linux-kernel - Re: [PATCH] string.h: Mark 34 functions with __must

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Date:   Wed, 9 Oct 2019 21:45:18 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Markus Elfring <Markus.Elfring@....de>,
        kernel-janitors@...r.kernel.org,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Joe Perches <joe@...ches.com>,
        Kees Cook <keescook@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] string.h: Mark 34 functions with __must_check

On Wed, Oct 09, 2019 at 09:31:41AM -0700, Nick Desaulniers wrote:
> On Wed, Oct 9, 2019 at 7:30 AM Dan Carpenter <dan.carpenter@...cle.com> wrote:
> >
> > On Wed, Oct 09, 2019 at 04:21:20PM +0200, Rasmus Villemoes wrote:
> > > On 09/10/2019 15.56, Dan Carpenter wrote:
> > > > That's because glibc strlen is annotated with __attribute_pure__ which
> > > > means it has no side effects.
> > >
> > > I know, except it has nothing to do with glibc headers. Just try the
> > > same thing in the kernel. gcc itself knows this about __builtin_strlen()
> > > etc. If anything, we could annotate some of our non-standard functions
> > > (say, memchr_inv) with __pure - then we'd both get the Wunused-value in
> > > the nonsense cases, and allow gcc to optimize or reorder the calls.
> >
> > Huh.  You're right.  GCC already knows.  So this patch is pointless like
> > you say.
> 
> Is it? None of the functions in include/linux/string.h are currently
> marked __pure today.

I've already embarrassed myself with my ignorance once so I may as well
keep talking now...  GCC did complain about the unused result even
though we don't declare them as __pure.  So GCC rule must have this rule
built in.

We were discussing in a different thread that standard says that
memcpy() pointers can't be NULL (even when we're copying zero bytes) so
GCC will assume that's true.  If you have:

	memcpy(foo, bar, 0);

	if (foo)
		*foo = 0;


GCC will sometimes remove the condition.  This doesn't affect the kernel
because we use -fno-delete-null-pointer-checks.
https://groups.google.com/forum/#!msg/syzkaller-netbsd-bugs/8B4CIKN0Xz8/wRvIUWxiAgAJ

Weird, huh?

regards,
dan carpenter