linux-kernel - Re: [PATCH 4/5] selinux: declare data arrays const

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Message-ID: <CAKwvOdkioR+7aebgzPu2Exe0oD9rwAeHK=CgM6vAkpBWdHnF2Q@mail.gmail.com>
Date:   Fri, 18 Feb 2022 09:24:39 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Christian Göttsche <cgzones@...glemail.com>,
        selinux@...r.kernel.org,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        Jeremy Kerr <jk@...econstruct.com.au>,
        "David S. Miller" <davem@...emloft.net>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        Yang Li <yang.lee@...ux.alibaba.com>,
        Austin Kim <austin.kim@....com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH 4/5] selinux: declare data arrays const

On Fri, Feb 18, 2022 at 8:13 AM Paul Moore <paul@...l-moore.com> wrote:
>
> On Thu, Feb 17, 2022 at 9:21 AM Christian Göttsche
> <cgzones@...glemail.com> wrote:
> >
> > diff --git a/security/selinux/include/initial_sid_to_string.h b/security/selinux/include/initial_sid_to_string.h
> > index 5d332aeb8b6c..915283cd89bd 100644
> > --- a/security/selinux/include/initial_sid_to_string.h
> > +++ b/security/selinux/include/initial_sid_to_string.h
> > @@ -1,5 +1,12 @@
> >  /* SPDX-License-Identifier: GPL-2.0 */
> > -static const char *initial_sid_to_string[] =
> > +
> > +#ifdef __SELINUX_GENHEADERS__
> > +# define const_qual
> > +#else
> > +# define const_qual const
> > +#endif
> > +
> > +static const char *const_qual initial_sid_to_string[] =
> >  {
> >         NULL,
> >         "kernel",
>
> Thanks for this Christian.  I generally like when we can const'ify
> things like this, but I'm not excited about the const_qual hack on
> core SELinux kernel code to satisfy genheaders.c.  I understand why it
> is needed, but I would rather clutter the genheaders.c code than the
> core SELinux kernel code.  If we can't cast away the const'ification
> in genheaders.c could we simply allocate duplicate arrays in
> genheaders.c and store the transformed strings into the new arrays?

Note: casting off const is UB. I've had to fix multiple bugs where
clang will drop writes to variables declared const but had const'ness
casted away.
-- 
Thanks,
~Nick Desaulniers