linux-kernel - Re: [PATCH] arm64: disable -fsanitize=shadow-call-stack for big-endian

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Date:   Wed, 27 May 2020 11:35:17 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tom Stellard <tstellar@...hat.com>
Subject: Re: [PATCH] arm64: disable -fsanitize=shadow-call-stack for
 big-endian

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@...glegroups.com> wrote:
>>
>> On Wed, May 27, 2020 at 8:24 AM Mark Rutland <mark.rutland@....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@...db.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?
>
>      Arnd

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)