linux-kernel - Re: objtool "'naked' return found in RETHUNK build" with clang + CONFIG

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Date:   Wed, 13 Jul 2022 09:13:29 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Josh Poimboeuf <jpoimboe@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: objtool "'naked' return found in RETHUNK build" with clang +
 CONFIG_K{A,C}SAN=y

On Wed, Jul 13, 2022 at 09:01:58AM -0700, Nick Desaulniers wrote:
> On Wed, Jul 13, 2022 at 8:48 AM Nathan Chancellor <nathan@...nel.org> wrote:
> >
> > Hi Josh and Peter,
> >
> > After commit 9bb2ec608a20 ("objtool: Update Retpoline validation") and
> > commit f43b9876e857 ("x86/retbleed: Add fine grained Kconfig knobs"), I
> > see a spew of objtool warnings when building certain configurations with
> > clang. Examples below, they appear to trigger in every single file.
> >
> > With x86_64_defconfig + CONFIG_KASAN=y:
> >
> > arch/x86/ia32/audit.o: warning: objtool: asan.module_ctor+0x11: 'naked' return found in RETHUNK build
> > arch/x86/ia32/audit.o: warning: objtool: asan.module_dtor+0x11: 'naked' return found in RETHUNK build
> > init/version.o: warning: objtool: asan.module_ctor+0x11: 'naked' return found in RETHUNK build
> > init/version.o: warning: objtool: asan.module_dtor+0x11: 'naked' return found in RETHUNK build
> > arch/x86/kernel/cpu/rdrand.o: warning: objtool: asan.module_ctor+0x11: 'naked' return found in RETHUNK build
> > arch/x86/kernel/cpu/rdrand.o: warning: objtool: asan.module_dtor+0x11: 'naked' return found in RETHUNK build
> > arch/x86/kernel/fpu/bugs.o: warning: objtool: asan.module_ctor+0x11: 'naked' return found in RETHUNK build
> > arch/x86/kernel/fpu/bugs.o: warning: objtool: asan.module_dtor+0x11: 'naked' return found in RETHUNK build
> > arch/x86/kernel/platform-quirks.o: warning: objtool: asan.module_ctor+0x0: 'naked' return found in RETHUNK build
> > init/calibrate.o: warning: objtool: asan.module_ctor+0x11: 'naked' return found in RETHUNK build
> > init/calibrate.o: warning: objtool: asan.module_dtor+0x11: 'naked' return found in RETHUNK build
> >
> > With x86_64_defconfig + CONFIG_KCSAN=y:
> >
> > arch/x86/ia32/audit.o: warning: objtool: tsan.module_ctor+0x5: 'naked' return found in RETHUNK build
> > init/calibrate.o: warning: objtool: tsan.module_ctor+0x5: 'naked' return found in RETHUNK build
> > init/version.o: warning: objtool: tsan.module_ctor+0x5: 'naked' return found in RETHUNK build
> > arch/x86/kernel/fpu/bugs.o: warning: objtool: tsan.module_ctor+0x5: 'naked' return found in RETHUNK build
> > arch/x86/kernel/cpu/rdrand.o: warning: objtool: tsan.module_ctor+0x5: 'naked' return found in RETHUNK build
> > arch/x86/events/probe.o: warning: objtool: tsan.module_ctor+0x5: 'naked' return found in RETHUNK build
> > arch/x86/kernel/apic/ipi.o: warning: objtool: tsan.module_ctor+0x5: 'naked' return found in RETHUNK build
> 
> Thanks for the report. I wonder if this might be a compiler bug; it
> seems like the {a|t}san.module_{c|d}tor functions are being emitted
> with ret instructions?
> 
> If you have one of these builds lying around still, can you provide:
> 
> $ llvm-objdump -dr --disassemble-symbols=tsan.module_ctor
> arch/x86/kernel/fpu/bugs.o

Sure thing.

With KASAN:

$ llvm-objdump -r --disassemble-symbols=asan.module_ctor,asan.module_dtor build/arch/x86/kernel/fpu/bugs.o

build/arch/x86/kernel/fpu/bugs.o:	file format elf64-x86-64

Disassembly of section .text.asan.module_ctor:

0000000000000000 <asan.module_ctor>:
       0: be 01 00 00 00               	movl	$1, %esi
       5: 48 c7 c7 00 00 00 00         	movq	$0, %rdi
		0000000000000008:  R_X86_64_32S	.data
       c: e8 00 00 00 00               	callq	0x11 <asan.module_ctor+0x11>
		000000000000000d:  R_X86_64_PLT32	__asan_register_globals-0x4
      11: c3                           	retq

Disassembly of section .text.asan.module_dtor:

0000000000000000 <asan.module_dtor>:
       0: be 01 00 00 00               	movl	$1, %esi
       5: 48 c7 c7 00 00 00 00         	movq	$0, %rdi
		0000000000000008:  R_X86_64_32S	.data
       c: e8 00 00 00 00               	callq	0x11 <asan.module_dtor+0x11>
		000000000000000d:  R_X86_64_PLT32	__asan_unregister_globals-0x4
      11: c3                           	retq

With KCSAN:

$ llvm-objdump -r --disassemble-symbols=tsan.module_ctor build/arch/x86/kernel/fpu/bugs.o

build/arch/x86/kernel/fpu/bugs.o:	file format elf64-x86-64

Disassembly of section .text.tsan.module_ctor:

0000000000000000 <tsan.module_ctor>:
       0: e8 00 00 00 00               	callq	0x5 <tsan.module_ctor+0x5>
		0000000000000001:  R_X86_64_PLT32	__tsan_init-0x4
       5: c3                           	retq

Cheers,
Nathan