linux-kernel - Re: [PATCH v10 3/4] random: introduce generic vDSO getrandom() implementation

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Message-ID: <87a649v0vr.ffs@tglx>
Date:   Tue, 29 Nov 2022 23:42:16 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        linux-crypto@...r.kernel.org, linux-api@...r.kernel.org,
        x86@...nel.org, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
        Carlos O'Donell <carlos@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v10 3/4] random: introduce generic vDSO getrandom()
 implementation

Jason!

On Tue, Nov 29 2022 at 22:06, Jason A. Donenfeld wrote:
> +/**
> + * struct vdso_rng_data - vdso RNG state information
> + * @generation:	a counter representing the number of RNG reseeds

A counter

> + * @is_ready:	whether the RNG is initialized

Signals whether ...

> + */
> +struct vdso_rng_data {
> +	unsigned long	generation;
> +	bool		is_ready;
> +};
> +
> +
> +#define MEMCPY_AND_ZERO_SRC(type, dst, src, len) do { \
> +	while (len >= sizeof(type)) { \
> +		__put_unaligned_t(type, __get_unaligned_t(type, src), dst); \
> +		__put_unaligned_t(type, 0, src); \
> +		dst += sizeof(type); \
> +		src += sizeof(type); \
> +		len -= sizeof(type); \
> +	} \
> +} while (0)

I'd appreciate it if you go back to the code I suggested to you and
compare and contrast it in terms of readability.

> +
> +static void memcpy_and_zero_src(void *dst, void *src, size_t len)
> +{
> +	if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) {
> +		if (IS_ENABLED(CONFIG_64BIT))
> +			MEMCPY_AND_ZERO_SRC(u64, dst, src, len);
> +		MEMCPY_AND_ZERO_SRC(u32, dst, src, len);
> +		MEMCPY_AND_ZERO_SRC(u16, dst, src, len);
> +	}
> +	MEMCPY_AND_ZERO_SRC(u8, dst, src, len);
> +}
> +
> +/**
> + * __cvdso_getrandom_data - generic vDSO implementation of getrandom() syscall
> + * @rng_info:		describes state of kernel RNG, memory shared with kernel
> + * @buffer:		destination buffer to fill with random bytes
> + * @len:		size of @buffer in bytes
> + * @flags:		zero or more GRND_* flags
> + * @opaque_state:	a pointer to an opaque state area

NIT. Please start the explanations with an uppercase letter

> +		/*
> +		 * Set @state->pos to beyond the end of the batch, so that the batch is refilled
> +		 * using the new key.
> +		 */
> +		state->pos = sizeof(state->batch);
> +	}
> +

This one is odd:

> +	len = ret;

@ret is not modified after the initialization at the top of the
function:

> +	ssize_t ret = min_t(size_t, INT_MAX & PAGE_MASK /* = MAX_RW_COUNT */, len);

so I really had to go up a page and figure out what the story is.

> +
> +	/* Since the batch was just refilled, set the position back to 0 to indicate a full batch. */
> +	state->pos = 0;
> +	goto more_batch;

Aside of the nitpicks above, thank you very much for making this
comprehensible.

The comments are well done and appreciated and I'm pretty sure that this
part:

> +	in_use = READ_ONCE(state->in_use);
> +	if (unlikely(in_use))
> +		goto fallback_syscall;
> +	WRITE_ONCE(state->in_use, true);

was very much induced by writing those comments :)

Thanks,

        tglx