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Date:   Thu, 29 Nov 2018 07:11:22 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>, julia@ni.com, jeyu@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64
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On Thu, Nov 29, 2018 at 10:42:10AM +0100, Peter Zijlstra wrote:
> On Wed, Nov 28, 2018 at 10:05:54PM -0800, Andy Lutomirski wrote:
> 
> > >> +static void static_call_bp_handler(struct pt_regs *regs, void *_data)
> > >> +{
> > >> +    struct static_call_bp_data *data = _data;
> > >> +
> > >> +    /*
> > >> +     * For inline static calls, push the return address on the stack so the
> > >> +     * "called" function will return to the location immediately after the
> > >> +     * call site.
> > >> +     *
> > >> +     * NOTE: This code will need to be revisited when kernel CET gets
> > >> +     *       implemented.
> > >> +     */
> > >> +    if (data->ret) {
> > >> +        regs->sp -= sizeof(long);
> > >> +        *(unsigned long *)regs->sp = data->ret;
> > >> +    }
> > 
> > You can’t do this.  Depending on the alignment of the old RSP, which
> > is not guaranteed, this overwrites regs->cs.  IRET goes boom.
> 
> I don't get it; can you spell that out?

I don't quite follow that either.  Maybe Andy is referring to x86-32,
for which regs->sp isn't actually saved: see kernel_stack_pointer().

This code is 64-bit only so that's not a concern.

> The way I understand it is that we're at a location where a "E8 - Near
> CALL" instruction should be, and thus RSP should be the regular kernel
> stack, and the above simply does "PUSH ret", which is what that CALL
> would've done too.

Right.

-- 
Josh