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From:   Bill Wendling <morbo@google.com>
Date:   Tue, 27 Jul 2021 13:22:21 -0700
Message-ID: <CAGG=3QVWxdYnjZshsYVwf+jVj8Mb9=44SZA64cL0g414JncWGw@mail.gmail.com>
Subject: Re: [PATCH v2 1/3] base: mark 'no_warn' as unused
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <nathan@kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-toolchains@vger.kernel.org
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On Tue, Jul 27, 2021 at 1:17 PM Segher Boessenkool
<segher@kernel.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.

-bw