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In-Reply-To: <20200527183517.uhph2pvnkyf6c5p7@google.com>
From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 27 May 2020 21:01:45 +0200
Message-ID: <CAK8P3a1tSKYeDDmAK3o6thazjbua-KmxRNQPX9448qjgp8iV=g@mail.gmail.com>
Subject: Re: [PATCH] arm64: disable -fsanitize=shadow-call-stack for big-endian
To:     Fangrui Song <maskray@google.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Kees Cook <keescook@chromium.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Tom Stellard <tstellar@redhat.com>
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On Wed, May 27, 2020 at 8:35 PM 'Fangrui Song' via Clang Built Linux
<clang-built-linux@googlegroups.com> wrote:
> On 2020-05-27, Arnd Bergmann wrote:
> >On Wed, May 27, 2020 at 7:28 PM 'Nick Desaulniers' via Clang Built
> >Linux <clang-built-linux@googlegroups.com> wrote:
> >>
> >> On Wed, May 27, 2020 at 8:24 AM Mark Rutland <mark.rutland@arm.com> wrote:
> >> >
> >> > On Wed, May 27, 2020 at 03:39:46PM +0200, Arnd Bergmann wrote:
> >> > > clang-11 and earlier do not support -fsanitize=shadow-call-stack
> >> > > in combination with -mbig-endian, but the Kconfig check does not
> >> > > pass the endianess flag, so building a big-endian kernel with
> >> > > this fails at build time:
> >> > >
> >> > > clang: error: unsupported option '-fsanitize=shadow-call-stack' for target 'aarch64_be-unknown-linux'
> >> > >
> >> > > Change the Kconfig check to let Kconfig figure this out earlier
> >> > > and prevent the broken configuration. I assume this is a bug
> >> > > in clang that needs to be fixed, but we also have to work
> >> > > around existing releases.
> >> > >
> >> > > Fixes: 5287569a790d ("arm64: Implement Shadow Call Stack")
> >> > > Link: https://bugs.llvm.org/show_bug.cgi?id=46076
> >> > > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> >> >
> >> > I suspect this is similar to the patchable-function-entry issue, and
> >> > this is an oversight that we'd rather fix toolchain side.
> >> >
> >> > Nick, Fangrui, thoughts?
> >>
> >> Exactly, Fangrui already has a fix: https://reviews.llvm.org/D80647.
> >> Thanks Fangrui!
> >
> >Ok, great! I had opened the bug first so I could reference it in the
> >commit changelog, it seems the fix came fast than I managed to
> >send out the kernel workaround.
> >
> >Do we still want the kernel workaround anyway to make it work
> >with older clang versions, or do we expect to fall back to not
> >use the integrated assembler for the moment?
>
> We can condition it on `CLANG_VERSION >= 100001` (assuming Tom (CCed)
> is happy (and there is still time) cherrying pick the two commits https://bugs.llvm.org/show_bug.cgi?id=46076 to clang 10.0.1)

Good idea. I assume we will keep requiring fairly recent clang versions
for a while now, so chances are that 10.1 or 11.0 becomes the minimum
supported version not too far in the future and then the workaround can
be dropped again.

       Arnd