linux-kernel - Re: [PATCH V4 16/17] riscv: compat: Add COMPAT Kbuild skeletal support

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Date:   Tue, 1 Feb 2022 10:36:01 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Guo Ren <guoren@...nel.org>
Cc:     Christoph Hellwig <hch@....de>,
        Christoph Hellwig <hch@...radead.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Arnd Bergmann <arnd@...db.de>,
        Anup Patel <anup@...infault.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        liush <liush@...winnertech.com>, Wei Fu <wefu@...hat.com>,
        Drew Fustini <drew@...gleboard.org>,
        Wang Junqiang <wangjunqiang@...as.ac.cn>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-csky@...r.kernel.org,
        linux-s390 <linux-s390@...r.kernel.org>,
        sparclinux <sparclinux@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Parisc List <linux-parisc@...r.kernel.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH V4 16/17] riscv: compat: Add COMPAT Kbuild skeletal support

On Tue, Feb 1, 2022 at 10:13 AM Guo Ren <guoren@...nel.org> wrote:
> On Tue, Feb 1, 2022 at 3:45 PM Christoph Hellwig <hch@....de> wrote:
> > On Mon, Jan 31, 2022 at 09:50:58PM +0800, Guo Ren wrote:
> > > On Mon, Jan 31, 2022 at 8:26 PM Christoph Hellwig <hch@...radead.org> wrote:
> > > >
> > > > Given that most rv64 implementations can't run in rv32 mode, what is the
> > > > failure mode if someone tries it with the compat mode enabled?
> > > A static linked simple hello_world could still run on a non-compat
> > > support hardware. But most rv32 apps would meet different userspace
> > > segment faults.
> > >
> > > Current code would let the machine try the rv32 apps without detecting
> > > whether hw support or not.
> >
> > Hmm, we probably want some kind of check for not even offer running
> > rv32 binaries.  I guess trying to write UXL some time during early
> > boot and catching the resulting exception would be the way to go?
>
> Emm... I think it's unnecessary. Free rv32 app running won't cause
> system problem, just as a wrong elf running. They are U-mode
> privileged.

While it's not a security issue, I think it would be helpful to get a
user-readable error message and a machine-readable /proc/cpuinfo
flag to see if a particular system can run rv32 binaries rather than
relying on SIGILL to kill a process.

        Arnd