Re: [PATCH v2 1/2] arm/build: Warn on orphan section placement

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, Jun 23, 2020 at 5:03 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Mon, Jun 22, 2020 at 1:49 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > --- a/arch/arm/kernel/vmlinux.lds.h
> > +++ b/arch/arm/include/asm/vmlinux.lds.h
> > @@ -1,4 +1,5 @@
> >  /* SPDX-License-Identifier: GPL-2.0 */
> > +#include <asm-generic/vmlinux.lds.h>
> >
> >  #ifdef CONFIG_HOTPLUG_CPU
> >  #define ARM_CPU_DISCARD(x)
> > @@ -37,6 +38,13 @@
> >                 *(.idmap.text)                                          \
> >                 __idmap_text_end = .;                                   \
> >
> > +#define ARM_COMMON_DISCARD                                             \
> > +               *(.ARM.attributes)                                      \
>
> I could have sworn that someone (Eli?) once told me that this section
> (.ARM.attributes) is used for disambiguating which ARM version or
> which optional extensions were used when compiling, and that without
> this section, one would not be able to disassemble 32b ARM precisely.
> If that's the case, we might not want to discard it?

Yep, looks like ELFObjectFileBase::getARMFeatures() in
llvm/lib/Object/ELFObjectFile.cpp does exactly that and more.
https://github.com/llvm/llvm-project/blob/8808574e7438c8768b78ae7dd0f029385c6df01d/llvm/lib/Object/ELFObjectFile.cpp#L359-L441
https://github.com/llvm/llvm-project/blob/8808574e7438c8768b78ae7dd0f029385c6df01d/llvm/lib/Object/ELFObjectFile.cpp#L159-L287

As a test, let's do:
$ ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make LLVM=1 -j71 defconfig
(so armv7)
$ ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make LLVM=1 -j71
(then pick any random object file)
$ llvm-readelf -S arch/arm/kernel/bugs.o | grep attri
  [15] .ARM.attributes   ARM_ATTRIBUTES  00000000 0000f7 000037 00      0   0  1
$ llvm-readelf --arch-specific arch/arm/kernel/bugs.o | grep -A 2 CPU_arch
        TagName: CPU_arch
        Description: ARM v7
      }
And let's see if this actually has a difference on the disassembly.
$ ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make LLVM=1 -j71
(full build, since we're talking about linker script changes for vmlinux)
$ llvm-objdump -d vmlinux > prepatch.txt
(apply your patch)
$ ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make LLVM=1 -j71 clean
$ ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make LLVM=1 -j71
$ llvm-objdump -d vmlinux > postpatch.txt
$ diff -u prepatch.txt postpatch.txt | less

No difference. Eh. Checking again with arm-linux-gnueabihf-objdump, it
seems some constants are slightly different for `movw`'s though.  Not
sure what's that about.

If I enable CONFIG_THUMB2_KERNEL=y, is where things become
interesting. llvm-objdump produces wildly different disassembly before
vs after removing .ARM.attributes.  There's also lots of decode errors
in the disassembly.

Repeating the thumb2 test with GNU objdump, I only see slight
differences in constants values for operands to `movw`.  So it looks
like GNU objdump doesn't rely on .ARM.attributes to disambiguate
between ARM vs THUMB2 instructions like llvm-objdump does.  We can
probably improve llvm-objdump, but I'd rather not discard this section
for now.

(also, I didn't test armv6, v5, etc, but those might be interesting
tests, too, should we want to discard this section.  Also, I think we
can explicitly specify --triple=thumbv7-linux-gnueabihf to
llvm-objdump, but I'd prefer it if my disassembler did the work for
me, since I'm lazy)

(oh man, the bytes are printed with different endianness between
arm-linux-gnueabihf-objdump and llvm-objdump...guessing that's a bug
in llvm).

--
Thanks,
~Nick Desaulniers