Re: [PATCH v2 11/28] kbuild: lto: postpone objtool

Kees Cook
2020-09-03T22:19:53Z

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

> ---
> 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?

> +
> +endif # CONFIG_LTO_CLANG
>
> quiet_cmd_ld_ko_o = LD [M] $@
> - cmd_ld_ko_o = \
> + cmd_ld_ko_o += \
> $(LD) -r $(KBUILD_LDFLAGS) \
> $(KBUILD_LDFLAGS_MODULE) $(LDFLAGS_MODULE) \
> $(addprefix -T , $(KBUILD_LDS_MODULE)) \
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index 3e99a19b9195..a352a5ad9ef7 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -93,8 +93,29 @@ objtool_link()
> {
> local objtoolopt;
>
> + if [ "${CONFIG_LTO_CLANG} ${CONFIG_STACK_VALIDATION}" = "y y" ]; then
> + # Don't perform vmlinux validation unless explicitly requested,
> + # but run objtool on vmlinux.o now that we have an object file.
> + if [ -n "${CONFIG_UNWINDER_ORC}" ]; then
> + objtoolopt="orc generate"
> + else
> + objtoolopt="check"
> + fi
> +
> + if [ -n ${USE_OBJTOOL_MCOUNT} ]; then
> + objtoolopt="${objtoolopt} --mcount"
> + fi
> + fi
> +
> if [ -n "${CONFIG_VMLINUX_VALIDATION}" ]; then
> - objtoolopt="check --vmlinux"
> + if [ -z "${objtoolopt}" ]; then
> + objtoolopt="check --vmlinux"
> + else
> + objtoolopt="${objtoolopt} --vmlinux"
> + fi
> + fi
> +
> + if [ -n "${objtoolopt}" ]; then
> if [ -z "${CONFIG_FRAME_POINTER}" ]; then
> objtoolopt="${objtoolopt} --no-fp"
> fi
> --
> 2.28.0.402.g5ffc5be6b7-goog
>

--
Kees Cook