linux-kernel - Re: [PATCH v2 1/3] base: mark 'no

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Date:   Tue, 27 Jul 2021 13:24:49 -0700
From:   Bill Wendling <morbo@...gle.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org
Subject: Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

On Tue, Jul 27, 2021 at 1:22 PM Bill Wendling <morbo@...gle.com> wrote:
>
> On Tue, Jul 27, 2021 at 1:17 PM Segher Boessenkool
> <segher@...nel.crashing.org> wrote:
> >
> > On Tue, Jul 27, 2021 at 07:59:24PM +0200, Greg Kroah-Hartman wrote:
> > > On Tue, Jul 27, 2021 at 10:39:49AM -0700, Nick Desaulniers wrote:
> > > > I think warn_unused_result should only really be used for functions
> > > > where the return value should be used 100% of the time.
> > >
> > > I too want a shiny new pony.
> > >
> > > But here in the real world, sometimes you have functions that for 99% of
> > > the users, you do want them to check the return value, but when you use
> > > them in core code or startup code, you "know" you are safe to ignore the
> > > return value.
> > >
> > > That is the case here.  We have other fun examples of where people have
> > > tried to add error handling to code that runs at boot that have actually
> > > introduced security errors and they justify it with "but you have to
> > > check error values!"
> > >
> > > > If there are
> > > > cases where it's ok to not check the return value, consider not using
> > > > warn_unused_result on function declarations.
> > >
> > > Ok, so what do you do when you have a function like this where 99.9% of
> > > the users need to check this?  Do I really need to write a wrapper
> > > function just for it so that I can use it "safely" in the core code
> > > instead?
> > >
> > > Something like:
> > >
> > > void do_safe_thing_and_ignore_the_world(...)
> > > {
> > >       __unused int error;
> > >
> > >       error = do_thing(...);
> > > }
> > >
> > > Or something else to get the compiler to be quiet about error being set
> > > and never used?
> >
> > The simplest is to write
> >         if (do_thing()) {
> >                 /* Nothing here, we can safely ignore the return value
> >                  * here, because of X and Y and I don't know, I have no
> >                  * idea actually why we can in this example.  Hopefully
> >                  * in real code people do have a good reason :-)
> >                  */
> >         }
> >
> > which should work in *any* compiler, doesn't need any extension, is
> > quite elegant, and encourages documenting why we ignore the return
> > value here.
> >
> Or better still, use sysfs_create_link_nowarn() instead of
> sysfs_create_link(). We'll just have to take the "__must_check"
> attribute off the sysfs_create_link_nowarn() declaration.
>
Strike that. I mistook the nowarn.
-bw