Re: [Qemu-devel] [PATCH 2/2] acpi-build: don't access unaligned addresses

[Richard Henderson <rth@twiddle.net>]

On 03/10/2014 12:43 PM, Michael S. Tsirkin wrote:
> casting an unaligned address to e.g.
> uint32_t can trigger undefined behaviour in C.
> Replace cast + assignment with memcpy.
> 
> Reported-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  hw/i386/acpi-build.c | 31 ++++++++++++++++---------------
>  1 file changed, 16 insertions(+), 15 deletions(-)
> 
> diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> index b667d31..e08f514 100644
> --- a/hw/i386/acpi-build.c
> +++ b/hw/i386/acpi-build.c
> @@ -466,9 +466,15 @@ static void acpi_align_size(GArray *blob, unsigned align)
>      g_array_set_size(blob, ROUND_UP(acpi_data_len(blob), align));
>  }
>  
> -/* Get pointer within table in a safe manner */
> -#define ACPI_BUILD_PTR(table, size, off, type) \
> -    ((type *)(acpi_data_get_ptr(table, size, off, sizeof(type))))
> +/* Set a value within table in a safe manner */
> +#define ACPI_BUILD_SET_LE(table, size, off, bits, val) \
> +    do { \
> +        uint64_t ACPI_BUILD_SET_LE_val = le_bswap(val, bits); \
> +        memcpy(acpi_data_get_ptr(table, size, off, \
> +                                 (bits) / BITS_PER_BYTE), \
> +               &ACPI_BUILD_SET_LE_val, \
> +               (bits) / BITS_PER_BYTE); \
> +    } while (0)
>  
>  static inline void *acpi_data_get_ptr(uint8_t *table_data, unsigned table_size,
>                                        unsigned off, unsigned size)
> @@ -974,22 +980,17 @@ static void build_pci_bus_end(PCIBus *bus, void *bus_state)
>  
>  static void patch_pci_windows(PcPciInfo *pci, uint8_t *start, unsigned size)
>  {
> -    *ACPI_BUILD_PTR(start, size, acpi_pci32_start[0], uint32_t) =
> -        cpu_to_le32(pci->w32.begin);
> +    ACPI_BUILD_SET_LE(start, size, acpi_pci32_start[0], 32, pci->w32.begin);

You're working too hard at this because you're using the wrong bswap interface.

Also, you missed one.

I'll follow up with a patch I wrote just recently, but hadn't yet gotten around
to posting.


r~