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From:   Bill Wendling <morbo@google.com>
Date:   Fri, 18 Mar 2022 11:46:14 -0700
Message-ID: <CAGG=3QV9+kakiTYSe+Oh1RTnSW1PLZNNO-KJaiQtincheQK+VQ@mail.gmail.com>
Subject: Re: [PATCH] gpiolib: acpi: use correct format characters
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        Linus Walleij <linus.walleij@linaro.org>,
        Bartosz Golaszewski <brgl@bgdev.pl>,
        Nathan Chancellor <nathan@kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, llvm@lists.linux.dev,
        Joe Perches <joe@perches.com>
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On Fri, Mar 18, 2022 at 11:41 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Fri, Mar 18, 2022 at 11:29 AM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > Should we add a note diagnostic to clang suggesting the explicit cast
> > as one method of silencing the warning?
>
> On the compiler side, I would love to see warnings about the ambiguity
> of the sign of 'char' in the general case.
>
> That said, I tried to add that to 'sparse' long long ago, and couldn't
> make it work sanely. All the approaches I tried all get _way_ too many
> false positives.
>
> I tried to come up with some way of figuring out "this code acts
> differently depending on whether 'char' is signed or not" and warning
> about it, and never could.
>
> And I suspect the same is true even for the much moire limited case of
> only format warnings.
>
> Because it's a *bad* idea to use '%d' (or almost any other format
> specifier) together with a 'char' argument, but only if you don't know
> the range of the char argument.
>
> But the other side of the argument is that quite often, people *do*
> know the range of the 'char' argument. If your 'char' type thing comes
> from some array or string that you control, and you used 'char' simply
> because you know you have small values (typical example: use it for an
> array of booleans etc), then it would be very annoying if the compiler
> warned you about using '%d'.
>
> There is no reason to use '%hhd' when you know your data range is [0,1].
>
> So honestly, I don't think you can come up with a sane warning that
> doesn't cause *way* too many false positives and just annoys people.
>
> I'd love to be proven wrong. In fact, I'd _really_ love to be proven
> wrong for that generic case. The "sometimes 'char' is signed,
> sometimes it is unsigned, and it depends on the architecture and the
> compiler flags" can be a real problem.
>
My first thought is that this might be better suited for a static
analyzer, like clang-tidy, that can do deeper analysis on code. It
might still be difficult to weed out all of the false positives, but
could be useful for specific offenders.

-bw