linux-kernel - Re: [clang] stack protector and f1f029c7bf

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Message-ID: <EE0F328A-4B60-44D9-A726-4CF5276CDBE7@zytor.com>
Date:   Fri, 25 May 2018 10:57:56 -0700
From:   hpa@...or.com
To:     Nick Desaulniers <ndesaulniers@...gle.com>, tstellar@...hat.com
CC:     Alistair Strachan <astrachan@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>,
        Matthias Kaehlcke <mka@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>, sedat.dilek@...il.com,
        LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...gle.com>
Subject: Re: [clang] stack protector and f1f029c7bf

On May 25, 2018 10:49:28 AM PDT, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>On Fri, May 25, 2018 at 10:35 AM Tom Stellard <tstellar@...hat.com>
>wrote:
>> On 05/25/2018 10:31 AM, Nick Desaulniers wrote:
>> > On Fri, May 25, 2018 at 9:53 AM <hpa@...or.com> wrote:
>> >> On May 25, 2018 9:46:42 AM PDT, Nick Desaulniers <
>ndesaulniers@...gle.com>
>> > wrote:
>> >>> On Fri, May 25, 2018 at 9:33 AM <hpa@...or.com> wrote:
>> >>>> On May 25, 2018 9:27:40 AM PDT, Nick Desaulniers
>> >>> <ndesaulniers@...gle.com> wrote:
>> >>> When you say
>> >>>
>> >>>> It still should be available as as inline, however, but now
>"extern
>> >>> inline".
>> >>>
>> >>> Am I understanding correctly that native_save_fl should be
>inlined
>into
>> >>> all
>> >>> call sites (modulo the problematic pv_irq_ops.save_fl case)? 
>Because
>> >>> for
>> >>> these two assembly implementations, it's not, but maybe there's
>> >>> something
>> >>> missing in my implementation?
>> >
>> >> Yes, that's what "extern inline" means. Maybe it needs a must
>inline
>> > annotation, but that's really messed up.
>> >
>
>> What about doing something like suggested here:
>> https://bugs.llvm.org/show_bug.cgi?id=37512#c17
>
>> This would keep the definition in C and make it easier for compilers
>> to inline.
>
>The GCC docs for __attribute__((naked)) seem to imply this is a machine
>specific constraint (of which x86 is not listed):
>https://gcc.gnu.org/onlinedocs/gcc-4.7.2/gcc/Function-Attributes.html
>
>But let's try with source:
>https://godbolt.org/g/aJ4gZB
>
>Clang errors:
><source>:3:3: error: non-ASM statement in naked function is not
>supported
>   unsigned long flags;
>   ^
>
>Is it valid to use assembly to place the results in %rax and mark the c
>function somehow?
>
>gcc doesn't support this attribute until 4.9 (but we can add a feature
>test
>for attributes with gcc (unlike builtins)), but warns that:
>
>warning: ‘naked’ attribute directive ignored [-Wattributes]
>
>gcc 8.1 and trunk inserts a `ud2` instruction (what?!) (let me see if I
>can
>repro outside of godbolt, and will file a bug report).

No, we found that the paravirt code can do the wrong thing for a C implementation.

Nick, could you forward the list of problems so we all have it?
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.