linux-kernel - Re: [PATCHv2] kcov: reject open when kernel not instrumented

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Date:	Thu, 16 Jun 2016 11:09:37 +0100
From:	James Morse <james.morse@....com>
To:	Mark Rutland <mark.rutland@....com>
CC:	linux-kernel@...r.kernel.org,
	Alexander Potapenko <glider@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	Michal Marek <mmarek@...e.com>
Subject: Re: [PATCHv2] kcov: reject open when kernel not instrumented

Hi Mark,

On 15/06/16 18:04, Mark Rutland wrote:
> If the toolchain does not support -fsanitize-coverage=trace-pc, we blat
> this option from CFLAGS_KCOV, and build the kernel without
> instrumentation, even if CONFIG_KCOV was selected. However, we still
> build the rest of the kcov infrastructure, and expose a kcov file under
> debugfs. This can be confusing, as the kernel will appear to support
> kcov, yet will never manage to sample any trace PC values. While we do
> note this fact at build time, this may be missed, and a user may not
> have access to build logs.
> 
> This patch ensures that CC_HAVE_SANCOV_TRACE_PC is defined when the
> toolchain supports -fsanitize-coverage=trace-pc, and is not defined
> otherwise. When CC_HAVE_SANCOV_TRACE_PC is not defined, the kernel will
> return -ENOTSUPP if userspace attempts to open the kcov debugfs file,
> indicating that kcov functionality is unavailable.

> diff --git a/Makefile b/Makefile
> index 0f70de6..3785a63 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -369,7 +369,7 @@ LDFLAGS_MODULE  =
>  CFLAGS_KERNEL	=
>  AFLAGS_KERNEL	=
>  CFLAGS_GCOV	= -fprofile-arcs -ftest-coverage -fno-tree-loop-im -Wno-maybe-uninitialized
> -CFLAGS_KCOV	= -fsanitize-coverage=trace-pc
> +CFLAGS_KCOV	= -fsanitize-coverage=trace-pc -DCC_HAVE_SANCOV_TRACE_PC
>  
>  
>  # Use USERINCLUDE when you must reference the UAPI directories only.
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index a02f2dd..0a0b164 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -3,6 +3,7 @@
>  #define DISABLE_BRANCH_PROFILING
>  #include <linux/compiler.h>
>  #include <linux/types.h>
> +#include <linux/errno.h>
>  #include <linux/file.h>
>  #include <linux/fs.h>
>  #include <linux/mm.h>
> @@ -160,6 +161,14 @@ static int kcov_open(struct inode *inode, struct file *filep)
>  {
>  	struct kcov *kcov;
>  
> +#ifndef CC_HAVE_SANCOV_TRACE_PC

I don't think this will work as kernel/kcov.c is listed in the Makefile as:
> # Don't self-instrument.
> KCOV_INSTRUMENT_kcov.o := n


This will cause the build machinery in scripts/Makefile.lib to not give
kernel/kcov.c the CFLAGS_KCOV contents:
> ifeq ($(CONFIG_KCOV),y)
> _c_flags += $(if $(patsubst n%,, \
> 	$(KCOV_INSTRUMENT_$(basetarget).o)$(KCOV_INSTRUMENT)y), \
> 	$(CFLAGS_KCOV))
> endif

... so kernel/kcov.c will never see anything in CFLAGS_KCOV ...


An alternative would be to add the flag to the compiler test that generates the
'not supported' warning, but it needs to go in another CFLAGS variable.
Something like:

-------------------%<-------------------
diff --git a/Makefile b/Makefile
@@ -687,6 +687,8 @@ ifdef CONFIG_KCOV
     $(warning Cannot use CONFIG_KCOV: \
              -fsanitize-coverage=trace-pc is not supported by compiler)
     CFLAGS_KCOV =
+  else
+    KBUILD_CFLAGS += -DCC_HAVE_SANCOV_TRACE_PC
   endif
 endif
-------------------%<-------------------


Thanks,

James