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Date:   Fri, 20 May 2022 01:12:04 +0200
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     tytso@mit.edu, hch@lst.de, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] random: convert to using fops->read_iter()
Message-ID: <YobOxLKnRDyYChXS@zx2c4.com>
References: <20220519193133.194138-1-axboe@kernel.dk>
 <20220519193133.194138-2-axboe@kernel.dk>
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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.

Jason