linux-kernel - Re: [PATCH] tracepoint: Fix CFI failures with tp_stub

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Date:   Thu, 23 Mar 2023 14:42:20 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
        mhiramat@...nel.org, "Jose E. Marchesi" <jose.marchesi@...cle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] tracepoint: Fix CFI failures with tp_stub_func

On Thu, 23 Mar 2023 14:37:15 -0400
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> > Repeated again so Mathieu doesn't have to search for it.
> > 
> >      [ Note, this version does use undefined compiler behavior (assuming that
> >        a stub function with no parameters or return, can be called by a location
> >        that thinks it has parameters but still no return value. Static calls
> >        do the same thing, so this trick is not without precedent.
> > 
> >        There's another solution that uses RCU tricks and is more complex, but
> >        can be an alternative if this solution becomes an issue.
> > 
> >        Link: https://lore.kernel.org/lkml/20210127170721.58bce7cc@gandalf.local.home/
> >      ]  
> 
> Ugh. The last thing we need there is more RCU complexity. My brain is still recovering
> from fixing the last time the introduction of static calls special-cases ended up subtly
> breaking tracepoints.

Well, we can go back to your approach with doing the check in the iterator.
Setting it to 0x1UL I think is what you wanted (to know it's a removed
function and not a random NULL).

I could write that up if you want.

-- Steve