Re: [RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool

Peter Zijlstra
2020-06-26T11:30:10Z

On Thu, Jun 25, 2020 at 03:40:42PM -0700, Sami Tolvanen wrote:

> > Not boot tested, but it generates the required sections and they look
> > more or less as expected, ymmv.

> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index a291823f3f26..189575c12434 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -174,7 +174,6 @@ config X86
> > select HAVE_EXIT_THREAD
> > select HAVE_FAST_GUP
> > select HAVE_FENTRY if X86_64 || DYNAMIC_FTRACE
> > - select HAVE_FTRACE_MCOUNT_RECORD
> > select HAVE_FUNCTION_GRAPH_TRACER
> > select HAVE_FUNCTION_TRACER
> > select HAVE_GCC_PLUGINS
>
> This breaks DYNAMIC_FTRACE according to kernel/trace/ftrace.c:
>
> #ifndef CONFIG_FTRACE_MCOUNT_RECORD
> # error Dynamic ftrace depends on MCOUNT_RECORD
> #endif
>
> And the build errors after that seem to confirm this. It looks like we might
> need another flag to skip recordmcount.

Hurm, Steve, how you want to do that?

> Anyway, since objtool is run before recordmcount, I just left this unchanged
> for testing and ignored the recordmcount warnings about __mcount_loc already
> existing. Something is a bit off still though, I see this at boot:
>
> ------------[ ftrace bug ]------------
> ftrace failed to modify
> [<ffffffff81000660>] __tracepoint_iter_initcall_level+0x0/0x40
> actual: 0f:1f:44:00:00
> Initializing ftrace call sites
> ftrace record flags: 0
> (0)
> expected tramp: ffffffff81056500
> ------------[ cut here ]------------
>
> Otherwise, this looks pretty good.

Ha! it is trying to convert the "CALL __fentry__" into a NOP and not
finding the CALL -- because objtool already made it a NOP...

Weird, I thought recordmcount would also write NOPs, it certainly has
code for that. I suppose we can use CC_USING_NOP_MCOUNT to avoid those,
but I'd rather Steve explain this before I wreck things further.