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SubjectRe: [PATCH v2 4/4] s390: fix clang -Wpointer-sign warnigns in boot code
On Mon, Apr 22, 2019 at 7:52 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:

> > @@ -20,7 +20,7 @@ extern __u8 _ebc_tolower[256]; /* EBCDIC -> lowercase */
> > extern __u8 _ebc_toupper[256]; /* EBCDIC -> uppercase */
> >
> > static inline void
> > -codepage_convert(const __u8 *codepage, volatile __u8 * addr, unsigned long nr)
> > +codepage_convert(const __u8 *codepage, volatile char* addr, unsigned long nr)
> > {
> > if (nr-- <= 0)
> > return;
>
> There are many call sites of ASCEBC which is defined in terms of this
> function. Do they all use `char*`? grep shows an explicit cast to
> `unsigned char*` in drivers/s390/char/tape_std.c for example.

Generally speaking, the kernel is full of Wpointer-sign warnings, that's why
this warning is disabled in the top-level Makefile by default. My patch fixes
the ones in the s390 boot code that is not built with those default flags, but
I made no attempt to fix the rest of the kernel.

Fun fact: on most architectures, 'char' is signed, but on s390 and 32-bit
arm it is unsigned. The compiler treats 'char', 'unsigned char' and
'signed char'
as three distinct types here for that reason.

> > diff --git a/arch/s390/include/asm/lowcore.h b/arch/s390/include/asm/lowcore.h
> > index cc0947e08b6f..f3a637afd485 100644
> > --- a/arch/s390/include/asm/lowcore.h
> > +++ b/arch/s390/include/asm/lowcore.h
> > @@ -128,7 +128,7 @@ struct lowcore {
> > /* SMP info area */
> > __u32 cpu_nr; /* 0x0398 */
> > __u32 softirq_pending; /* 0x039c */
> > - __u32 preempt_count; /* 0x03a0 */
> > + __s32 preempt_count; /* 0x03a0 */
>
> This change is less obvious. Do you still have the warning for that
> this hunk fixes?

I can't find it now, but there are multiple users that would cause
this, like:

static inline void preempt_count_set(int pc)
{
int old, new;

do {
old = READ_ONCE(S390_lowcore.preempt_count);
new = (old & PREEMPT_NEED_RESCHED) |
(pc & ~PREEMPT_NEED_RESCHED);
} while (__atomic_cmpxchg(&S390_lowcore.preempt_count,
old, new) != old);
}


Arnd

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