linux-kernel - Re: objtool clac/stac handling change..

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Message-ID: <20200703222516.GW2786714@ZenIV.linux.org.uk>
Date:   Fri, 3 Jul 2020 23:25:16 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Christophe Leroy <christophe.leroy@....fr>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: objtool clac/stac handling change..

On Fri, Jul 03, 2020 at 02:41:43PM -0700, Andy Lutomirski wrote:

> I still feel like the ex_handler-automatically-does-CLAC thing is an
> optimization that isn't worth it.  Once we pull our heads out of the
> giant pile of macros and inlined functions, we're talking about
> changing:

> clac; jmp.  But on the flip side, the jump folding pattern looks
> better like this:
> 
> unsafe_uaccess_begin();
> if (unsafe_get_user(...))
>   goto fail;
> if (unsafe_get_user(...))
>   goto fail;
> unsafe_uaccess_end();
> 
> fail:
>   unsafe_uaccess_end();
> 
> than like:
> 
> unsafe_uaccess_begin();
> if (unsafe_get_user(...))
>   goto fail;
> if (unsafe_get_user(...))
>   goto fail;
> unsafe_uaccess_end();
> 
> fail:
>   /* not unsafe_uaccess_end(); because unsafe_get_user() has
> conditional-CLAC semantics */

First of all, user_access_begin() itself can bloody well fail.  So you need
to handle that as well.  And then it becomes nowhere near as pretty.

We can pretend that it's normal C; however, that's not true at all - there
are shitloads of things you can't do in such areas, starting with "call anything
other than a very small list of functions".  It's not a normal C environment
at all.

My problem is not with having AC turned off in exception handler - it leads
to saner patterns, no arguments here.  I'm not happy with doing doing that
on *every* exception, with no way to specify whether it should or should not
be done.  It's not like it would've cost us anything to be able to specify
that - we have the third argument of _ASM_EXTABLE_HANDLE(), after all.