On Tue, Jun 23, 2020 at 5:03 PM Nick Desaulniers
[off-list ref] wrote:

On Mon, Jun 22, 2020 at 1:49 PM Kees Cook [off-list ref] wrote:

quoted

--- 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

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel