[PATCH] objtool: Warn if libelf is missing

[Josh Poimboeuf]

With CONFIG_STACK_VALIDATION enabled, if the host system doesn't have
a development version of libelf installed, the build fails with errors
like:

  elf.h:22:18: fatal error: gelf.h: No such file or directory compilation terminated.

Instead of failing to build, instead just print a warning and disable
stack validation.

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
Based on tip/master.

 Makefile               | 13 ++++++++++++-
 scripts/Makefile.build |  2 ++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 750df29..1fd8328 100644
--- a/Makefile
+++ b/Makefile
@@ -998,8 +998,19 @@ prepare0: archprepare FORCE
 # All the preparing..
 prepare: prepare0 prepare-objtool
 
+ifdef CONFIG_STACK_VALIDATION
+  has_libelf := $(shell echo "int main() {}" | $(HOSTCC) -xc -o /dev/null -lelf - &> /dev/null && echo 1 || echo 0)
+  ifeq ($(has_libelf),1)
+    objtool_target := tools/objtool FORCE
+  else
+    $(warning "Cannot use CONFIG_STACK_VALIDATION, please install libelf-dev or elfutils-libelf-devel")
+    SKIP_STACK_VALIDATION := 1
+    export SKIP_STACK_VALIDATION
+  endif
+endif
+
 PHONY += prepare-objtool
-prepare-objtool: $(if $(CONFIG_STACK_VALIDATION), tools/objtool FORCE)
+prepare-objtool: $(objtool_target)
 
 # Generate some files
 # ---------------------------------------------------------------------------
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 130a452..e1bc190 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -242,6 +242,7 @@ cmd_record_mcount =						\
 endif
 
 ifdef CONFIG_STACK_VALIDATION
+ifneq ($(SKIP_STACK_VALIDATION),1)
 
 __objtool_obj := $(objtree)/tools/objtool/objtool
 
@@ -260,6 +261,7 @@ objtool_obj = $(if $(patsubst y%,, \
 	$(OBJECT_FILES_NON_STANDARD_$(basetarget).o)$(OBJECT_FILES_NON_STANDARD)n), \
 	$(__objtool_obj))
 
+endif # SKIP_STACK_VALIDATION
 endif # CONFIG_STACK_VALIDATION
 
 define rule_cc_o_c
-- 
2.4.3