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From: David Howells <dhowells@redhat.com>
In-Reply-To: <CAPAsAGx5Kmb4k1pyUn_vgSy+rhjfcWAadFerjBpsH00Ne3HCAA@mail.gmail.com>
References: <CAPAsAGx5Kmb4k1pyUn_vgSy+rhjfcWAadFerjBpsH00Ne3HCAA@mail.gmail.com> <147997746306.9316.227351186233378585.stgit@warthog.procyon.org.uk> <147997747766.9316.11606802798600210253.stgit@warthog.procyon.org.uk> <5323.1479986239@warthog.procyon.org.uk>
To: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: dhowells@redhat.com, jmorris@namei.org,
        Dmitry Kasatkin <dmitry.kasatkin@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, stable@vger.kernel.org,
        linux-security-module@vger.kernel.org, keyrings@vger.kernel.org,
        linux-ima-devel@lists.sourceforge.net
Subject: Re: [PATCH 2/2] MPI: Fix mpi_powm() when exponent is 0 and the result has no limbs
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Date: Thu, 24 Nov 2016 11:33:34 +0000
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Andrey Ryabinin <ryabinin.a.a@gmail.com> wrote:

> > David Howells <dhowells@redhat.com> wrote:
> >
> >> +             if (!rp) {
> >> +                     if (mpi_resize(res, 1) < 0)
> >
> > This is better done with RESIZE_IF_NEEDED().
> >
> 
> mpi_resize() is equal to RESIZE_IF_NEEDED(), it also checks for allocated space:
> 
> int mpi_resize(MPI a, unsigned nlimbs)
> {
> ....
>    if (nlimbs <= a->alloced)
>     return 0; /* no need to do it */

Hmmm...  In that case, should your patch use mpi_resize() rather than
RESIZE_IF_NEEDED()?  It's a trivial case that we should perhaps weed out much
earlier (ie. reject the key if exp<2 or mod<2), but it would make the object
file slightly smaller not to do the test twice.

David