Re: [PATCH] string.h: Mark 34 functions with __must_check

Dan Carpenter
2019-10-09T18:48:36Z

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@oracle.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