linux-kernel - Re: [PATCH v2] lkdtm: support llvm-objcopy

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Message-ID: <20190515181909.GA11401@archlinux-i9>
Date:   Wed, 15 May 2019 11:19:09 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     gregkh@...uxfoundation.org, clang-built-linux@...glegroups.com,
        stable@...r.kernel.org, Nathan Chancellor <nathanchance@...il.com>,
        Alan Modra <amodra@...il.com>,
        Jordan Rupprect <rupprecht@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] lkdtm: support llvm-objcopy

On Wed, May 15, 2019 at 11:12:04AM -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. Adding the load flag is required for GNU objcopy
> to mark .rodata Type as PROGBITS after the rename.
> 
> 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 that .text is now renamed .rodata, the alloc flag A is set,
> the type is PROGBITS, and the section is not flagged as writeable W.
> 
> Cc: stable@...r.kernel.org
> Link: https://sourceware.org/bugzilla/show_bug.cgi?id=24554
> Link: https://github.com/ClangBuiltLinux/linux/issues/448
> Reported-by: Nathan Chancellor <nathanchance@...il.com>

Doesn't look like this got updated. I don't want to make you send a v3
just for that though since it's purely cosmetic and adding my tag below
will ensure I get copied on any backports and such.

> Suggested-by: Alan Modra <amodra@...il.com>
> Suggested-by: Jordan Rupprect <rupprecht@...gle.com>
> Suggested-by: Kees Cook <keescook@...omium.org>
> Acked-by: Kees Cook <keescook@...omium.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>

> ---
> Changes from v1 -> v2:
> * add load flag, as per Kees and Alan.
> * update commit message to mention reason for load flag.
> * add Kees' and Alan's suggested by.
> * carry Kees' Ack.
> * cc stable.
> 
>  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..fb10eafe9bde 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,load
>  targets += rodata.o rodata_objcopy.o
>  $(obj)/rodata_objcopy.o: $(obj)/rodata.o FORCE
>  	$(call if_changed,objcopy)
> -- 
> 2.21.0.1020.gf2820cf01a-goog
>