Patch "lkdtm: turn off kcov for lkdtm_rodata_do_nothing:" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

lkdtm: turn off kcov for lkdtm_rodata_do_nothing:

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
lkdtm-turn-off-kcov-for-lkdtm_rodata_do_nothing.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Arnd Bergmann <[email protected]>
Date: Tue, 28 Mar 2017 11:57:27 +0200
Subject: lkdtm: turn off kcov for lkdtm_rodata_do_nothing:

From: Arnd Bergmann <[email protected]>

[ 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).

---
 drivers/misc/Makefile |    2 ++
 1 file changed, 2 insertions(+)

@@ -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 \

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/lkdtm-turn-off-kcov-for-lkdtm_rodata_do_nothing.patch queue-4.9/soc-tegra-fix-link-errors-with-pmc-disabled.patch