linux-kernel - Re: [PATCH] lkdtm: support llvm-objcopy

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Date:   Mon, 13 May 2019 16:29:10 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     keescook@...omium.org, clang-built-linux@...glegroups.com,
        Nathan Chancellor <nathanchance@...il.com>,
        Jordan Rupprect <rupprecht@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [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@...il.com>

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

> Suggested-by: Jordan Rupprect <rupprecht@...gle.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.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}