Re: [Qemu-devel] [PATCH v2] acpi-build: don't access unaligned addresses

[Peter Maydell <peter.maydell@linaro.org>]

On 10 March 2014 19:56, Michael S. Tsirkin <mst@redhat.com> 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>

This does fix the clang warnings.

> -/* 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 = cpu_to_le64(val); \
> +        memcpy(acpi_data_get_ptr(table, size, off, \
> +                                 (bits) / BITS_PER_BYTE), \
> +               &ACPI_BUILD_SET_LE_val, \
> +               (bits) / BITS_PER_BYTE); \
> +    } while (0)

Personally I would have done:

#define acpi_stb(table, size, off, val) \
    stb_le_p(acpi_data_get_ptr(table, size, off, 1), val)
#define acpi_stw(table, size, off, val) \
    stw_le_p(acpi_data_get_ptr(table, size, off, 2), val)
#define acpi_stl(table, size, off, val) \
    stl_le_p(acpi_data_get_ptr(table, size, off, 4), val)
#define acpi_stq(table, size, off, val) \
    stq_le_p(acpi_data_get_ptr(table, size, off, 8), val)

which keeps the grubby details of memcpy and byteswapping
in bswap.h. However since it's purely inside this file and
not specifying an API to the rest of QEMU I don't object
if you prefer the approach you've taken.

thanks
-- PMM