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Subject[PATCH] Kbuild: implement support for DWARF5
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DWARF5 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. Further -gdwarf-X where X is an unsupported value doesn't
produce an error in $(CC). 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

Make CONFIG_DEBUG_INFO_DWARF4 part of a Kconfig choice to preserve
forward compatibility.

Link: http://www.dwarfstd.org/doc/DWARF5.pdf
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
RFC because this patch is super half baked, but I'm looking for
feedback.

I would logically split this into a series of patches;
1. disable -Wa,gdwarf-2 for LLVM_IAS=1, see also
https://github.com/ClangBuiltLinux/linux/issues/716
https://github.com/ClangBuiltLinux/continuous-integration/blob/master/patches/llvm-all/linux-next/arm64/silence-dwarf2-warnings.patch
that way we can backport for improved LLVM_IAS support.
2. move CONFIG_DEBUG_INFO_DWARF4 to choice.
3. implement the rest on top.

I'm pretty sure GNU `as` only recently gained the ability to specify
-gdwarf-4 without erroring in binutils 2.35, so that part likely needs
to be fixed.

Makefile | 19 ++++++++++++++++---
include/asm-generic/vmlinux.lds.h | 6 +++++-
lib/Kconfig.debug | 29 +++++++++++++++++++++++++----
scripts/test_dwarf5_support.sh | 4 ++++
4 files changed, 50 insertions(+), 8 deletions(-)
create mode 100755 scripts/test_dwarf5_support.sh

diff --git a/Makefile b/Makefile
index e71979882e4f..0862df5b1a24 100644
--- a/Makefile
+++ b/Makefile
@@ -828,10 +828,23 @@ else
DEBUG_CFLAGS += -g
endif

-KBUILD_AFLAGS += -Wa,-gdwarf-2
-
+DWARF_VERSION=2
ifdef CONFIG_DEBUG_INFO_DWARF4
-DEBUG_CFLAGS += -gdwarf-4
+DWARF_VERSION=4
+endif
+ifdef CONFIG_DEBUG_INFO_DWARF5
+DWARF_VERSION=5
+endif
+DEBUG_CFLAGS += -gdwarf-$(DWARF_VERSION)
+
+ifneq ($(DWARF_VERSION)$(LLVM_IAS),21)
+KBUILD_AFLAGS += -Wa,-gdwarf-$(DWARF_VERSION)
+endif
+
+ifdef CONFIG_CC_IS_CLANG
+ifneq ($(LLVM_IAS),1)
+KBUILD_CFLAGS += -Wa,-gdwarf-$(DWARF_VERSION)
+endif
endif

ifdef CONFIG_DEBUG_INFO_REDUCED
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index cd14444bf600..0382808ef9fe 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -828,7 +828,11 @@
.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 537cf3c2937d..6b01f0e2dad8 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -256,14 +256,35 @@ config DEBUG_INFO_SPLIT
to know about the .dwo files and include them.
Incompatible with older versions of ccache.

+choice
+prompt "DWARF version"
+ depends on DEBUG_INFO
+ default DEBUG_INFO_DWARF2
+ help
+ Which version of DWARF debug info to emit.
+
+config DEBUG_INFO_DWARF2
+ bool "Generate dwarf2 debuginfo"
+ help
+ Generate dwarf2 debug info.
+
config DEBUG_INFO_DWARF4
bool "Generate dwarf4 debuginfo"
depends on $(cc-option,-gdwarf-4)
help
- Generate dwarf4 debug info. This requires recent versions
- of gcc and gdb. It makes the debug information larger.
- But it significantly improves the success of resolving
- variables in gdb on optimized code.
+ Generate dwarf4 debug info. This requires gcc 4.5+ and gdb 7.0+.
+ 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 DEBUG_INFO
+ 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
bool "Generate BTF typeinfo"
diff --git a/scripts/test_dwarf5_support.sh b/scripts/test_dwarf5_support.sh
new file mode 100755
index 000000000000..82c0eea45845
--- /dev/null
+++ b/scripts/test_dwarf5_support.sh
@@ -0,0 +1,4 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+set -eu
+echo ".file 0 \"asdf\"" | $* -Wa,-gdwarf-5 -c -x assembler -o /dev/null -
--
2.29.0.rc1.297.gfa9743e501-goog
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