Re: [PATCH v16 5/5] x86: vdso: Wire up getrandom() vDSO implementation

Eric Biggers
2024-05-31T03:38:18Z

On Tue, May 28, 2024 at 02:19:54PM +0200, Jason A. Donenfeld wrote:
> diff --git a/arch/x86/entry/vdso/vgetrandom-chacha.S b/arch/x86/entry/vdso/vgetrandom-chacha.S
> new file mode 100644
> index 000000000000..d79e2bd97598
> --- /dev/null
> +++ b/arch/x86/entry/vdso/vgetrandom-chacha.S
> @@ -0,0 +1,178 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
> + */
> +
> +#include <linux/linkage.h>
> +#include <asm/frame.h>
> +
> +.section .rodata, "a"
> +.align 16
> +CONSTANTS: .octa 0x6b20657479622d323320646e61707865
> +.text
> +
> +/*
> + * Very basic SSE2 implementation of ChaCha20. Produces a given positive number
> + * of blocks of output with a nonce of 0, taking an input key and 8-byte
> + * counter. Importantly does not spill to the stack. Its arguments are:
> + *
> + * rdi: output bytes
> + * rsi: 32-byte key input
> + * rdx: 8-byte counter input/output
> + * rcx: number of 64-byte blocks to write to output
> + */
> +SYM_FUNC_START(__arch_chacha20_blocks_nostack)
> +
> +.set output, %rdi
> +.set key, %rsi
> +.set counter, %rdx
> +.set nblocks, %rcx
> +.set i, %al
> +/* xmm registers are *not* callee-save. */
> +.set state0, %xmm0
> +.set state1, %xmm1
> +.set state2, %xmm2
> +.set state3, %xmm3
> +.set copy0, %xmm4
> +.set copy1, %xmm5
> +.set copy2, %xmm6
> +.set copy3, %xmm7
> +.set temp, %xmm8
> +.set one, %xmm9

An "interesting" x86_64 quirk: in SSE instructions, registers xmm0-xmm7 take
fewer bytes to encode than xmm8-xmm15.

Since 'temp' is used frequently, moving it into the lower range (and moving one
of the 'copy' registers, which isn't used as frequently, into the higher range)
decreases the code size of __arch_chacha20_blocks_nostack() by 5%.

- Eric