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Date:   Mon, 18 Jul 2022 23:18:40 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Johannes Wikner <kwikner@ethz.ch>,
        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        Jann Horn <jannh@google.com>, "H.J. Lu" <hjl.tools@gmail.com>,
        Joao Moreira <joao.moreira@intel.com>,
        Joseph Nuzman <joseph.nuzman@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Juergen Gross <jgross@suse.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>, samitolvanen@google.com
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
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On Mon, Jul 18, 2022 at 10:44:14PM +0200, Thomas Gleixner wrote:

> > I do think that for generality, the "-mforce-function-padding" option
> > should perhaps take as an argument how much padding (and how much
> > alignment) to force:
> >
> >     -mforce-function-padding=5:16
> >
> > would force 5 bytes of minimum padding, and align functions to 16
> > bytes. It should be easy to generate (no more complexity than your
> > current one) by just making the output do
> >
> >         .skip 5,0xcc
> >         .palign 4,0xcc
> >
> > and now you can specify that you only need X bytes of padding, for example.
> 
> Yes, I know. But it was horrible enough to find the right spot in that
> gcc maze. Then I was happy that I figured how to add the boolean
> option. I let real compiler people take care of the rest. HJL???
> 
> And we need input from the Clang folks because their CFI work also puts
> stuff in front of the function entry, which nicely collides.

Right, I need to go look at the latest kCFI patches, that sorta got
side-tracked for working on all the retbleed muck :/

Basically kCFI wants to preface every (indirect callable) function with:

__cfi_\func:
	int3
        movl $0x12345678, %rax
        int3
        int3
\func:
        endbr
\func_direct:

Ofc, we can still put the whole:

	sarq	$5, PER_CPU_VAR(__x86_call_depth);
	jmp	\func_direct

thing in front of that. But it does somewhat destroy the version I had
that only needs the 10 bytes padding for the sarq.