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

Sami Tolvanen
2020-06-25T22:40:51Z

On Thu, Jun 25, 2020 at 10:02:35PM +0200, Peter Zijlstra wrote:
> On Thu, Jun 25, 2020 at 09:15:03AM -0700, Sami Tolvanen wrote:
> > On Thu, Jun 25, 2020 at 09:45:30AM +0200, Peter Zijlstra wrote:
>
> > > At least for x86_64 I can do a really quick take for a recordmcount pass
> > > in objtool, but I suppose you also need this for ARM64 ?
> >
> > Sure, sounds good. arm64 uses -fpatchable-function-entry with clang, so we
> > don't need recordmcount there.
>
> This is on top of my local pile:
>
> git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git master
>
> which notably includes the static_call series.
>
> Not boot tested, but it generates the required sections and they look
> more or less as expected, ymmv.
>
> ---
> arch/x86/Kconfig | 1 -
> scripts/Makefile.build | 3 ++
> scripts/link-vmlinux.sh | 2 +-
> tools/objtool/builtin-check.c | 9 ++---
> tools/objtool/builtin.h | 2 +-
> tools/objtool/check.c | 81 +++++++++++++++++++++++++++++++++++++++++++
> tools/objtool/check.h | 1 +
> tools/objtool/objtool.h | 1 +
> 8 files changed, 91 insertions(+), 9 deletions(-)
>
> 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.

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.

Sami