On Thu, Jul 28, 2022 at 4:14 PM Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Thu, Jul 28, 2022 at 5:35 AM Guo Ren <guoren@xxxxxxxxxx> wrote:
> >
> > Hi Palmer,
> >
> > The warning is from a clang problem.
> >
> > drivers/net/wireguard/queueing.c:
> > static void __wg_prev_queue_enqueue(struct prev_queue *queue, struct
> > sk_buff *skb)
> > {
> > WRITE_ONCE(NEXT(skb), NULL);
> > WRITE_ONCE(NEXT(xchg_release(&queue->head, skb)), skb);
> > }
> >
> > The queue->head is 64bit pointer size.
> >
> > #define __xchg_relaxed(ptr, new, size) \
> > ({ \
> > __typeof__(ptr) __ptr = (ptr); \
> > __typeof__(new) __new = (new); \
> > __typeof__(*(ptr)) __ret; \
> > switch (size) { \
> > case 2: { \
> > ... Clang shouldn't give warning from here, because code won't enter the path.
> > break; \
> > } \
> > case 4: \
> > ...
> > break; \
> > case 8: \
> > ... The case would enter this path.
> > break; \
> > default: \
> > BUILD_BUG(); \
> > } \
> > __ret; \
> > })
>
> I assume it's this warning you are referring to?
>
> >> drivers/net/wireguard/queueing.c:68:18: warning: cast to 'typeof (*((__ai_ptr)))' (aka 'struct sk_buff *') from smaller integer type 'unsigned int' [-Wint-to-pointer-cast]
> WRITE_ONCE(NEXT(xchg_release(&queue->head, skb)), skb);
>
> I don't consider this a bug in clang, it just performs the normal type checking
> before dead code elimination and complains about code that clearly violates
> the type rules.
>
> I would suggest you split out the 16-bit xchg() into a properly typed inline
> function and add type casts when calling it.
Okay, I would try that style.
> In fact, I would love to
> completely eliminate the 8-bit and 16-bit cases from the regular xchg()
> and cmpxchg() interface and require all callers to explicitly call the
> xchg16()/cmpxchg16() instead, as we require for cmpxchg64() on 32-bit
> architectures already. This is something to do for another time though.
>
> > > include/asm-generic/io.h:547:31: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
> > > val = __raw_readb(PCI_IOBASE + addr);
> > > ~~~~~~~~~~ ^
> > > include/asm-generic/io.h:560:61: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
> > > val = __le16_to_cpu((__le16 __force)__raw_readw(PCI_IOBASE + addr));
>
> Not your bug, but I see that CONFIG_MMU=n risc-v has the same bug that s390 has
> with missing I/O space support. The correct workaround for this is to mark all
> drivers using PCI I/O space as 'depends on HAS_IO_PORT' or similar and then
> leaving out the definitions from the asm-generic header. Niklas Schnelle has
> spent a lot of time working on patches for this, but they are somewhat stuck
> in review. If RISC-V has the same problem, I hope we can get more people
> interested in it. I think OpenRISC and C-Sky have this as well, but I'm not
> sure if there is any plan to upstream clang support for those.
C-SKY hasn't any plan to support clang, as I know.
>
> Arnd
--
Best Regards
Guo Ren
