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

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Message-ID: <20210121074928.GA1346795@gmail.com>
Date:   Thu, 21 Jan 2021 08:49:28 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     linux-tip-commits@...r.kernel.org,
        Andrea Righi <andrea.righi@...onical.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org
Subject: Re: [tip: x86/entry] x86/entry: Build thunk_$(BITS) only if
 CONFIG_PREEMPTION=y


* 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