[PATCH v2 4/4] Kbuild: implement support for DWARF v5

Nick Desaulniers
2020-11-04T00:54:03Z

DWARF v5 is the latest standard of the DWARF debug info format.

Feature detection of DWARF5 is onerous, especially given that we've
removed $(AS), so we must query $(CC) for DWARF5 assembler directive
support. GNU `as` only recently gained support for specifying
-gdwarf-5.

The DWARF version of a binary can be validated with:
$ llvm-dwarfdump vmlinux | head -n 5 | grep version
or
$ readelf --debug-dump=info vmlinux 2>/dev/null | grep Version

DWARF5 wins significantly in terms of size when mixed with compression
(CONFIG_DEBUG_INFO_COMPRESSED).

363M vmlinux.clang12.dwarf5.compressed
434M vmlinux.clang12.dwarf4.compressed
439M vmlinux.clang12.dwarf2.compressed
457M vmlinux.clang12.dwarf5
536M vmlinux.clang12.dwarf4
548M vmlinux.clang12.dwarf2

Link: http://www.dwarfstd.org/doc/DWARF5.pdf
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Suggested-by: Jakub Jelinek <jakub@redhat.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Makefile | 1 +
include/asm-generic/vmlinux.lds.h | 6 +++++-
lib/Kconfig.debug | 8 ++++++++
scripts/test_dwarf5_support.sh | 9 +++++++++
4 files changed, 23 insertions(+), 1 deletion(-)
create mode 100755 scripts/test_dwarf5_support.sh

diff --git a/Makefile b/Makefile
index e23786a4c1c7..9056bac0ff85 100644
--- a/Makefile
+++ b/Makefile
@@ -828,6 +828,7 @@ endif

dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
+dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
DEBUG_CFLAGS += -gdwarf-$(dwarf-version-y)
ifneq ($(dwarf-version-y)$(LLVM_IAS),21)
# Binutils 2.35+ required for -gdwarf-4+ support.
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index b2b3d81b1535..76ce62c77029 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -829,7 +829,11 @@
.debug_types 0 : { *(.debug_types) } \
/* DWARF 5 */ \
.debug_macro 0 : { *(.debug_macro) } \
- .debug_addr 0 : { *(.debug_addr) }
+ .debug_addr 0 : { *(.debug_addr) } \
+ .debug_line_str 0 : { *(.debug_line_str) } \
+ .debug_loclists 0 : { *(.debug_loclists) } \
+ .debug_rnglists 0 : { *(.debug_rnglists) } \
+ .debug_str_offsets 0 : { *(.debug_str_offsets) }

/* Stabs debugging sections. */
#define STABS_DEBUG \
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 03c494eefabd..c5b54ba51060 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -274,6 +274,14 @@ config DEBUG_INFO_DWARF4
It makes the debug information larger, but it significantly
improves the success of resolving variables in gdb on optimized code.

+config DEBUG_INFO_DWARF5
+ bool "Generate DWARF5 debuginfo"
+ depends on $(cc-option,-gdwarf-5)
+ depends on $(success,$(srctree)/scripts/test_dwarf5_support.sh $(CC) $(CLANG_FLAGS))
+ help
+ Genereate dwarf5 debug info. Requires binutils 2.35+, gcc 5.1+, and
+ gdb 8.0+.
+
endchoice # "DWARF version"

config DEBUG_INFO_BTF
diff --git a/scripts/test_dwarf5_support.sh b/scripts/test_dwarf5_support.sh
new file mode 100755
index 000000000000..156ad5ec4274
--- /dev/null
+++ b/scripts/test_dwarf5_support.sh
@@ -0,0 +1,9 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+
+# Test that assembler accepts -gdwarf-5 and .file 0 directives, which were bugs
+# in binutils < 2.35.
+# https://sourceware.org/bugzilla/show_bug.cgi?id=25612
+# https://sourceware.org/bugzilla/show_bug.cgi?id=25614
+set -e
+echo '.file 0 "filename"' | $* -Wa,-gdwarf-5 -c -x assembler -o /dev/null -
--
2.29.1.341.ge80a0c044ae-goog