[v2] gpiolib: acpi: use correct format characters

When compiling with -Wformat, clang emits the following warning:

drivers/gpio/gpiolib-acpi.c:393:4: warning: format specifies type
'unsigned char' but the argument has type 'int' [-Wformat]
                        pin);
                        ^~~

The types of these arguments are unconditionally defined, so this patch
updates the format character to the correct ones casts to unsigned to
retain the behavior or the "hh" modifier..

Link: https://github.com/ClangBuiltLinux/linux/issues/378
Signed-off-by: Bill Wendling <morbo@google.com>
---
v2 - Cast "pin" to retain the same width as the original.
---
 drivers/gpio/gpiolib-acpi.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

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

On Sat, Mar 19, 2022 at 3:22 PM Bill Wendling <morbo@google.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

On Sat, Mar 19, 2022 at 3:54 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> So warning that '%hhX' is paired with an 'int' is all just completely
> mindless and wrong.

Sadly, I can see a different bogus warning reason why people would
want to use '%02hhX'.

Again, the *sane* thing from a human perspective is to use '%02X. But
if the compiler doesn't do any range analysis at all, it could decide
that "Oh, that print format could need up to 8 bytes of space in the
result". Using '%02hhX' would cut that down to two.

And since we use

        char ev_name[5];

and currently use "_%c%02hhX" as the format string, even a compiler
that doesn't notice that "pin <= 255" test that guards this all will
go "ok, that's at most 4 bytes and the final NUL termination, so it's
fine".

While a compiler - like gcc - that only sees that the original source
of the 'pin' value is a 'unsigned short' array, and then doesn't take
the "pin <= 255" into account, will warn like this:

    drivers/gpio/gpiolib-acpi.c: In function 'acpi_gpiochip_request_interrupt':
    drivers/gpio/gpiolib-acpi.c:206:24: warning: '%02X' directive
writing between 2 and 4 bytes into a region of size 3
[-Wformat-overflow=]
       sprintf(ev_name, "_%c%02X",
                            ^~~~
    drivers/gpio/gpiolib-acpi.c:206:20: note: directive argument in
the range [0, 65535]

because gcc isn't being very good at that argument range analysis either.

In other words, the original use of 'hhx' was bogus to begin with, and
due to *another* compiler warning being bad, and we had that bad code
being written back in 2016 to work around _that_ compiler warning
(commit e40a3ae1f794: "gpio: acpi: work around false-positive
-Wstring-overflow warning").

Sadly, two different bad compiler warnings together does not make for
one good one.

It just makes for even more pain.

End result: I think the simplest and cleanest option is simply this:

  --- a/drivers/gpio/gpiolib-acpi.c
  +++ b/drivers/gpio/gpiolib-acpi.c
  @@ -387,8 +387,8 @@ static acpi_status
acpi_gpiochip_alloc_event(struct acpi_resource *ares,
        pin = agpio->pin_table[0];

        if (pin <= 255) {
  -             char ev_name[5];
  -             sprintf(ev_name, "_%c%02hhX",
  +             char ev_name[8];
  +             sprintf(ev_name, "_%c%02X",
                        agpio->triggering == ACPI_EDGE_SENSITIVE ? 'E' : 'L',
                        pin);
                if (ACPI_SUCCESS(acpi_get_handle(handle, ev_name, &evt_handle)))

which undoes that '%hhX' change for gcc, and replaces it with just
using a slightly bigger stack allocation. It's not like a 5-byte
allocation is in any way likely to have saved any actual stack, since
all the other variables in that function are 'int' or bigger.

False-positive compiler warnings really do make people write worse
code, and that's a problem. But on a scale of bad code, I feel that
extending the buffer trivially is better than adding a pointless cast
that literally makes no sense.

At least in this case the end result isn't unreadable or buggy. We've
had several cases of bad compiler warnings that caused changes that
were actually horrendously wrong.

                  Linus

On Sat, Mar 19, 2022 at 3:55 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> So I think that clang warning is only annoying, not helpful, but:
>
> On Sat, Mar 19, 2022 at 3:22 PM Bill Wendling <morbo@google.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.
>
You're right. I was trying to ensure that the patch didn't change
behavior. But the cast achieves nothing. Thanks!

-bw

> 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