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Subject: Re: [RFC PATCH 00/17] objtool: add base support for arm64
To:     Mark Brown <broonie@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Mark Rutland <mark.rutland@arm.com>,
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        Catalin Marinas <catalin.marinas@arm.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
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        Will Deacon <will@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Kees Cook <keescook@chromium.org>
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From:   "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Message-ID: <bebccb15-1195-c004-923e-74d8444250e1@linux.microsoft.com>
Date:   Fri, 22 Jan 2021 15:15:12 -0600
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On 1/22/21 11:43 AM, Mark Brown wrote:
> On Thu, Jan 21, 2021 at 12:54:52PM -0600, Josh Poimboeuf wrote:
> 
>> 2) The shadow stack idea sounds promising -- how hard would it be to
>>    make a prototype reliable unwinder?
> 
> In theory it doesn't look too hard and I can't see a particular reason
> not to try doing this - there's going to be edge cases but hopefully for
> reliable stack trace they're all in areas where we would be happy to
> just decide the stack isn't reliable anyway, things like nesting which
> allocates separate shadow stacks for each nested level for example.
> I'll take a look.
> 

I am a new comer to this discussion and I am learning. Just have some
questions. Pardon me if they are obvious or if they have already been
asked and answered.

Doesn't Clang already have support for a shadow stack implementation for ARM64?
We could take a look at how Clang does it.

Will there not be a significant performance hit? May be, some of it can be
mitigated by using a parallel shadow stack rather than a compact one.

Are there any longjmp style situations in the kernel where the stack is
unwound by several frames? In these cases, the shadow stack must be unwound
accordingly.

Madhavan

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