linux-kernel - Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e

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Date:   Thu, 17 Sep 2020 12:04:27 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Daniel Kiss <Daniel.Kiss@....com>
Cc:     Marco Elver <elver@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Rong Chen <rong.a.chen@...el.com>,
        kernel test robot <lkp@...el.com>,
        "Li, Philip" <philip.li@...el.com>, x86-ml <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Momchil Velikov <Momchil.Velikov@....com>
Subject: Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING
 e6eb15c9ba3165698488ae5c34920eea20eaa38e

On Wed, Sep 16, 2020 at 10:30:42PM +0100, Daniel Kiss wrote:
> 
>     Thanks for the summary -- yeah, that was my suspicion, that some
>     attribute was being lost somewhere. And I think if we generalize this,
>     and don't just try to attach "frame-pointer" attr to the function, we
>     probably also solve the BTI issue that Mark still pointed out with
>     these module_ctor/dtors.
> 
>     I was trying to see if there was a generic way to attach all the
>     common attributes to the function generated here:
>     https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/Utils/
>     ModuleUtils.cpp#L122
>     -- but we probably can't attach all attributes, and need to remove a
>     bunch of them again like the sanitizers (or alternatively just select
>     the ones we need). But, I'm still digging for the function that
>     attaches all the common attributes…
> 
> 
> We had the problem with not just the sanitisers.  Same problem pops with
> functions 
> that created elsewhere in clang (e.g _clang_call_terminate ) or llvm.
> 
> In case of BTI the flag even controllable by function attributes which makes it
> more trickier so
> the module flags found the only reliable way to pass this information down. 
> Scanning existing functions is fragile for data only compilation units for
> example.
> 
> Our solution, not generic enough but might help.
> https://reviews.llvm.org/D85649 

Thanks for the pointer -- I've subscribed to that now.

Just to check my understanding, is the issue that generated functions
don't implicitly get function attributes like
"branch-target-enforcement", and so the BTI insertion pass skips those?

I'm guessing that it's unlikely this'll be fixed for an LLVM 11 release?
On the kernel side I guess we'll have to guard affected features as
being incompatible with BTI until there's a viable fix on the compiler
side. :/

Thanks,
Mark.