linux-kernel - [PATCH v2 4/5] Makefile.debug: re-enable debug info for .S files

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Date:   Wed, 31 Aug 2022 11:44:07 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Michal Marek <michal.lkml@...kovi.net>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, x86@...nel.org,
        Dmitrii Bundin <dmitrii.bundin.a@...il.com>,
        Fangrui Song <maskray@...gle.com>,
        Alexey Alexandrov <aalexand@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: [PATCH v2 4/5] Makefile.debug: re-enable debug info for .S files

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")

This patch does a few things:
1. Add -g to KBUILD_AFLAGS. This will instruct the compiler to instruct
   the assembler to emit debug info. But this can cause an issue for
   folks using a newer compiler but older assembler, because the
   implicit default DWARF version changed from v4 to v5 in gcc-11 and
   clang-14.
2. If the user is using CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT, use a
   version check to explicitly set -Wa,-gdwarf-<version> for the
   assembler. There's another problem with this; GAS only gained support
   for explicit DWARF versions 3-5 in the 2.36 GNU binutils release.
3. Wrap -Wa,-gdwarf-<version> in as-option call to test whether the
   assembler supports that explicit DWARF version.

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@...gle.com>
Reported-by: Bill Wendling <morbo@...gle.com>
Reported-by: Greg Thelen <gthelen@...gle.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
Changes v1 -> v2:
* Use newly added compiler-specific macros, as per Bill.

 scripts/Makefile.debug | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/scripts/Makefile.debug b/scripts/Makefile.debug
index 9f39b0130551..46e88f0ca998 100644
--- a/scripts/Makefile.debug
+++ b/scripts/Makefile.debug
@@ -4,18 +4,32 @@ ifdef CONFIG_DEBUG_INFO_SPLIT
 DEBUG_CFLAGS	+= -gsplit-dwarf
 else
 DEBUG_CFLAGS	+= -g
+KBUILD_AFLAGS	+= -g
 endif
 
-ifndef CONFIG_AS_IS_LLVM
-KBUILD_AFLAGS	+= -Wa,-gdwarf-2
+ifdef CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
+# gcc-11+, clang-14+
+ifeq ($(call cc-min-version, 110000, 140000),y)
+dwarf-version-y := 5
+else
+dwarf-version-y := 4
 endif
-
-ifndef CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
+else # !CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
 dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
 dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
 DEBUG_CFLAGS	+= -gdwarf-$(dwarf-version-y)
 endif
 
+# Binutils 2.35+ (or clang) required for -gdwarf-{4|5}.
+# https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=31bf18645d98b4d3d7357353be840e320649a67d
+ifneq ($(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y)),)
+KBUILD_AFLAGS	+= -Wa,-gdwarf-$(dwarf-version-y)
+else
+ifndef CONFIG_AS_IS_LLVM
+KBUILD_AFLAGS	+= -Wa,-gdwarf-2
+endif
+endif
+
 ifdef CONFIG_DEBUG_INFO_REDUCED
 DEBUG_CFLAGS	+= -fno-var-tracking
 ifdef CONFIG_CC_IS_GCC
-- 
2.37.2.672.g94769d06f0-goog