Re: [dm-devel] [PATCH 5.20 1/4] block: add bio_rewind() API

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Jun 24, 2022 at 10:12:52PM +0800, Ming Lei wrote:
> diff --git a/block/blk-crypto.c b/block/blk-crypto.c
> index a496aaef85ba..caae2f429fc7 100644
> --- a/block/blk-crypto.c
> +++ b/block/blk-crypto.c
> @@ -134,6 +134,21 @@ void bio_crypt_dun_increment(u64 dun[BLK_CRYPTO_DUN_ARRAY_SIZE],
>  	}
>  }
>  
> +/* Decrements @dun by @dec, treating @dun as a multi-limb integer. */
> +void bio_crypt_dun_decrement(u64 dun[BLK_CRYPTO_DUN_ARRAY_SIZE],
> +			     unsigned int dec)
> +{
> +	int i;
> +
> +	for (i = 0; dec && i < BLK_CRYPTO_DUN_ARRAY_SIZE; i++) {
> +		dun[i] -= dec;
> +		if (dun[i] > inc)
> +			dec = 1;
> +		else
> +			dec = 0;
> +	}
> +}

This doesn't compile.  Also this doesn't handle underflow into the next limb
correctly.  A correct version would be:

		u64 prev = dun[i];

		dun[i] -= dec;
		if (dun[i] > prev)
			dec = 1;
		else
			dec = 0;

- Eric