Arnd Bergmann wrote:
> On Saturday 31 January 2009, Andrew Morton wrote:
>> Is this written in a standard somewhere? Is it guaranteed?
>
> Alignment is defined in the architecture psABI documents.
> Unfortunately, many of them were written before the 'long long'
> type became part of the C standard, so it's not strictly guaranteed.
> AFAICT, the alignment of __u64 on x86 is the same as the alignment
> of 'double' by convention.
>
> However, the problem is well-understood: x86 is the only one
> that has a problem in 32/64 bit compat mode. m68k has similar
> issues with 16/32 bit integers, but those don't apply here.
>
>> If some (perhaps non-gcc) compiler were to lay this out differently
>> (perhaps with suitable command-line options) then that's liveable
>> with - as long as the kernel never changes the layout. Of course
>> it would be better to avoid this if poss.
>
> If a compiler was using irregular structure alignment, all sorts of
> library interfaces would break. The kernel ABI is only a small part
> of the problem then.
>
>> The other potential issue with a structure like this is that there's a
>> risk that it will lead us to copy four bytes of uninitialised kernel
>> memory out to userspace.
>>
>> IOW, it seems a generally bad idea to rely upon compiler-added padding
>> for this sort of thing.
>
> Agreed in general, but the whole point of this particular patch was to
> provide compatibility with an interface that has been part of XFS for
> many years.
> Linux already has a better interface for new users (sys_fallocate), so
> changing the patch would not be helpful and not provide any advantage.
>
> There is also no leak of uninitialized data here, because this structure
> is only read, never written.
>
> Arnd <><
Arnd Bergmann wrote:
> +struct space_resv {
> + __s16 l_type;
> + __s16 l_whence;
> + __s64 l_start;
> + __s64 l_len; /* len == 0 means until end of file */
> + __s32 l_sysid;
> + __u32 l_pid;
> + __s32 l_pad[4]; /* reserve area */
> +};
What about telling the compiler exactly what you said above, just
to be sure we all mean the same thing. (And as documentation for new
comers):
+struct space_resv_64 {
+ __s16 l_type;
+ __s16 l_whence;
+ __u32 reserved;
+ __s64 l_start;
+ __s64 l_len; /* len == 0 means until end of file */
+ __s32 l_sysid;
+ __u32 l_pid;
+ __s32 l_pad[4]; /* reserve area */
+} __packed;
And define another one for x86_32
Boaz
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