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SubjectRe: [PATCH] lkdtm: support llvm-objcopy
On Mon, May 13, 2019 at 03:21:09PM -0700, 'Nick Desaulniers' via Clang Built Linux wrote:
> With CONFIG_LKDTM=y and make OBJCOPY=llvm-objcopy, llvm-objcopy errors:
> llvm-objcopy: error: --set-section-flags=.text conflicts with
> --rename-section=.text=.rodata
>
> Rather than support setting flags then renaming sections vs renaming
> then setting flags, it's simpler to just change both at the same time
> via --rename-section.
>
> This can be verified with:
> $ readelf -S drivers/misc/lkdtm/rodata_objcopy.o
> ...
> Section Headers:
> [Nr] Name Type Address Offset
> Size EntSize Flags Link Info Align
> ...
> [ 1] .rodata PROGBITS 0000000000000000 00000040
> 0000000000000004 0000000000000000 A 0 0 4
> ...
>
> Which shows in the Flags field that .text is now renamed .rodata, the
> append flag A is set, and the section is not flagged as writeable W.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/448
> Reported-by: Nathan Chancellor <nathanchance@gmail.com>

This should be natechancellor@gmail.com (although I think I do own that
email, just haven't been into it for 10+ years...)

> Suggested-by: Jordan Rupprect <rupprecht@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> drivers/misc/lkdtm/Makefile | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
> index 951c984de61a..89dee2a9d88c 100644
> --- a/drivers/misc/lkdtm/Makefile
> +++ b/drivers/misc/lkdtm/Makefile
> @@ -15,8 +15,7 @@ KCOV_INSTRUMENT_rodata.o := n
>
> OBJCOPYFLAGS :=
> OBJCOPYFLAGS_rodata_objcopy.o := \
> - --set-section-flags .text=alloc,readonly \
> - --rename-section .text=.rodata
> + --rename-section .text=.rodata,alloc,readonly
> targets += rodata.o rodata_objcopy.o
> $(obj)/rodata_objcopy.o: $(obj)/rodata.o FORCE
> $(call if_changed,objcopy)
> --
> 2.21.0.1020.gf2820cf01a-goog
>

I ran this script to see if there was any change for GNU objcopy and it
looks like .rodata's type gets changed, is this intentional? Otherwise,
this works for llvm-objcopy like you show.

-----------

1c1
< There are 11 section headers, starting at offset 0x240:
---
> There are 11 section headers, starting at offset 0x230:
8c8
< [ 1] .rodata PROGBITS 0000000000000000 00000040
---
> [ 1] .rodata NOBITS 0000000000000000 00000040
10c10

-----------

#!/bin/bash

TMP1=$(mktemp)
TMP2=$(mktemp)

git checkout next-20190513

make -j$(nproc) ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- O=out mrproper allyesconfig drivers/misc/lkdtm/
readelf -S out/drivers/misc/lkdtm/rodata_objcopy.o > ${TMP1}

curl -LSs https://lore.kernel.org/lkml/20190513222109.110020-1-ndesaulniers@google.com/raw | git am -3

make -j$(nproc) ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- O=out mrproper allyesconfig drivers/misc/lkdtm/
readelf -S out/drivers/misc/lkdtm/rodata_objcopy.o > ${TMP2}

diff ${TMP1} ${TMP2}

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