linux-kernel - Re: [PATCH v4 0/3] asm-generic/io.h: Silence -Wnull-pointer-arithmetic warning on PCI

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Date:   Tue, 4 May 2021 09:42:42 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>
Cc:     Vineet Gupta <vgupta@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        "open list:SYNOPSYS ARC ARCHITECTURE" 
        <linux-snps-arc@...ts.infradead.org>,
        sparclinux <sparclinux@...r.kernel.org>
Subject: Re: [PATCH v4 0/3] asm-generic/io.h: Silence -Wnull-pointer-arithmetic
 warning on PCI_IOBASE

On Tue, May 4, 2021 at 9:40 AM Niklas Schnelle <schnelle@...ux.ibm.com> wrote:
> On Mon, 2021-05-03 at 18:07 +0200, Arnd Bergmann wrote:
> > - for the risc-v patch, I would suggest explaining that this fixes
> >   an existing runtime bug, not just a compiler error:
> >   | This is already broken, as accessing a fixed I/O port number of
> >   | an ISA device on NOMMU RISC-V would turn into a NULL pointer
> >   | dereference.
> >   Feel free to either copy this, or use your own explanation.
>
> I mixed the above in with the current commit message:
>
>     Without MMU support PCI_IOBASE is left undefined because PCI_IO_END is
>     VMEMMAP_START. Nevertheless the in*()/out*() helper macros are left
>     defined with uses of PCI_IOBASE. At the moment this only compiles
>     because asm-generic/io.h defines PCI_IOBASE as 0 if it is undefined and
>     so at macro expansion PCI_IOBASE is defined. This leads to compilation
>     errors when asm-generic/io.h is changed to leave PCI_IOBASE undefined.
>     More importantly it is currently broken at runtime, as accessing a fixed
>     I/O port number of an ISA device on NOMMU RISC-V would turn into a NULL
>     pointer dereference. Instead only define the in*()/out*() helper macros
>     with MMU support and fall back to the asm-generic/io.h helper stubs
>     otherwise.

Looks good, thanks. Maybe split into two or three paragraphs for readability.

     Arnd