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

Petr Mladek
2021-11-25T08:20:14Z

On Wed 2021-11-24 09:42:13, Josh Poimboeuf wrote:
> 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@suse.cz/
> > 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.

Grr, sure. It was off-by-one error on my side. /o\

Thanks for explanation.

Best Regards,
Petr