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SubjectRe: [PATCH] include/linux/unaligned: Pack the whole struct rather than just the field.
On Wed, 1 Dec 2010 22:11:53 +0000 Will Newton <will.newton@gmail.com> wrote:
> The current packed struct implementation of unaligned access adds
> the packed attribute only to the field within the unaligned struct
> rather than to the struct as a whole. This is not sufficient to
> enforce proper behaviour on architectures with a default struct
> alignment of more than one byte.
>
> For example, the current implementation of __get_unaligned_cpu16
> when compiled for arm with gcc -O1 -mstructure-size-boundary=32
> assumes the struct is on a 4 byte boundary so performs the load
> of the 16bit packed field as if it were on a 4 byte boundary:
>
> __get_unaligned_cpu16:
> ldrh r0, [r0, #0]
> bx lr
>
> Moving the packed attribute to the struct rather than the field
> causes the proper unaligned access code to be generated:
>
> __get_unaligned_cpu16:
> ldrb r3, [r0, #0] @ zero_extendqisi2
> ldrb r0, [r0, #1] @ zero_extendqisi2
> orr r0, r3, r0, asl #8
> bx lr
>
> Signed-off-by: Will Newton <will.newton@gmail.com>
> ---
> include/linux/unaligned/packed_struct.h | 6 +++---
> 1 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/unaligned/packed_struct.h
> b/include/linux/unaligned/packed_struct.h
> index 2498bb9..c9a6abd 100644
> --- a/include/linux/unaligned/packed_struct.h
> +++ b/include/linux/unaligned/packed_struct.h
> @@ -3,9 +3,9 @@
>
> #include <linux/kernel.h>
>
> -struct __una_u16 { u16 x __attribute__((packed)); };
> -struct __una_u32 { u32 x __attribute__((packed)); };
> -struct __una_u64 { u64 x __attribute__((packed)); };
> +struct __una_u16 { u16 x; } __attribute__((packed));
> +struct __una_u32 { u32 x; } __attribute__((packed));
> +struct __una_u64 { u64 x; } __attribute__((packed));
>

Yes, that was wrong.

Do you think this bug affects 2.6.36 or earlier?

Even if it doesn't, it looks like a bit of a hand-grenade to leave it
unfixed in earlier kernels.



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