On Friday 15 June 2007, Andi Kleen wrote: > > On Friday 15 June 2007 11:31:37 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. > > Why does it break them? It should just make them a little slower. > > The network code requires unaligned accesses to work > anyways so if your architecture doesn't support them it is already > remotely crashable. >
It doesn't break in all cases, but quite often, you have something like: struct foo { __u32 a; __u64 b; }; If you define a struct compat_foo { __u32 a; __u64 b; } __attribute__((packed)); That is broken on all non-x86 architectures, because it removes the padding that is inserted on the respective 32 bit platforms, while struct compat_foo { __u32 a; compat_u64 b; }; Is a correct definition on all architectures. It also produces somewhat better code if the architecture does not support unaligned data access, but that is just an unintended side-effect. Arnd <>< - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/