linux-kernel - Re: [PATCH v5 09/12] crypto: ecdsa - Rename keylen to bufsize where necessary

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Date: Fri, 8 Mar 2024 10:48:42 +0300
From: Vitaly Chikunov <vt@...linux.org>
To: Jarkko Sakkinen <jarkko@...nel.org>,
	Stefan Berger <stefanb@...ux.ibm.com>
Cc: keyrings@...r.kernel.org, linux-crypto@...r.kernel.org,
	herbert@...dor.apana.org.au, davem@...emloft.net,
	linux-kernel@...r.kernel.org, saulo.alessandre@....jus.br,
	lukas@...ner.de
Subject: Re: [PATCH v5 09/12] crypto: ecdsa - Rename keylen to bufsize where
 necessary

Jarkko,

On Thu, Mar 07, 2024 at 02:20:12PM -0500, Stefan Berger wrote:
> On 3/7/24 14:13, Jarkko Sakkinen wrote:
> > On Thu Mar 7, 2024 at 12:22 AM EET, Stefan Berger wrote:
> > > In some cases the name keylen does not reflect the purpose of the variable
> > > anymore once NIST P521 is used but it is the size of the buffer. There-
> > > for, rename keylen to bufsize where appropriate.
> > > 
> > > Signed-off-by: Stefan Berger <stefanb@...ux.ibm.com>
> > > Tested-by: Lukas Wunner <lukas@...ner.de>
> > > ---
> > >   crypto/ecdsa.c | 12 ++++++------
> > >   1 file changed, 6 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c
> > > index 4daefb40c37a..4e847b59622a 100644
> > > --- a/crypto/ecdsa.c
> > > +++ b/crypto/ecdsa.c
> > > @@ -35,8 +35,8 @@ struct ecdsa_signature_ctx {
> > >   static int ecdsa_get_signature_rs(u64 *dest, size_t hdrlen, unsigned char tag,
> > >   				  const void *value, size_t vlen, unsigned int ndigits)
> > >   {
> > > -	size_t keylen = ndigits * sizeof(u64);
> > 
> > nit: still don't get why "* sizeof(u64)" would ever be more readable
> > thean "* 8".
> 
> Because existing code in crypto uses sizeof(u64) when converting ndigits to
> number of bytes and '8' is not used for converting to bytes. Do we need to
> change this now ? No, I think it's better to conform to existing code.

`sizeof(u64)` is easily read as `8` by reviewers, but just `8` will
require inline comments because it's magic number isn't it? So this will
not even decrease number of letters.

`sizeof(u64)` is self-documenting code and you don't even need to
interpret it as `8` for review as you don't need number from any
sizeof(struct ..).

Also, possible we need to calculate number of bits in the big number, so
this would become `* 8 * 8`, in that case how would you distinguish
omission of one `* 8` by a typo.

Overall it's quite common in the whole tree

  linux/torvalds$ git grep -e '\* sizeof(u64)' -e 'sizeof(u64) \*' | wc -l
  551

So this is perhaps acceptable and depends on point of view. crypto
subsystem coders seems to prefer not to save on letters and type
`sizeof(u64)`.

Thanks,

> 
> # grep -rI ndigits crypto/ | grep sizeof\(u64\)
> crypto/ecrdsa.c:        unsigned int ndigits = req->dst_len / sizeof(u64);
> crypto/ecrdsa.c:            req->dst_len != ctx->curve->g.ndigits *
> sizeof(u64) ||
> crypto/ecrdsa.c:        vli_from_be64(r, sig + ndigits * sizeof(u64),
> ndigits);
> crypto/ecrdsa.c:            ctx->curve->g.ndigits * sizeof(u64) !=
> ctx->digest_len)
> crypto/ecrdsa.c:            ctx->key_len != ctx->curve->g.ndigits *
> sizeof(u64) * 2)
> crypto/ecrdsa.c:        ndigits = ctx->key_len / sizeof(u64) / 2;
> crypto/ecrdsa.c:        vli_from_le64(ctx->pub_key.y, ctx->key + ndigits *
> sizeof(u64),
> crypto/ecrdsa.c:        return ctx->pub_key.ndigits * sizeof(u64);
> crypto/ecdh.c:      params.key_size > sizeof(u64) * ctx->ndigits)
> crypto/ecc.c:   size_t len = ndigits * sizeof(u64);
> crypto/ecc.c:           num_bits = sizeof(u64) * ndigits * 8 + 1;
> crypto/ecdsa.c: size_t bufsize = ndigits * sizeof(u64);
> crypto/ecdsa.c: size_t bufsize = ctx->curve->g.ndigits * sizeof(u64);
> crypto/ecdsa.c: ndigits = DIV_ROUND_UP(digitlen, sizeof(u64));
> 
>    Stefan
> 
> > 
> > BR, Jarkko