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

From: Daniel Kiss <Daniel.Kiss@arm.com>
To: Mark Rutland <Mark.Rutland@arm.com>
Cc: Marco Elver <elver@google.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Borislav Petkov <bp@alien8.de>, Rong Chen <rong.a.chen@intel.com>,
	kernel test robot <lkp@intel.com>,
	"Li, Philip" <philip.li@intel.com>, x86-ml <x86@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Kees Cook <keescook@chromium.org>,
	Masahiro Yamada <masahiroy@kernel.org>,
	kasan-dev <kasan-dev@googlegroups.com>,
	Momchil Velikov <Momchil.Velikov@arm.com>
Subject: Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING e6eb15c9ba3165698488ae5c34920eea20eaa38e
Date: Thu, 17 Sep 2020 11:16:32 +0000	[thread overview]
Message-ID: <B5C0B917-BB17-460B-9CC3-51E7D49F04A7@arm.com> (raw)
In-Reply-To: <20200917110427.GA98505@C02TD0UTHF1T.local>

> On 17 Sep 2020, at 13:04, Mark Rutland <mark.rutland@arm.com> wrote:
>
> 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?
Yes, that is correct.

>
> 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. :/
I don’t know but I have motivation to backport all PAC/BTI fixes to LLVM11.0.1.

>
> Thanks,
> Mark.

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