netdev - Re: [PATCH net-next 09/10] nvme-tcp: use crc32c() and skb_copy_and_crc32c_datagram

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Message-ID: <aCbAsCkTPMNE6Ogb@infradead.org>
Date: Thu, 15 May 2025 21:36:00 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Eric Biggers <ebiggers@...nel.org>
Cc: netdev@...r.kernel.org, linux-nvme@...ts.infradead.org,
	linux-sctp@...r.kernel.org, linux-rdma@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Daniel Borkmann <daniel@...earbox.net>,
	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
	Sagi Grimberg <sagi@...mberg.me>, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH net-next 09/10] nvme-tcp: use crc32c() and
 skb_copy_and_crc32c_datagram_iter()

On Sat, May 10, 2025 at 05:41:09PM -0700, Eric Biggers wrote:
> +static inline void nvme_tcp_ddgst_init(u32 *crcp)
>  {
> +	*crcp = ~0;
>  }

This helper looks really odd.  It coud just return the value, or
we could just assign it there using a seed define, e.g. this is what
XFS does:

#define XFS_CRC_SEED    (~(uint32_t)0)

nd that might in fact be worth lifting to common code with a good
comment on why all-d is used as the typical crc32 seed.

> +static inline void nvme_tcp_ddgst_final(u32 *crcp, __le32 *ddgst)
>  {
> +	*ddgst = cpu_to_le32(~*crcp);
> +}

Just return the big endian version calculated here?

> +static inline void nvme_tcp_hdgst(void *pdu, size_t len)
> +{
> +	put_unaligned_le32(~crc32c(~0, pdu, len), pdu + len);
>  }

This could also use the seed define mentioned above.

>  	recv_digest = *(__le32 *)(pdu + hdr->hlen);
> -	nvme_tcp_hdgst(queue->rcv_hash, pdu, pdu_len);
> +	nvme_tcp_hdgst(pdu, pdu_len);
>  	exp_digest = *(__le32 *)(pdu + hdr->hlen);
>  	if (recv_digest != exp_digest) {

This code looks really weird, as it samples the on-the-wire digest
first and then overwrites it.  I'd expect to just check the on-the-wire
on against one calculated in a local variable.

Sagi, any idea what is going on here?

Otherwise this looks great.  It's always good to get rid of the
horrendous crypto API calls.