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Date:   Tue, 7 May 2019 13:30:50 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     David Laight <David.Laight@ACULAB.COM>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
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        Nicolai Stange <nstange@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
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Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions
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On Tue, May 07, 2019 at 09:18:51AM +0000, David Laight wrote:
> From: Peter Zijlstra
> > Sent: 07 May 2019 09:58
> ...
> > +	/*
> > +	 * When we're here from kernel mode; the (exception) stack looks like:
> > +	 *
> > +	 * 4*4(%esp) - <previous context>
> > +	 * 3*4(%esp) - flags
> > +	 * 2*4(%esp) - cs
> > +	 * 1*4(%esp) - ip
> > +	 * 0*4(%esp) - orig_eax
> 
> Am I right in thinking that this is the only 'INT3' stack frame that
> needs to be 'fiddled' with?
> And that the 'emulate a call instruction' has verified that is the case??
> So the %cs is always the kernel %cs.

Only the INT3 thing needs 'the gap', but the far bigger change here is
that kernel frames now have a complete pt_regs set and all sorts of
horrible crap can go away.

For 32bit 'the gap' happens naturally when building a 5 entry frame. Yes
it is possible to build a 5 entry frame on top of the old 3 entry one,
but why bother...