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Date:   Mon, 22 Jun 2020 15:52:37 -0700
From:   Fangrui Song <maskray@google.com>
To:     Kees Cook <keescook@chromium.org>
Cc:     Borislav Petkov <bp@suse.de>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        clang-built-linux@googlegroups.com, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/3] vmlinux.lds.h: Add .gnu.version* to DISCARDS
Message-ID: <20200622225237.ybol4qmz4mhkmlqc@google.com>
References: <20200622205341.2987797-1-keescook@chromium.org>
 <20200622205341.2987797-2-keescook@chromium.org>
 <20200622220043.6j3vl6v7udmk2ppp@google.com>
 <202006221524.CEB86E036B@keescook>
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On 2020-06-22, Kees Cook wrote:
>On Mon, Jun 22, 2020 at 03:00:43PM -0700, Fangrui Song wrote:
>> On 2020-06-22, Kees Cook wrote:
>> > For vmlinux linking, no architecture uses the .gnu.version* section,
>> > so remove it via the common DISCARDS macro in preparation for adding
>> > --orphan-handling=warn more widely.
>> >
>> > Signed-off-by: Kees Cook <keescook@chromium.org>
>> > ---
>> > include/asm-generic/vmlinux.lds.h | 1 +
>> > 1 file changed, 1 insertion(+)
>> >
>> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
>> > index db600ef218d7..6fbe9ed10cdb 100644
>> > --- a/include/asm-generic/vmlinux.lds.h
>> > +++ b/include/asm-generic/vmlinux.lds.h
>> > @@ -934,6 +934,7 @@
>> > 	*(.discard)							\
>> > 	*(.discard.*)							\
>> > 	*(.modinfo)							\
>> > +	*(.gnu.version*)						\
>> > 	}
>> >
>> > /**
>> > --
>> > 2.25.1
>>
>> I wonder what lead to .gnu.version{,_d,_r} sections in the kernel.
>
>This looks like a bug in bfd.ld? There are no versioned symbols in any
>of the input files (and no output section either!)
>
>The link command is:
>$ ld -m elf_x86_64 --no-ld-generated-unwind-info -z noreloc-overflow -pie \
>--no-dynamic-linker   --orphan-handling=warn -T \
>arch/x86/boot/compressed/vmlinux.lds \
>arch/x86/boot/compressed/kernel_info.o \
>arch/x86/boot/compressed/head_64.o arch/x86/boot/compressed/misc.o \
>arch/x86/boot/compressed/string.o arch/x86/boot/compressed/cmdline.o \
>arch/x86/boot/compressed/error.o arch/x86/boot/compressed/piggy.o \
>arch/x86/boot/compressed/cpuflags.o \
>arch/x86/boot/compressed/early_serial_console.o \
>arch/x86/boot/compressed/kaslr.o arch/x86/boot/compressed/kaslr_64.o \
>arch/x86/boot/compressed/mem_encrypt.o \
>arch/x86/boot/compressed/pgtable_64.o arch/x86/boot/compressed/acpi.o \
>-o arch/x86/boot/compressed/vmlinux
>
>None of the inputs have the section:
>
>$ for i in arch/x86/boot/compressed/kernel_info.o \
>arch/x86/boot/compressed/head_64.o arch/x86/boot/compressed/misc.o \
>arch/x86/boot/compressed/string.o arch/x86/boot/compressed/cmdline.o \
>arch/x86/boot/compressed/error.o arch/x86/boot/compressed/piggy.o \
>arch/x86/boot/compressed/cpuflags.o \
>arch/x86/boot/compressed/early_serial_console.o \
>arch/x86/boot/compressed/kaslr.o arch/x86/boot/compressed/kaslr_64.o \
>arch/x86/boot/compressed/mem_encrypt.o \
>arch/x86/boot/compressed/pgtable_64.o arch/x86/boot/compressed/acpi.o \
>; do echo -n $i": "; readelf -Vs $i | grep 'version'; done
>arch/x86/boot/compressed/kernel_info.o: No version information found in this file.
>arch/x86/boot/compressed/head_64.o: No version information found in this file.
>arch/x86/boot/compressed/misc.o: No version information found in this file.
>arch/x86/boot/compressed/string.o: No version information found in this file.
>arch/x86/boot/compressed/cmdline.o: No version information found in this file.
>arch/x86/boot/compressed/error.o: No version information found in this file.
>arch/x86/boot/compressed/piggy.o: No version information found in this file.
>arch/x86/boot/compressed/cpuflags.o: No version information found in this file.
>arch/x86/boot/compressed/early_serial_console.o: No version information found in this file.
>arch/x86/boot/compressed/kaslr.o: No version information found in this file.
>arch/x86/boot/compressed/kaslr_64.o: No version information found in this file.
>arch/x86/boot/compressed/mem_encrypt.o: No version information found in this file.
>arch/x86/boot/compressed/pgtable_64.o: No version information found in this file.
>arch/x86/boot/compressed/acpi.o: No version information found in this file.
>
>And it's not in the output:
>
>$ readelf -Vs arch/x86/boot/compressed/vmlinux | grep version
>No version information found in this file.
>
>So... for the kernel we need to silence it right now.

Re-link with -M (or -Map file) to check where .gnu.version{,_d,_r} input
sections come from?

If it is a bug, we should probably figure out which version of binutils
has fixed the bug.