linux-kernel - Re: [PATCH 1/4] lib/crypto: Add ML-DSA verification support

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Message-ID: <2755899.1763728901@warthog.procyon.org.uk>
Date: Fri, 21 Nov 2025 12:41:41 +0000
From: David Howells <dhowells@...hat.com>
To: Eric Biggers <ebiggers@...nel.org>
Cc: dhowells@...hat.com, linux-crypto@...r.kernel.org,
    Herbert Xu <herbert@...dor.apana.org.au>,
    Luis Chamberlain <mcgrof@...nel.org>,
    Petr Pavlu <petr.pavlu@...e.com>, Daniel Gomez <da.gomez@...nel.org>,
    Sami Tolvanen <samitolvanen@...gle.com>,
    "Jason A . Donenfeld" <Jason@...c4.com>,
    Ard Biesheuvel <ardb@...nel.org>,
    Stephan Mueller <smueller@...onox.de>,
    Lukas Wunner <lukas@...ner.de>,
    Ignat Korchagin <ignat@...udflare.com>, keyrings@...r.kernel.org,
    linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] lib/crypto: Add ML-DSA verification support

Eric Biggers <ebiggers@...nel.org> wrote:

> On Thu, Nov 20, 2025 at 01:55:18PM +0000, David Howells wrote:
> > Eric Biggers <ebiggers@...nel.org> wrote:
> > 
> > > +	/* Compute d = (c mod 2^32) * (q^-1 mod 2^32). */
> > > +	s32 d = (s32)c * QINV_MOD_R;
> > 
> > Hmmm...  is "(s32)c" actually "(c mod 2^32)"?  Should that be:
> > 
> > 	u32 d = (u32)c * QINV_MOD_R;
> > 
> > This is followed up by casting 'd' to "s64".  I don't think that should
> > sign-extend it, but...
> 
> It selects the representative in the range [INT32_MIN, INT32_MAX],
> rather than the representative in the range [0, UINT32_MAX].  The sign
> extension is intentional.

I'm concerned about the basis on which it becomes positive or negative.  It
looks like the sign bit ends up being chosen arbitrarily.

> > > +		/* Reduce to [0, q), then tmp = w'_1 = UseHint(h, w'_Approx) */
> > 
> > Bracket mismatch.  "[0, q]"
> 
> It's intentional, since it denotes a mathematical range.  Elsewhere I
> used the words "the range" explicitly, so I'll add that above too.  (Or
> maybe reword it differently.)

I meant you have an opening square bracket and a closing round bracket in
"[0, q)".

David