linux-kernel - Re: [PATCH 12/15] x86/static_call: Add out-of-line static call implementation

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Date:   Fri, 7 Jun 2019 10:38:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...LAB.COM>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 12/15] x86/static_call: Add out-of-line static call
 implementation

On Fri, Jun 07, 2019 at 06:13:58AM +0000, Nadav Amit wrote:
> > On Jun 5, 2019, at 6:08 AM, Peter Zijlstra <peterz@...radead.org> wrote:

> > +void arch_static_call_transform(void *site, void *tramp, void *func)
> > +{
> > +	unsigned char opcodes[CALL_INSN_SIZE];
> > +	unsigned char insn_opcode;
> > +	unsigned long insn;
> > +	s32 dest_relative;
> > +
> > +	mutex_lock(&text_mutex);
> > +
> > +	insn = (unsigned long)tramp;
> > +
> > +	insn_opcode = *(unsigned char *)insn;
> > +	if (insn_opcode != 0xE9) {
> > +		WARN_ONCE(1, "unexpected static call insn opcode 0x%x at %pS",
> > +			  insn_opcode, (void *)insn);
> > +		goto unlock;
> 
> This might happen if a kprobe is installed on the call, no?
> 
> I don’t know if you want to be more gentle handling of this case (or perhaps
> modify can_probe() to prevent such a case).
> 

yuck.. yes, that's something that needs consideration.