linux-kernel - Re: [PATCH v2] gpiolib: acpi: use correct format characters

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Date:   Sat, 19 Mar 2022 15:54:43 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Bill Wendling <morbo@...gle.com>
Cc:     Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v2] gpiolib: acpi: use correct format characters

So I think that clang warning is only annoying, not helpful, but:

On Sat, Mar 19, 2022 at 3:22 PM Bill Wendling <morbo@...gle.com> wrote:
>
> diff --git a/drivers/gpio/gpiolib-acpi.c b/drivers/gpio/gpiolib-acpi.c
> index a5495ad31c9c..92dd9b8784f2 100644
> --- a/drivers/gpio/gpiolib-acpi.c
> +++ b/drivers/gpio/gpiolib-acpi.c
> @@ -388,9 +388,9 @@ static acpi_status acpi_gpiochip_alloc_event(struct acpi_resource *ares,
>
>         if (pin <= 255) {
>                 char ev_name[5];
> -               sprintf(ev_name, "_%c%02hhX",
> +               sprintf(ev_name, "_%c%02X",

This part I approve of.

>                         agpio->triggering == ACPI_EDGE_SENSITIVE ? 'E' : 'L',
> -                       pin);
> +                       (unsigned char)pin);

But this cast seems pointless and wrong.

Casts in general are bad, and should be avoided unless there's a real
reason for them. And that reason doesn't seem to exist. We don't
actually want to truncate the value of 'pin', and just a few lines
earlier actually checked that it is in range.

And if 'pin' can't be negative - it comes from a 'u16' table
dereference - but even if it could have been that would have been a
different bug here anyway (and should have been fixed by tightening
the check).

So the cast doesn't add anything - not for humans, and not for a
compiler that could just optimize it away because it saw the range
check.

End result: just fix the pointless 'hh' in the print specifier. It
doesn't add anything, and only causes problems. Anybody who uses '%02X
to print a byte should only use it for byte values - and the code
already does.

Of course, the _reason_ for this all was a warning that was pointless
to begin with, and should never have existed. Clang was not smart
enough to take the range knowledge that it _could_ have taken into
account, and instead wrote out a completely bogus warning.

It's completely bogus not just because clang didn't do a sufficiently
good job of range analysis - it's completely bogus because a 'varargs'
function DOES NOT TAKE arguments of type 'char'.

So the *only* reason to use '%hhX' in the first place is that you
*want* the sprintf() to actually limit the value to a byte for you
(possibly because you have a signed char, know it will sign-extend to
'int', and want to limit it back to 8 bits).

If you *actually* had a 'unsigned char' to begin with, you'd be
completely insane to use %hhX. It's just pointless.

So warning that '%hhX' is paired with an 'int' is all just completely
mindless and wrong.

              Linus