Re: [PATCH v10 3/4] random: introduce generic vDSO getrandom() implementation

[Jason A. Donenfeld]

Hi Arnd,

On Wed, Nov 30, 2022 at 4:07 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > +#ifdef CONFIG_64BIT
> > +typedef u64 vdso_kernel_ulong;
> > +#else
> > +typedef u32 vdso_kernel_ulong;
> > +#endif
>
> This does not address the ABI concern: to allow 32-bit and 64-bit
> tasks to share the same data page, it has to be the same width on
> both, either u32 or 64, but not depending on a configuration
> option.

I think it does address the issue. CONFIG_64BIT is a .config setting,
not a compiler-derived setting. So a 64-bit kernel will get a u64 in
kernel mode, and then it will get a u64 for the 64-bit vdso usermode
compile, and finally it will get a u64 for the 32-bit vdso usermode
compile. So in all three cases, the size is the same.

> > struct vdso_rng_data {
> >       vdso_kernel_ulong       generation;
> >       bool                    is_ready;
> > };
>
> There is another problem with this: you have implicit padding
> in the structure because the two members have different size
> and alignment requirements. The easiest fix is to make them
> both u64, or you could have a u32 is_ready and an explit u32
> for the padding.

There's padding at the end of the structure, yes. But both
`generation` and `is_ready` will be at the same offset. If the
structure grows, then sure, that'll have to be taken into account. But
that's not a problem because this is a private implementation detail
between the vdso code and the kernel.

Jason