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

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Message-ID: <9e52895227515143bf3cd9197876ff1ed596682b.camel@linux.ibm.com>
Date:   Tue, 04 May 2021 09:39:40 +0200
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Arnd Bergmann <arnd@...db.de>
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 Mon, 2021-05-03 at 18:07 +0200, Arnd Bergmann wrote:
> On Fri, Apr 30, 2021 at 1:16 PM Niklas Schnelle <schnelle@...ux.ibm.com> wrote:
> > From: Niklas Schnelle <niklas@...ani.de>
> > 
> > This is version 4 of my attempt to get rid of a clang
> > -Wnull-pointer-arithmetic warning for the use of PCI_IOBASE in
> > asm-generic/io.h. This was originally found on s390 but should apply to
> > all platforms leaving PCI_IOBASE undefined while making use of the inb()
> > and friends helpers from asm-generic/io.h.
> > 
> > This applies cleanly and was compile tested on top of v5.12 for the
> > previously broken ARC, nds32, h8300 and risc-v architecture
> > 
> > I did boot test this only on x86_64 and s390x the former implements
> > inb() itself while the latter would emit a WARN_ONCE() but no drivers
> > use inb().
> 
> This looks all fine to me, but with the merge window open right now, I
> can't add it into linux-next yet, and it wouldn't qualify as a bugfix for 5.13.
> 
> Please resend them to me after -rc1 is out so I can merge it for
> 5.14 through the asm-generic tree.

Thanks for the great feedback I appreciate it. Will do the resend of
course.

> 
> Please add two small changes to the changelog texts:
> 
> - for patch 3, please include a 'Link: tag' to the lore archive of the
>   previous discussion, that should cover any questions that people
>   may have

Done

> 
> - 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.



> 
>        Arnd