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Date:   Tue, 27 Nov 2018 09:50:49 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andy Lutomirski <luto@kernel.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 Cartwright <julia@ni.com>, Jessica Yu <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 Tue, Nov 27, 2018 at 09:43:30AM +0100, Peter Zijlstra wrote:
> Now; if I'm not mistaken, the below @site is in fact @regs->ip - 1, no?
> 
> We already patched site with INT3, which is what we just trapped on. So
> we could in fact write something like:
> 
> static void static_call_bp_handler(struct pt_regs *regs, void *data)
> {
> 	struct static_call_bp_data *scd = data;
> 
> 	switch (data->type) {
> 	case CALL_INSN: /* emulate CALL instruction */
> 		regs->sp -= sizeof(unsigned long);
> 		*(unsigned long *)regs->sp = regs->ip + CALL_INSN_SIZE - 1;
> 		regs->ip = data->func;
> 		break;
> 
> 	case JMP_INSN: /* emulate JMP instruction */
> 		regs->ip = data->func;
> 		break;
> 	}
> }

> 	handler_data = (struct static_call_bp_data){
> 		.type = IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) ? CALL_INSN : JMP_INSN,
> 		.func = func,
> 	};

Heck; check this:

static void static_call_bp_handler(struct pt_regs *regs, void *data)
{
#ifdef CONFIG_HAVE_STATIC_CALL_INLINE

	/* emulate CALL instruction */
	regs->sp -= sizeof(unsigned long);
	*(unsigned long *)regs->sp = regs->ip + CALL_INSN_SIZE - 1;
	regs->ip = data;

#else /* !CONFIG_HAVE_STATIC_CALL_INLINE */

	/* emulate JMP instruction */
	regs->ip = data;

#endif
}