linux-kernel - Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups

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Message-ID: <4553dfb68285a686737969e117bf48cc@overdrivepizza.com>
Date:   Tue, 15 Feb 2022 15:38:33 -0800
From:   Joao Moreira <joao@...rdrivepizza.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        X86 ML <x86@...nel.org>, hjl.tools@...il.com,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        andrew.cooper3@...rix.com, LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        llvm@...ts.linux.dev
Subject: Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups

>> 
>> Mostly so that we can deduce the address of the trap from the 
>> retpoline
>> site, also the above has a fairly high chance of using jcc.d32 which 
>> is
>> actually larger than jcc.d8+ud2.
> 
> Ah, yeah, that's an interesting point.
> 
> Still, I worry about finding ways to convinces Clang to emit precisely
> cmp/je/trap/call, but I guess we'll catch it immediately if it doesn't.
> :P

This can probably be done more easily/precisely if implemented directly
in the compiler's arch-specific backend. At least for x86 it wasn't a
hassle to emit a defined sequence of instructions in the past. The price
is that it will require a pass specific to each supported architecture,
but I guess this isn't that bad.

Perhaps this is discussion for a different mailing list, idk... but
just pointing that it is not a huge wall.