Hi Thomas,

On Sat, Nov 26, 2022 at 12:08:41AM +0100, Thomas Gleixner wrote:

Jason!

On Thu, Nov 24 2022 at 17:55, Jason A. Donenfeld wrote:

quoted

+++ b/arch/x86/entry/vdso/vgetrandom-chacha.S
+/*
+ * 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:

Basic or not.

Heh, FYI I didn't mean "basic" here as in "doesn't need a review", but
just that it's a straightforward technique and doesn't do any
complicated multiblock pyrotechnics (which frankly aren't really
needed).

This needs a Reviewed-by from someone who understands SSE2
and ChaCha20 before this can go anywhere near the x86 tree.

No problem. I'll see to it that somebody qualified gives this a review.

quoted

+#include <linux/kernel.h>

Why do you need kernel.h here?

Turns out I don't, thanks.

quoted

+static __always_inline ssize_t
+getrandom_syscall(void *buffer, size_t len, unsigned int flags)

static __always_inline ssize_t getrandom_syscall(void *buffer, size_t len, unsigned int flags)

please. We expanded to 100 quite some time ago.

Some kernel-doc compliant comment for this would be appreciated as well.

Will do.

quoted

+{
+	long ret;
+
+	asm ("syscall" : "=a" (ret) :
+	     "0" (__NR_getrandom), "D" (buffer), "S" (len), "d" (flags) :
+	     "rcx", "r11", "memory");
+
+	return ret;
+}
+
+#define __vdso_rng_data (VVAR(_vdso_rng_data))
+
+static __always_inline const struct vdso_rng_data *__arch_get_vdso_rng_data(void)
+{
+	if (__vdso_data->clock_mode == VDSO_CLOCKMODE_TIMENS)
+		return (void *)&__vdso_rng_data +
+		       ((void *)&__timens_vdso_data - (void *)&__vdso_data);
+	return &__vdso_rng_data;

So either bite the bullet and  write it:

	if (__vdso_data->clock_mode == VDSO_CLOCKMODE_TIMENS)
		return (void *)&__vdso_rng_data + ((void *)&__timens_vdso_data - (void *)&__vdso_data);

Seems fine to me. I'll write it like that.

quoted

+/*
+ * Generates a given positive number of block of ChaCha20 output with nonce=0,
+ * and does not write to any stack or memory outside of the parameters passed
+ * to it. This way, we don't need to worry about stack data leaking into forked
+ * child processes.

Please use proper kernel-doc

quoted

+ */
+static __always_inline void __arch_chacha20_blocks_nostack(u8 *dst_bytes, const u32 *key, u32 *counter, size_t nblocks)
+{
+	extern void chacha20_blocks_nostack(u8 *dst_bytes, const u32 *key, u32 *counter, size_t nblocks);
+	return chacha20_blocks_nostack(dst_bytes, key, counter, nblocks);

The above aside, can you please explain the value of this __arch_()
wrapper?

It's just voodoo for no value because it hands through the arguments
1:1. So where are you expecting that that __arch...() version of this is
any different than invoking the architecture specific version of
chacha20_blocks_nostack().

I'll just name the assembly function with __arch...(). The idea behind
the wrapper was just to keep all of the non-generic code called from the
generic code prefixed with __arch_, but there's no reason I need to name
it like that from C alone. Will fix for v8.

Thanks again,
Jason