On Mon, Aug 26, 2024 at 10:01:17AM +0200, Christophe Leroy wrote: > > > Le 26/08/2024 à 09:50, Jason A. Donenfeld a écrit : > > 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://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2F20240528122352.2485958-5-Jason%40zx2c4.com%2F%23Z31include%3Avdso%3Atypes.h&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C41747dd989164267c1cc08dcc5a3c424%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C638602554376441761%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C0%7C%7C%7C&sdata=Tf9ShSN6aOOFZ1HymAmHhj0xhQ6BUtHJX95t50gsp9k%3D&reserved=0 > > > > 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. > > So yes I really really want it on ppc32 because this is the only type of > boards I have and this is really were we need getrandom() to be > optimised, indeed ppc64 was sherry-on-the-cake in my series, I just > added it because it was easy to do after doing ppc32. I saw that you did in fact find a bit on ppc32 for VM_DROPPABLE. So it looks at least possible. Because of this generation counter business, I still think it might make sense to do in two steps, though, first doing 64-bit, and then doing 32-bit after. As for the generation counter error you're seeing, I guess what we want is smp_store_release memory ordering semantics, but letting tearing happen (since the upper 32-bits will be zero anyway). I'm not sure the best way to do this, whether it's a new helper, or doing a WRITE_ONCE together with an smp barrier, or what. But I imagine it's something like that. Jason
