On Fri, Jun 17, 2016 at 5:42 PM, Kees Cook <keesc...@chromium.org> wrote: > On Fri, Jun 17, 2016 at 2:39 AM, Mark Rutland <mark.rutl...@arm.com> 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. > > Do you want to refuse to build if the compiler doesn't support the > flag? I finally figured out how to do this, I think, for > -fstack-protector: > > http://git.kernel.org/cgit/linux/kernel/git/kees/linux.git/commit/?h=kbuild/stackprotector&id=600c1bd5f8647a8470dc2fc5a8697e3eafb5fd52 > > If you wanted, the CONFIG_KCOV test could live under the same > prepare-compiler-check target. > > -Kees > I suspect the intention here is not to abort the build, but to have an opportunity to report at run time that the kernel has been built without kcov support. >> >> 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. >> >> As uninstrumented files (e.g. kernel/kcov.c) need to know when this >> compiler feature is in use, wee pass the define via KBUILD_CFLAGS rather >> than CFLAGS_KCOV. >> >> Signed-off-by: Mark Rutland <mark.rutl...@arm.com> >> Cc: Alexander Potapenko <gli...@google.com> >> Cc: Andrew Morton <a...@linux-foundation.org> >> Cc: Dmitry Vyukov <dvyu...@google.com> >> Cc: James Morse <james.mo...@arm.com> >> Cc: Kees Cook <keesc...@chromium.org> >> Cc: Michal Marek <mma...@suse.com> >> Cc: linux-kernel@vger.kernel.org >> --- >> Makefile | 2 ++ >> kernel/kcov.c | 9 +++++++++ >> 2 files changed, 11 insertions(+) >> >> Since v1 [1]: >> * Use CC_HAVE_SANCOV_TRACE_PC rather than CONFIG_KCOV_CC >> Since v2 [2]: >> * Use KBUILD_CFLAGS so kernel/kcov.c gets the flag >> >> [1] >> http://lkml.kernel.org/r/1466005756-15626-1-git-send-email-mark.rutl...@arm.com >> [2] >> http://lkml.kernel.org/r/1466010285-2772-1-git-send-email-mark.rutl...@arm.com >> >> diff --git a/Makefile b/Makefile >> index b409076..699d363 100644 >> --- 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 >> >> 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 >> + /* >> + * CONFIG_KCOV was selected, but the compiler does not support the >> + * options KCOV requires. >> + */ >> + return -ENOTSUPP; >> +#endif /* CC_HAVE_SANCOV_TRACE_PC */ >> + >> kcov = kzalloc(sizeof(*kcov), GFP_KERNEL); >> if (!kcov) >> return -ENOMEM; >> -- >> 1.9.1 >> > > > > -- > Kees Cook > Chrome OS & Brillo Security
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