Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage

[Josh Poimboeuf]

On Mon, Nov 22, 2021 at 06:46:44PM +0100, Petr Mladek wrote:
> On Thu 2021-11-11 17:50:03, Josh Poimboeuf wrote:
> > On Wed, Nov 10, 2021 at 12:20:47PM +0000, David Laight wrote:
> > > > > Wouldn't moving part of a function to .text.cold (or .text.unlikely)
> > > > > generate the same problems with the stack backtrace code as the
> > > > > .text.fixup section you are removing had??
> > > > 
> > > > GCC can already split a function into func and func.cold today (or
> > > > worse: func, func.isra.N, func.cold, func.isra.N.cold etc..).
> > > > 
> > > > I'm assuming reliable unwind and livepatch know how to deal with this.
> > > 
> > > They'll have 'proper' function labels at the top - so backtrace
> > > stands a chance.
> > > Indeed you (probably) want it to output "func.irsa.n.cold" rather
> > > than just "func" to help show which copy it is in.  > 
> > > I guess that livepatch will need separate patches for each
> > > version of the function - which might be 'interesting' if
> > > all the copies actually need patching at the same time.
> > > You'd certainly want a warning if there seemed to be multiple
> > > copies of the function.
> > 
> > Hm, I think there is actually a livepatch problem here.
> > 
> > If the .cold (aka "child") function actually had a fentry hook then we'd
> > be fine.  Then we could just patch both "parent" and "child" functions
> > at the same time.  We already have the ability to patch multiple
> > functions having dependent interface changes.
> > 
> > But there's no fentry hook in the child, so we can only patch the
> > parent.
> > 
> > If the child schedules out, and then the parent gets patched, things can
> > go off-script if the child later jumps back to the unpatched version of
> > the parent, and then for example the old parent tries to call another
> > patched function with a since-changed ABI.
> 
> This thread seems to be motivation for the patchset
> https://lore.kernel.org/all/20211119090327.12811-1-mbenes@xxxxxxx/
> I am trying to understand the problem here, first. And I am
> a bit lost.
> 
> How exactly is child called in the above scenario, please?
> How could parent get livepatched when child is sleeping?
> 
> I imagine it the following way:
> 
>     parent_func()
>        fentry
> 
>        /* some parent code */
>        jmp child
> 	   /* child code */
> 	   jmp back_to_parent
>        /* more parent code */
>        ret

Right.

> In the above example, parent_func() would be on stack and could not
> get livepatched even when the process is sleeping in the child code.
> 
> The livepatching is done via ftrace. Only code with fentry could be
> livepatched. And code called via fentry must be visible on stack.

How would parent_func() be on the stack?  If it jumps to the child then
it leaves no trace on the stack.

-- 
Josh