Re: [PATCH v2 06/17] vdso: Change getrandom's generation to unsigned long

Jason A. Donenfeld
2024-08-26T07:50:32Z

On Thu, Aug 22, 2024 at 09:13:14AM +0200, Christophe Leroy wrote:
> Performing SMP atomic operations on u64 fails on powerpc32.
>
> Random driver generation is handled as unsigned long not u64,
> see for instance base_cnrg or struct crng.
>
> Use the same type for vDSO's getrandom as it gets copied
> from the above. This is also in line with the local
> current_generation which is already an unsigned long.

This isn't going to work when 32-bit userspace tries to access a 64-bit
kernel.

I had "fixed" this with a vdso_kernel_ulong type way back in an earlier
version: https://lore.kernel.org/lkml/20240528122352.2485958-5-Jason@zx2c4.com/#Z31include:vdso:types.h

But tglx pointed out in that thread that this actually isn't necessary:

| All of this is pointless because if a 32-bit application runs on a
| 64-bit kernel it has to use the 64-bit 'generation'. So why on earth do
| we need magic here for a 32-bit kernel?
|
| Just use u64 for both and spare all this voodoo. We're seriously not
| "optimizing" for 32-bit kernels.
|
| All what happens on a 32-bit kernel is that the RNG will store the
| unsigned long (32bit) generation into a 64bit variable:
|
| smp_store_release(&_vdso_rng_data.generation, next_gen + 1);
|
| As the upper 32bit are always zero, there is no issue vs. load store
| tearing at all. So there is zero benefit for this aside of slightly
| "better" user space code when running on a 32-bit kernel. Who cares?

So I just got rid of it and used a u64 as he suggested.

However, there's also an additional reason why it's not worth churning
further over this - because VM_DROPPABLE is 64-bit only (due to flags in
vma bits), likely so is vDSO getrandom() for the time being. So I think
it makes more sense to retool this series to be ppc64, and then if you
really really want 32-bit and can convince folks it matters, then all of
these parts (for example, here, the fact that the smp helper doesn't
want to tear) can be fixed up in a separate series.

Jason