty(qxl, &ext.info->next);
> -        *item = id;
> +        qxl_release_ring_set_prod(qxl, id);
>          qxl_ring_set_dirty(qxl);
>      } else {
>          /* append item to the list */
>