linux-kernel - Re: [RFC PATCH 2/2] livepatch: Clear relocation targets on a module removal

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Message-ID: <20190905132344.byfybt6s42cajtfz@treble>
Date:   Thu, 5 Sep 2019 08:23:44 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] livepatch: Clear relocation targets on a module
 removal

On Thu, Sep 05, 2019 at 01:19:06PM +0200, Jiri Kosina wrote:
> On Thu, 5 Sep 2019, Petr Mladek wrote:
> 
> > > I don't have a number, but it's very common to patch a function which 
> > > uses jump labels or alternatives.
> > 
> > Really? My impression is that both alternatives and jump_labels
> > are used in hot paths. I would expect them mostly in core code
> > that is always loaded.
> > 
> > Alternatives are often used in assembly that we are not able
> > to livepatch anyway.
> > 
> > Or are they spread widely via some macros or inlined functions?
> 
> All the indirect jumps are turned into alternatives when retpolines are in 
> place.

Actually in C code those are done by the compiler as calls/jumps to
__x86_indirect_thunk_*.

But there are still a bunch of paravirt patched instructions and
alternatives used throughout the kernel.

-- 
Josh