Re: [PATCH RFC v1] random: implement getrandom() in vDSO

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Sun, Jul 31, 2022 at 01:45:43AM +0200, Jason A. Donenfeld wrote:
> So, anyway, if I do muster a v2 of this (perhaps just to see the idea
> through), the API might split in two to something like:
> 
>   void *getrandom_allocate_states([inout] size_t *number_of_states, [out] size_t *length_per_state);
>   ssize_t getrandom(void *state, void *buffer, size_t len, unsigned long flags);
> 
> User code will call getrandom_allocate_state(), which will allocate
> enough pages to hold *number_of_states, and return the size of each one
> in length_per_state and the number actually allocated back in
> number_of_states. The result can then be sliced up by that size, and
> passed to getrandom(). So glibc or whatever would presumably allocate
> one per thread, and handle any reentrancy/locking around it.
> 
> Or some other variation on that. I'm sure you hate those function
> signatures. Everybody loves to bikeshed APIs, right? There's plenty to
> be tweaked. But that's anyhow about where my thinking is for a potential
> v2.

Doing this also doubled performance, perhaps unsurprisingly, as that
getcpu() operation wasn't free.

For uint32_t generation:

   vdso: 25000000 times in 0.289876265 seconds
syscall: 25000000 times in 4.296636025 seconds