Re: [PATCH] Introduce compat_u64 and compat_s64 types

From: Matthew Wilcox
Date: Fri Jun 15 2007 - 08:13:49 EST

On Fri, Jun 15, 2007 at 11:31:37AM +0200, Arnd Bergmann wrote:
> One common problem with 32 bit system call and ioctl emulation
> is the different alignment rules between i386 and 64 bit machines.
> A number of drivers work around this by marking the compat
> structures as 'attribute((packed))', which is not the right
> solution because it breaks all the non-x86 architectures that
> want to use the same compat code.
> Hopefully, this patch improves the situation, it introduces two
> new types, compat_u64 and compat_s64. These are defined on all
> architectures to have the same size and alignment as the 32 bit
> version of u64 and s64.

You're relying on compat_[us]64 being only used in structures which are
already packed. If someone uses them in a non-packed struct, they won't
decrease the alignment. I think it would be more effective to specify
it as:

__attribute__((aligned(4), packed))

The other problem is that if someone defines a struct like this:

struct foo {
short bar;
compat_s64 baz;
} __attribute__((packed))

it'll have different definitions on x86 and ia64.

So I think we should be aiming for the ((aligned, packed)) definition and
remove the __attribute__((packed)) from the struct definitions. What do
you think?
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