linux-kernel - [PATCH 4.9 126/241] lkdtm: turn off kcov for lkdtm_rodata_do

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Date:   Mon, 19 Mar 2018 19:06:31 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Sasha Levin <alexander.levin@...rosoft.com>
Subject: [PATCH 4.9 126/241] lkdtm: turn off kcov for lkdtm_rodata_do_nothing:

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Arnd Bergmann <arnd@...db.de>


[ Upstream commit 7064dc7fc13b2994d33ae540ffb7a3a05ac463bf ]

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).

Fixes: 5c9a8750a640 ("kernel: add kcov code coverage")
Fixes: 9a49a528dcf3 ("lkdtm: add function for testing .rodata section")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
Acked-by: Dmitry Vyukov <dvyukov@...gle.com>
Acked-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/misc/Makefile |    2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/misc/Makefile
+++ b/drivers/misc/Makefile
@@ -61,6 +61,8 @@ 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 \