linux-kernel - Re: [tip: x86/core] objtool: Find unused ENDBR instructions

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Date:   Fri, 18 Mar 2022 11:49:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     David Laight <David.Laight@...lab.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-tip-commits@...r.kernel.org" 
        <linux-tip-commits@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [tip: x86/core] objtool: Find unused ENDBR instructions

On Thu, Mar 17, 2022 at 03:22:54PM -0700, Josh Poimboeuf wrote:
> On Tue, Mar 15, 2022 at 03:39:52PM +0000, David Laight wrote:
> > From: Peter Zijlstra
> > > 
> > > objtool: Find unused ENDBR instructions
> > > 
> > > Find all ENDBR instructions which are never referenced and stick them
> > > in a section such that the kernel can poison them, sealing the
> > > functions from ever being an indirect call target.
> > 
> > Thought, what happens if the only indirect call is from
> > code in a module?
> 
> Then <boom>, I guess.  Is it safe to assume in-tree modules don't need
> to do indirect calls to exported functions?  I guess we'll find out :-)

So exported functions will keep their ENDBR. Specifically, their address
is taken by the EXPORT_SYMBOL thing.

Sealing them might work, but let's not do that just now ;-)

Any unexported function discovered through kallsyms OTOH... those
deservedly will go *boom*.