linux-kernel - Re: [RFC PATCH 31/56] x86/alternative: Prepend nops with retpolines

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Message-ID: <20251016111041.GP1386988@noisy.programming.kicks-ass.net>
Date: Thu, 16 Oct 2025 13:10:41 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: David Kaplan <david.kaplan@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H . Peter Anvin" <hpa@...or.com>, Alexander Graf <graf@...zon.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 31/56] x86/alternative: Prepend nops with retpolines

On Thu, Oct 16, 2025 at 01:07:17PM +0200, Peter Zijlstra wrote:
> On Mon, Oct 13, 2025 at 09:34:19AM -0500, David Kaplan wrote:
> > When patching retpolines, nops may be required for padding such as when
> > turning a 5-byte direct call into a 2-byte indirect call.  Previously,
> > these were appended at the end so the code becomes "call *reg;nop;nop;nop"
> > for example.  This was fine because it's always going from a larger
> > instruction to a smaller one.
> > 
> > But this is a problem if the sequence is transformed from a 2-byte indirect
> > to the 5-byte direct call version at runtime because when the called
> > function returns, it will be in the middle of the 5-byte call instruction.
> > 
> > To fix this, prepend the nops instead of appending them.  Consequently, the
> > return site of the called function is always the same.
> > 
> 
> So this results in:
> 
> NOP3; call *%r11

Also possible:

  lfence; call *r11

(which is why we needed 6 bytes for reg>8)

> And you're saying a task can be on the other side of that call and then
> return lines up. But what if the task is preempted right after that
> NOP3?
> 
> Same for all the alternative patching; what ensures no task is currently
> having a register state that is in the middle of things?