[PATCH v4] Makefile.debug: re-enable debug info for .S files

[Nick Desaulniers <ndesaulniers@google.com>]

Alexey reported that the fraction of unknown filename instances in
kallsyms grew from ~0.3% to ~10% recently; Bill and Greg tracked it down
to assembler defined symbols, which regressed as a result of:

commit b8a9092330da ("Kbuild: do not emit debug info for assembly with LLVM_IAS=1")

In that commit, I allude to restoring debug info for assembler defined
symbols in a follow up patch, but it seems I forgot to do so in

commit a66049e2cf0e ("Kbuild: make DWARF version a choice")

Link: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=31bf18645d98b4d3d7357353be840e320649a67d
Fixes: b8a9092330da ("Kbuild: do not emit debug info for assembly with LLVM_IAS=1")
Reported-by: Alexey Alexandrov <aalexand@google.com>
Reported-by: Bill Wendling <morbo@google.com>
Reported-by: Greg Thelen <gthelen@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes v3 -> v4:
* Break out into its own patch.
* Move addition of debug-flags-y to DEBUG_CFLAGS and KBUILD_AFLAGS up
  grouping the -gdwarf-* handling together.

 lib/Kconfig.debug      |  4 +++-
 scripts/Makefile.debug | 18 +++++++++---------
 2 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index bcbe60d6c80c..d3e5f36bb01e 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -264,8 +264,10 @@ config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
 config DEBUG_INFO_DWARF4
 	bool "Generate DWARF Version 4 debuginfo"
 	select DEBUG_INFO
+	depends on !CC_IS_CLANG || (CC_IS_CLANG && (AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502)))
 	help
-	  Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
+	  Generate DWARF v4 debug info. This requires gcc 4.5+, binutils 2.35.2
+	  if using clang without clang's integrated assembler, and gdb 7.0+.
 
 	  If you have consumers of DWARF debug info that are not ready for
 	  newer revisions of DWARF, you may wish to choose this or have your
diff --git a/scripts/Makefile.debug b/scripts/Makefile.debug
index 26d6a9d97a20..d6aecd78b942 100644
--- a/scripts/Makefile.debug
+++ b/scripts/Makefile.debug
@@ -4,15 +4,15 @@ ifdef CONFIG_DEBUG_INFO_SPLIT
 DEBUG_CFLAGS	+= -gsplit-dwarf
 endif
 
-ifndef CONFIG_AS_IS_LLVM
-KBUILD_AFLAGS	+= -Wa,-gdwarf-2
-endif
-
-ifndef CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
-dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
-dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
-DEBUG_CFLAGS	+= -gdwarf-$(dwarf-version-y)
+debug-flags-$(CONFIG_DEBUG_INFO_DWARF4)	+= -gdwarf-4
+debug-flags-$(CONFIG_DEBUG_INFO_DWARF5)	+= -gdwarf-5
+ifeq ($(CONFIG_CC_IS_CLANG)$(CONFIG_AS_IS_GNU),yy)
+# Clang does not pass -g or -gdwarf-* option down to GAS.
+# Add -Wa, prefix to explicitly specify the flags.
+KBUILD_AFLAGS	+= $(addprefix -Wa$(comma), $(debug-flags-y))
 endif
+DEBUG_CFLAGS	+= $(debug-flags-y)
+KBUILD_AFLAGS	+= $(debug-flags-y)
 
 ifdef CONFIG_DEBUG_INFO_REDUCED
 DEBUG_CFLAGS	+= -fno-var-tracking
@@ -27,5 +27,5 @@ KBUILD_AFLAGS	+= -gz=zlib
 KBUILD_LDFLAGS	+= --compress-debug-sections=zlib
 endif
 
-KBUILD_CFLAGS += $(DEBUG_CFLAGS)
+KBUILD_CFLAGS	+= $(DEBUG_CFLAGS)
 export DEBUG_CFLAGS
-- 
2.37.3.968.ga6b4b080e4-goog