linux-kernel - Re: [RFC PATCH v2 5/7] objtool: Enable objtool to run only on files with ftrace enabled

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Date:   Tue, 24 May 2022 21:50:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "aik@...abs.ru" <aik@...abs.ru>,
        "sv@...ux.ibm.com" <sv@...ux.ibm.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "naveen.n.rao@...ux.vnet.ibm.com" <naveen.n.rao@...ux.vnet.ibm.com>,
        "mbenes@...e.cz" <mbenes@...e.cz>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [RFC PATCH v2 5/7] objtool: Enable objtool to run only on files
 with ftrace enabled

On Tue, May 24, 2022 at 06:59:50PM +0000, Christophe Leroy wrote:
> 
> 
> Le 24/05/2022 à 20:02, Peter Zijlstra a écrit :
> > On Tue, May 24, 2022 at 08:01:39PM +0200, Peter Zijlstra wrote:
> >> On Tue, May 24, 2022 at 03:17:45PM +0200, Christophe Leroy wrote:
> >>> From: Sathvika Vasireddy <sv@...ux.ibm.com>
> >>>
> >>> This patch makes sure objtool runs only on the object files
> >>> that have ftrace enabled, instead of running on all the object
> >>> files.
> >>>
> >>> Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> >>> Signed-off-by: Sathvika Vasireddy <sv@...ux.ibm.com>
> >>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> >>> ---
> >>>   scripts/Makefile.build | 4 ++--
> >>>   1 file changed, 2 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> >>> index 2e0c3f9c1459..06ceffd92921 100644
> >>> --- a/scripts/Makefile.build
> >>> +++ b/scripts/Makefile.build
> >>> @@ -258,8 +258,8 @@ else
> >>>   # 'OBJECT_FILES_NON_STANDARD_foo.o := 'y': skip objtool checking for a file
> >>>   # 'OBJECT_FILES_NON_STANDARD_foo.o := 'n': override directory skip for a file
> >>>   
> >>> -$(obj)/%.o: objtool-enabled = $(if $(filter-out y%, \
> >>> -	$(OBJECT_FILES_NON_STANDARD_$(basetarget).o)$(OBJECT_FILES_NON_STANDARD)n),y)
> >>> +$(obj)/%.o: objtool-enabled = $(and $(if $(filter-out y%, $(OBJECT_FILES_NON_STANDARD_$(basetarget).o)$(OBJECT_FILES_NON_STANDARD)n),y),        \
> >>> +        $(if $(findstring $(strip $(CC_FLAGS_FTRACE)),$(_c_flags)),y),y)
> >>
> >> I think this breaks x86, quite a bit of files have ftrace disabled but
> >> very much must run objtool anyway.
> > 
> > Also; since the Changelog gives 0 clue as to what problem it's trying to
> > solve, I can't suggest anything.
> 
> I asked Sathvika on the previous series, see 
> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20220523175548.922671-3-sv@linux.ibm.com/
> 
> He says it is to solve the problem I reported at 
> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20220318105140.43914-4-sv@linux.ibm.com/#2861128

So on x86 we have:

arch/x86/entry/vdso/Makefile:OBJECT_FILES_NON_STANDARD       := y

to kill objtool for the whole of the VDSO. When we run objtool on
vmlinux it isn't a problem, because the VDSO ends up as a data section
through linker scripts.