On Thu, Sep 03, 2020 at 03:19:43PM -0700, Kees Cook wrote:

On Thu, Sep 03, 2020 at 01:30:36PM -0700, Sami Tolvanen wrote:

quoted

With LTO, LLVM bitcode won't be compiled into native code until
modpost_link, or modfinal for modules. This change postpones calls
to objtool until after these steps.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

For a "fail fast" style of building, it makes sense to have objtool run
as early as possible, so it makes sense to keep the current behavior in
non-LTO mode. I do wonder, though, if there is a real benefit to having
"fail fast" case. I imagine a lot of automated builds are using
--keep-going with make, and actually waiting until the end to do the
validation means more code will get build-tested before objtool rejects
the results. *shrug*

quoted

---
 arch/Kconfig              |  2 +-
 scripts/Makefile.build    |  2 ++
 scripts/Makefile.modfinal | 24 ++++++++++++++++++++++--
 scripts/link-vmlinux.sh   | 23 ++++++++++++++++++++++-
 4 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 71392e4a8900..7a418907e686 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig

@@ -599,7 +599,7 @@ config LTO_CLANG
 	depends on $(success,$(NM) --help | head -n 1 | grep -qi llvm)
 	depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm)
 	depends on ARCH_SUPPORTS_LTO_CLANG
-	depends on !FTRACE_MCOUNT_RECORD
+	depends on HAVE_OBJTOOL_MCOUNT || !(X86_64 && DYNAMIC_FTRACE)
 	depends on !KASAN
 	depends on !GCOV_KERNEL
 	select LTO

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index c348e6d6b436..b8f1f0d65a73 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build

@@ -218,6 +218,7 @@ cmd_record_mcount = $(if $(findstring $(strip $(CC_FLAGS_FTRACE)),$(_c_flags)),
 endif # USE_RECORDMCOUNT
 
 ifdef CONFIG_STACK_VALIDATION
+ifndef CONFIG_LTO_CLANG
 ifneq ($(SKIP_STACK_VALIDATION),1)
 
 __objtool_obj := $(objtree)/tools/objtool/objtool

@@ -253,6 +254,7 @@ objtool_obj = $(if $(patsubst y%,, \
 	$(__objtool_obj))
 
 endif # SKIP_STACK_VALIDATION
+endif # CONFIG_LTO_CLANG
 endif # CONFIG_STACK_VALIDATION
 
 # Rebuild all objects when objtool changes, or is enabled/disabled.

diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 1005b147abd0..909bd509edb4 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal

@@ -34,10 +34,30 @@ ifdef CONFIG_LTO_CLANG
 # With CONFIG_LTO_CLANG, reuse the object file we compiled for modpost to
 # avoid a second slow LTO link
 prelink-ext := .lto
-endif
+
+# ELF processing was skipped earlier because we didn't have native code,
+# so let's now process the prelinked binary before we link the module.
+
+ifdef CONFIG_STACK_VALIDATION
+ifneq ($(SKIP_STACK_VALIDATION),1)
+cmd_ld_ko_o +=								\
+	$(objtree)/tools/objtool/objtool				\
+		$(if $(CONFIG_UNWINDER_ORC),orc generate,check)		\
+		--module						\
+		$(if $(CONFIG_FRAME_POINTER),,--no-fp)			\
+		$(if $(CONFIG_GCOV_KERNEL),--no-unreachable,)		\
+		$(if $(CONFIG_RETPOLINE),--retpoline,)			\
+		$(if $(CONFIG_X86_SMAP),--uaccess,)			\
+		$(if $(USE_OBJTOOL_MCOUNT),--mcount,)			\
+		$(@:.ko=$(prelink-ext).o);
+
+endif # SKIP_STACK_VALIDATION
+endif # CONFIG_STACK_VALIDATION

I wonder if objtool_args could be reused here instead of having two
places to keep in sync? It looks like that might mean moving things
around a bit before this patch, since I can't quite see if
Makefile.build's variables are visible to Makefile.modfinal?

It doesn't look like they are. I suppose we could move objtool_args to
Makefile.lib. Masahiro, any thoughts?

Sami

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