linux-kernel - Re: [PATCH v5 06/12] S.A.R.A.: WX protection

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Date:   Sun, 7 Jul 2019 17:49:35 +0200
From:   Salvatore Mesoraca <s.mesoraca16@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-kernel@...r.kernel.org,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-mm@...ck.org, linux-security-module@...r.kernel.org,
        Brad Spengler <spender@...ecurity.net>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Christoph Hellwig <hch@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        PaX Team <pageexec@...email.hu>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        James Morris <jmorris@...ei.org>
Subject: Re: [PATCH v5 06/12] S.A.R.A.: WX protection

Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Sat, Jul 06, 2019 at 12:54:47PM +0200, Salvatore Mesoraca wrote:
>
> > +#define sara_warn_or_return(err, msg) do {           \
> > +     if ((sara_wxp_flags & SARA_WXP_VERBOSE))        \
> > +             pr_wxp(msg);                            \
> > +     if (!(sara_wxp_flags & SARA_WXP_COMPLAIN))      \
> > +             return -err;                            \
> > +} while (0)
> > +
> > +#define sara_warn_or_goto(label, msg) do {           \
> > +     if ((sara_wxp_flags & SARA_WXP_VERBOSE))        \
> > +             pr_wxp(msg);                            \
> > +     if (!(sara_wxp_flags & SARA_WXP_COMPLAIN))      \
> > +             goto label;                             \
> > +} while (0)
>
> No.  This kind of "style" has no place in the kernel.
>
> Don't hide control flow.  It's nasty enough to reviewers,
> but it's pure hell on anyone who strays into your code while
> chasing a bug or doing general code audit.  In effect, you
> are creating your oh-so-private C dialect and assuming that
> everyone who ever looks at your code will start with learning
> that *AND* incorporating it into their mental C parser.
> I'm sorry, but you are not that important.
>
> If it looks like a function call, a casual reader will assume
> that this is exactly what it is.  And when one is scanning
> through a function (e.g. to tell if handling of some kind
> of refcounts is correct, with twentieth grep through the
> tree having brought something in your code into the view),
> the last thing one wants is to switch between the area-specific
> C dialects.  Simply because looking at yours is sandwiched
> between digging through some crap in drivers/target/ and that
> weird thing in kernel/tracing/, hopefully staying limited
> to 20 seconds of glancing through several functions in your
> code.
>
> Don't Do That.  Really.

I understand your concerns.
The first version of SARA didn't use these macros,
they were added because I was asked[1] to do so.

I have absolutely no problems in reverting this change.
I just want to make sure that there is agreement on this matter.
Maybe Kees can clarify his stance.

Thank you for your suggestions.

[1] https://lkml.kernel.org/r/CAGXu5jJuQx2qOt_aDqDQDcqGOZ5kmr5rQ9Zjv=MRRCJ65ERfGw@mail.gmail.com