netdev - Re: [PATCH bpf-next v3 1/2] bpf: add skcipher API support to TC/XDP programs

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Date: Wed, 1 Nov 2023 15:59:56 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org,
 linux-crypto@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
 Andrii Nakryiko <andrii@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
 Mykola Lysenko <mykolal@...com>, Vadim Fedorenko <vadfed@...a.com>,
 "David S. Miller" <davem@...emloft.net>,
 Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH bpf-next v3 1/2] bpf: add skcipher API support to TC/XDP
 programs

On 11/1/23 3:50 PM, Vadim Fedorenko wrote:
>>> +static int bpf_crypto_skcipher_crypt(struct crypto_sync_skcipher *tfm,
>>> +                     const struct bpf_dynptr_kern *src,
>>> +                     struct bpf_dynptr_kern *dst,
>>> +                     const struct bpf_dynptr_kern *iv,
>>> +                     bool decrypt)
>>> +{
>>> +    struct skcipher_request *req = NULL;
>>> +    struct scatterlist sgin, sgout;
>>> +    int err;
>>> +
>>> +    if (crypto_sync_skcipher_get_flags(tfm) & CRYPTO_TFM_NEED_KEY)
>>> +        return -EINVAL;
>>> +
>>> +    if (__bpf_dynptr_is_rdonly(dst))
>>> +        return -EINVAL;
>>> +
>>> +    if (!__bpf_dynptr_size(dst) || !__bpf_dynptr_size(src))
>>> +        return -EINVAL;
>>> +
>>> +    if (__bpf_dynptr_size(iv) != crypto_sync_skcipher_ivsize(tfm))
>>> +        return -EINVAL;
>>> +
>>> +    req = skcipher_request_alloc(&tfm->base, GFP_ATOMIC);
>>
>> Doing alloc per packet may kill performance. Is it possible to optimize it 
>> somehow? What is the usual size of the req (e.g. the example in the selftest)?
>>
> 
> In ktls code aead_request is allocated every time encryption is invoked, see 
> tls_decrypt_sg(), apparently per skb. Doesn't look like performance
> killer. For selftest it's only sizeof(struct skcipher_request).

ktls is doing the en/decrypt on the userspace behalf to compensate the cost.

When this kfunc is used in xdp to decrypt a few bytes for each packet and then 
XDP_TX out, this extra alloc will be quite noticeable. If the size is usually 
small, can it be done in the stack memory?