linux-kernel - Re: [PATCH] lkdtm: turn off kcov for lkdtm_rodata_do

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Date:   Tue, 28 Mar 2017 12:00:15 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] lkdtm: turn off kcov for lkdtm_rodata_do_nothing:

On Tue, Mar 28, 2017 at 11:57 AM, Arnd Bergmann <arnd@...db.de> wrote:
> I ran into a link error on ARM64 for lkdtm_rodata_do_nothing:
>
> drivers/misc/built-in.o: In function `lkdtm_rodata_do_nothing':
> :(.rodata+0x68c8): relocation truncated to fit: R_AARCH64_CALL26 against symbol `__sanitizer_cov_trace_pc' defined in .text section in kernel/built-in.o
>
> I did not analyze this further, but my theory is that we would need a trampoline
> to call __sanitizer_cov_trace_pc(), but the linker (correctly) only adds trampolines
> for callers in executable sections.
>
> Disabling KCOV for this one file avoids the build failure with no
> other practical downsides I can think of.
>
> The problem can only happen on kernels that contain both kcov and
> lkdtm, so if we want to backport this, it should be in the earliest
> version that has both (v4.8).
>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Kees Cook <keescook@...omium.org>
> Fixes: 5c9a8750a640 ("kernel: add kcov code coverage")
> Fixes: 9a49a528dcf3 ("lkdtm: add function for testing .rodata section")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/misc/Makefile | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
> index 4925ea8e1952..7a321047bfbe 100644
> --- a/drivers/misc/Makefile
> +++ b/drivers/misc/Makefile
> @@ -63,6 +63,8 @@ lkdtm-$(CONFIG_LKDTM)         += lkdtm_perms.o
>  lkdtm-$(CONFIG_LKDTM)          += lkdtm_rodata_objcopy.o
>  lkdtm-$(CONFIG_LKDTM)          += lkdtm_usercopy.o
>
> +KCOV_INSTRUMENT_lkdtm_rodata.o := n
> +
>  OBJCOPYFLAGS :=
>  OBJCOPYFLAGS_lkdtm_rodata_objcopy.o := \
>                         --set-section-flags .text=alloc,readonly \
> --
> 2.9.0


Acked-by: Dmitry Vyukov <dvyukov@...gle.com>