linux-kernel - Re: [RFC][PATCH 1/6] x86: Annotate _THIS_IP

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Message-ID: <YZz3VUh2czlD1aWQ@hirez.programming.kicks-ass.net>
Date:   Tue, 23 Nov 2021 15:14:45 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        jpoimboe@...hat.com, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, ndesaulniers@...gle.com,
        keescook@...omium.org, samitolvanen@...gle.com
Subject: Re: [RFC][PATCH 1/6] x86: Annotate _THIS_IP_

On Tue, Nov 23, 2021 at 01:53:49PM +0000, Mark Rutland wrote:
> On Mon, Nov 22, 2021 at 06:03:02PM +0100, Peter Zijlstra wrote:
> > In order to find _THIS_IP_ code references in objtool, annotate them.
> 
> Just to check my understanding, IIUC this is because in later patches
> you'll look at text relocations to spot missing ENDBRs, and when doing
> so you need to filter out _THIS_IP_ instances, since those don't need an
> ENDBR. Is that right?
> 
> Just checking I haven't missed some other concern that might apply to
> arm64's BTI (Branch Target Identifier), which are analagous to ENDBR.

Correct; since _THIS_IP_ is never used for control flow (afaik, let's
hope to $deity etc..) we can ignore any relocation resulting from it
(lots!).