Re: [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility with legacy xfs ioctls

[Geert Uytterhoeven <geert@linux-m68k.org>]

On Sun, 1 Feb 2009, Boaz Harrosh wrote:
> 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;

Because the compiler will assume all fields are always unaligned and will use very
suboptimal code to access them?

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds