[tip:core/objtool] objtool: Detect and warn if libelf is missing and don't break the build

[tip-bot for Josh Poimboeuf]

Commit-ID:  3b27a0c85d7068130ed8e3977a2e977ade986841
Gitweb:     http://git.kernel.org/tip/3b27a0c85d7068130ed8e3977a2e977ade986841
Author:     Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate: Thu, 3 Mar 2016 11:39:30 -0600
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Sat, 5 Mar 2016 09:20:44 +0100

objtool: Detect and warn if libelf is missing and don't break the build

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>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
Cc: Sudip Mukherjee <sudipm.mukherjee@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-next@xxxxxxxxxxxxxxx
Cc: linux@xxxxxxxxxxxx
Cc: live-patching@xxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/8c27fe00face60f42e888ddb3142c97e45223165.1457026550.git.jpoimboe@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 Makefile               | 13 ++++++++++++-
 scripts/Makefile.build |  2 ++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 2c27a01..60b7295 100644
--- a/Makefile
+++ b/Makefile
@@ -995,8 +995,19 @@ 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