Re: [PATCH 7/7] s390/vdso: Wire up getrandom() vdso implementation

Jason A. Donenfeld
2024-09-13T13:53:48Z

On Fri, Sep 13, 2024 at 03:05:43PM +0200, Heiko Carstens wrote:
> Provide the s390 specific vdso getrandom() architecture backend.
>
> _vdso_rng_data required data is placed within the _vdso_data vvar page, by
> using a hardcoded offset larger than vdso_data.
>
> As required the chacha20 implementation does not write to the stack.
>
> The implementation follows more or less the arm64 implementations and
> makes use of vector instructions. It has a fallback to the getrandom()
> system call for machines where the vector facility is not
> installed.
> The check if the vector facility is installed, as well as an
> optimization for machines with the vector-enhancements facility 2,
> is implemented with alternatives, avoiding runtime checks.
>
> Note that __kernel_getrandom() is implemented without the vdso user wrapper
> which would setup a stack frame for odd cases (aka very old glibc variants)
> where the caller has not done that. All callers of __kernel_getrandom() are
> required to setup a stack frame, like the C ABI requires it.
>
> The vdso testcases vdso_test_getrandom and vdso_test_chacha pass.

I'd be curious to see the results of ./vdso_test_getrandom bench-single
and such.

Jason