linux-kernel - Re: [tip: x86/entry] x86/entry: Build thunk_$(BITS) only if CONFIG

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Date:   Thu, 21 Jan 2021 09:51:59 +0100
From:   Andrea Righi <andrea.righi@...onical.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org
Subject: Re: [tip: x86/entry] x86/entry: Build thunk_$(BITS) only if
 CONFIG_PREEMPTION=y

On Thu, Jan 21, 2021 at 08:49:28AM +0100, Ingo Molnar wrote:
> 
> * tip-bot2 for Andrea Righi <tip-bot2@...utronix.de> wrote:
> 
> > The following commit has been merged into the x86/entry branch of tip:
> > 
> > Commit-ID:     e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89
> > Gitweb:        https://git.kernel.org/tip/e6d92b6680371ae1aeeb6c5eb2387fdc5d9a2c89
> > Author:        Andrea Righi <andrea.righi@...onical.com>
> > AuthorDate:    Thu, 14 Jan 2021 12:48:35 +01:00
> > Committer:     Ingo Molnar <mingo@...nel.org>
> > CommitterDate: Thu, 21 Jan 2021 08:11:52 +01:00
> > 
> > x86/entry: Build thunk_$(BITS) only if CONFIG_PREEMPTION=y
> > 
> > With CONFIG_PREEMPTION disabled, arch/x86/entry/thunk_64.o is just an
> > empty object file.
> > 
> > With the newer binutils (tested with 2.35.90.20210113-1ubuntu1) the GNU
> > assembler doesn't generate a symbol table for empty object files and
> > objtool fails with the following error when a valid symbol table cannot
> > be found:
> > 
> >   arch/x86/entry/thunk_64.o: warning: objtool: missing symbol table
> > 
> > To prevent this from happening, build thunk_$(BITS).o only if
> > CONFIG_PREEMPTION is enabled.
> > 
> >   BugLink: https://bugs.launchpad.net/bugs/1911359
> > 
> > Fixes: 320100a5ffe5 ("x86/entry: Remove the TRACE_IRQS cruft")
> > Signed-off-by: Andrea Righi <andrea.righi@...onical.com>
> > Signed-off-by: Ingo Molnar <mingo@...nel.org>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Link: https://lore.kernel.org/r/YAAvk0UQelq0Ae7+@xps-13-7390
> 
> Hm, this fails to build on UML defconfig:
> 
>  /home/mingo/gcc/cross/lib/gcc/x86_64-linux/9.3.1/../../../../x86_64-linux/bin/ld: arch/x86/um/../entry/thunk_64.o: in function `preempt_schedule_thunk':
>  /home/mingo/tip.cross/arch/x86/um/../entry/thunk_64.S:34: undefined reference to `preempt_schedule'
>  /home/mingo/gcc/cross/lib/gcc/x86_64-linux/9.3.1/../../../../x86_64-linux/bin/ld: arch/x86/um/../entry/thunk_64.o: in function `preempt_schedule_notrace_thunk':
>  /home/mingo/tip.cross/arch/x86/um/../entry/thunk_64.S:35: undefined reference to `preempt_schedule_notrace'
> 
> Thanks,
> 
> 	Ingo

I've been able to reproduce it, I'm looking at this right now. Thanks!

-Andrea