Re: [PATCH 1/2] random: convert to using fops->read_iter()

Jason A. Donenfeld
2022-05-19T23:22:18Z

On Fri, May 20, 2022 at 1:21 AM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 5/19/22 5:20 PM, Jason A. Donenfeld wrote:
> > On Fri, May 20, 2022 at 01:12:04AM +0200, Jason A. Donenfeld wrote:
> >> Hi Jens,
> >>
> >> On Thu, May 19, 2022 at 01:31:32PM -0600, Jens Axboe wrote:
> >>> for (;;) {
> >>> chacha20_block(chacha_state, output);
> >>> if (unlikely(chacha_state[12] == 0))
> >>> ++chacha_state[13];
> >>>
> >>> block_len = min_t(size_t, len, CHACHA_BLOCK_SIZE);
> >>> - left = copy_to_user(ubuf, output, block_len);
> >>> - if (left) {
> >>> - ret += block_len - left;
> >>> + block_len = copy_to_iter(output, block_len, to);
> >>> + if (!block_len)
> >>> break;
> >>> - }
> >>>
> >>> - ubuf += block_len;
> >>> ret += block_len;
> >>> len -= block_len;
> >>> - if (!len)
> >>> - break;
> >>>
> >>> BUILD_BUG_ON(PAGE_SIZE % CHACHA_BLOCK_SIZE != 0);
> >>> if (ret % PAGE_SIZE == 0) {
> >>> if (signal_pending(current))
> >>> break;
> >>> cond_resched();
> >>> }
> >>> }
> >>
> >> This isn't quite the same, is it? Before, it would immediately break out
> >> of the loop on any short copy. Now, it will only break out on a zero
> >> copy, which means it's possible that ret % PAGE_SIZE == 0, and there'll
> >> be an unnecessary cond_resched() before copy_to_iter() runs again and
> >> then breaks.
> >
> > Maybe something like the below would do the trick?
> >
> >
> > static ssize_t get_random_bytes_user(struct iov_iter *to)
> > {
> > size_t block_len, copied, ret = 0, len = iov_iter_count(to);
> > u32 chacha_state[CHACHA_STATE_WORDS];
> > u8 output[CHACHA_BLOCK_SIZE];
> >
> > if (!len)
> > return 0;
> >
> > /*
> > * Immediately overwrite the ChaCha key at index 4 with random
> > * bytes, in case userspace causes copy_to_user() below to sleep
> > * forever, so that we still retain forward secrecy in that case.
> > */
> > crng_make_state(chacha_state, (u8 *)&chacha_state[4], CHACHA_KEY_SIZE);
> > /*
> > * However, if we're doing a read of len <= 32, we don't need to
> > * use chacha_state after, so we can simply return those bytes to
> > * the user directly.
> > */
> > if (len <= CHACHA_KEY_SIZE) {
> > ret = copy_to_iter(&chacha_state[4], len, to);
> > goto out_zero_chacha;
> > }
> >
> > for (;;) {
> > chacha20_block(chacha_state, output);
> > if (unlikely(chacha_state[12] == 0))
> > ++chacha_state[13];
> >
> > block_len = min_t(size_t, len, CHACHA_BLOCK_SIZE);
> > copied = copy_to_iter(output, block_len, to);
> > ret += copied;
> > if (block_len != copied)
> > break;
> > len -= copied;
>
> Yep, that looks good! Do you still want a v2?

Yes please, thanks.

Jason