Re: [PATCH] lkdtm: turn off kcov for lkdtm_rodata_do_nothing:

[Greg Kroah-Hartman]

On Tue, Mar 28, 2017 at 09:57:34AM -0700, Kees Cook wrote:
> On Tue, Mar 28, 2017 at 3:00 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> > On Tue, Mar 28, 2017 at 11:57 AM, Arnd Bergmann <arnd@xxxxxxxx> 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.
> 
> Yup, this function should be inherently uncallable (which is what
> lkdtm is testing), so it makes sense to turn off kcov for it.
> 
> >>
> >> 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@xxxxxxxxxx>
> >> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> >> Fixes: 5c9a8750a640 ("kernel: add kcov code coverage")
> >> Fixes: 9a49a528dcf3 ("lkdtm: add function for testing .rodata section")
> >> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> >> ---
> >>  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@xxxxxxxxxx>
> 
> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
> 
> Greg, can you please add this to the drivers/misc tree when you get a chance?

Will do.

thanks,

greg k-h