linux-kernel - Re: [PATCH 2/2] Discard .note.gnu.property sections in generic NOTES

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Message-ID: <e15de60c-8133-3d93-eb1c-c6b1b5389887@csgroup.eu>
Date:   Sat, 17 Sep 2022 06:31:20 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Omar Sandoval <osandov@...ndov.com>,
        "H.J. Lu" <hjl.tools@...il.com>
CC:     "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Paul Mackerras <paulus@...ba.org>,
        "Naveen N . Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Borislav Petkov <bp@...e.de>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 2/2] Discard .note.gnu.property sections in generic NOTES



Le 16/09/2022 à 21:40, Omar Sandoval a écrit :
> [Vous ne recevez pas souvent de courriers de osandov@...ndov.com. D?couvrez pourquoi ceci est important ? https://aka.ms/LearnAboutSenderIdentification ]
> 
> On Tue, Apr 28, 2020 at 06:21:05AM -0700, H.J. Lu wrote:
>> With the command-line option, -mx86-used-note=yes, the x86 assembler
>> in binutils 2.32 and above generates a program property note in a note
>> section, .note.gnu.property, to encode used x86 ISAs and features.  But
>> kernel linker script only contains a single NOTE segment:
>>
>> PHDRS {
>>   text PT_LOAD FLAGS(5);
>>   data PT_LOAD FLAGS(6);
>>   percpu PT_LOAD FLAGS(6);
>>   init PT_LOAD FLAGS(7);
>>   note PT_NOTE FLAGS(0);
>> }
>> SECTIONS
>> {
>> ...
>>   .notes : AT(ADDR(.notes) - 0xffffffff80000000) { __start_notes = .; KEEP(*(.not
>> e.*)) __stop_notes = .; } :text :note
>> ...
>> }
>>
>> The NOTE segment generated by kernel linker script is aligned to 4 bytes.
>> But .note.gnu.property section must be aligned to 8 bytes on x86-64 and
>> we get
>>
>> [hjl@...-skx-1 linux]$ readelf -n vmlinux
>>
>> Displaying notes found in: .notes
>>    Owner                Data size Description
>>    Xen                  0x00000006 Unknown note type: (0x00000006)
>>     description data: 6c 69 6e 75 78 00
>>    Xen                  0x00000004 Unknown note type: (0x00000007)
>>     description data: 32 2e 36 00
>>    xen-3.0              0x00000005 Unknown note type: (0x006e6558)
>>     description data: 08 00 00 00 03
>> readelf: Warning: note with invalid namesz and/or descsz found at offset 0x50
>> readelf: Warning:  type: 0xffffffff, namesize: 0x006e6558, descsize:
>> 0x80000000, alignment: 8
>> [hjl@...-skx-1 linux]$
>>
>> Since note.gnu.property section in kernel image is never used, this patch
>> discards .note.gnu.property sections in kernel linker script by adding
>>
>> /DISCARD/ : {
>>    *(.note.gnu.property)
>> }
>>
>> before kernel NOTE segment in generic NOTES.
>>
>> Signed-off-by: H.J. Lu <hjl.tools@...il.com>
>> Reviewed-by: Kees Cook <keescook@...omium.org>
>> ---
>>   include/asm-generic/vmlinux.lds.h | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
>> index 71e387a5fe90..95cd678428f4 100644
>> --- a/include/asm-generic/vmlinux.lds.h
>> +++ b/include/asm-generic/vmlinux.lds.h
>> @@ -833,7 +833,14 @@
>>   #define TRACEDATA
>>   #endif
>>
>> +/*
>> + * Discard .note.gnu.property sections which are unused and have
>> + * different alignment requirement from kernel note sections.
>> + */
>>   #define NOTES                                                                \
>> +     /DISCARD/ : {                                                   \
>> +             *(.note.gnu.property)                                   \
>> +     }                                                               \
>>        .notes : AT(ADDR(.notes) - LOAD_OFFSET) {                       \
>>                __start_notes = .;                                      \
>>                KEEP(*(.note.*))                                        \
>> --
>> 2.25.4
>>
> 
> Hi, H.J.,
> 
> I recently ran into this same .notes corruption when building kernels on
> Arch Linux.
> 
> What ended up happening to this patch? It doesn't appear to have been
> merged, and I couldn't find any further discussion about it. I'm happy
> to resend it for you if you need a hand.

As far as I can see, ARM64 is doing something with that section, see 
arch/arm64/include/asm/assembler.h

Instead of discarding that section, would it be enough to force 
alignment of .notes to 8 bytes ?

Thanks
Christophe


> 
> Thanks,
> Omar