5.4 and 4.19 warning fix for LLVM_IAS

[Nick Desaulniers <ndesaulniers@google.com>]

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Dear stable kernel maintainers,
(Woah, two in one day; have I exceeded my limit?)

Please consider the attached patch for 5.4 and 4.19 for commit
b8a9092330da ("Kbuild: do not emit debug info for assembly with
LLVM_IAS=1"), which fixes a significant number of warnings under arch/
when assembling a kernel with Clang.

These backports have already been shipped in Android; I would like to
revert them and take them from syncing with stable. CrOS also has the
patches staged, but I would prefer for them to sync them from stable
as well.

b8a9092330da just landed in v5.10-rc7.  I recently read
https://lwn.net/Articles/838819/, which mentions a discussion about
letting patches have more time to soak in mainline, so I accept if
your decision is to wait, though I'll note these have been soaking in
Android for 2 days shy of one month (Nov 10).

There were minor conflicts due to missing Kbuild support for
compressed debug info, which is a feature I implemented but don't plan
to backport to stable.

We plan to use Clang's integrated assembler for Android and CrOS for 4.19+.

See also: https://github.com/ClangBuiltLinux/linux/issues/716.
-- 
Thanks,
~Nick Desaulniers

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From 125bcd1867e6b59f4eb364522276bc273e2648e2 Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers@google.com>
Date: Mon, 9 Nov 2020 10:35:28 -0800
Subject: [PATCH] Kbuild: do not emit debug info for assembly with LLVM_IAS=1

commit b8a9092330da2030496ff357272f342eb970d51b upstream.

Clang's integrated assembler produces the warning for assembly files:

warning: DWARF2 only supports one section per compilation unit

If -Wa,-gdwarf-* is unspecified, then debug info is not emitted for
assembly sources (it is still emitted for C sources).  This will be
re-enabled for newer DWARF versions in a follow up patch.

Enables defconfig+CONFIG_DEBUG_INFO to build cleanly with
LLVM=1 LLVM_IAS=1 for x86_64 and arm64.

Cc: <stable@vger.kernel.org>
Link: https://github.com/ClangBuiltLinux/linux/issues/716
Reported-by: Dmitry Golovin <dima@golovin.in>
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Suggested-by: Dmitry Golovin <dima@golovin.in>
Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
Suggested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Fangrui Song <maskray@google.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
[nd: backport to avoid conflicts from:
  commit 10e68b02c861 ("Makefile: support compressed debug info")
  commit 7b16994437c7 ("Makefile: Improve compressed debug info support detection")
  commit 695afd3d7d58 ("kbuild: Simplify DEBUG_INFO Kconfig handling")]
---
 Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Makefile b/Makefile
index e520dee34490..f8c766b35b71 100644
--- a/Makefile
+++ b/Makefile
@@ -802,8 +802,11 @@ DEBUG_CFLAGS	+= -gsplit-dwarf
 else
 DEBUG_CFLAGS	+= -g
 endif
+ifneq ($(LLVM_IAS),1)
 KBUILD_AFLAGS	+= -Wa,-gdwarf-2
 endif
+endif
+
 ifdef CONFIG_DEBUG_INFO_DWARF4
 DEBUG_CFLAGS	+= -gdwarf-4
 endif
-- 
2.29.2.576.ga3fc446d84-goog


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From 17369ed6faa2ebdf4c10accfd6ca9075fdcbb4a2 Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers@google.com>
Date: Mon, 9 Nov 2020 10:35:28 -0800
Subject: [PATCH] Kbuild: do not emit debug info for assembly with LLVM_IAS=1

commit b8a9092330da2030496ff357272f342eb970d51b upstream.

Clang's integrated assembler produces the warning for assembly files:

warning: DWARF2 only supports one section per compilation unit

If -Wa,-gdwarf-* is unspecified, then debug info is not emitted for
assembly sources (it is still emitted for C sources).  This will be
re-enabled for newer DWARF versions in a follow up patch.

Enables defconfig+CONFIG_DEBUG_INFO to build cleanly with
LLVM=1 LLVM_IAS=1 for x86_64 and arm64.

Cc: <stable@vger.kernel.org>
Link: https://github.com/ClangBuiltLinux/linux/issues/716
Reported-by: Dmitry Golovin <dima@golovin.in>
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Suggested-by: Dmitry Golovin <dima@golovin.in>
Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
Suggested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Fangrui Song <maskray@google.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
[nd: backport to avoid conflicts from:
  commit 10e68b02c861 ("Makefile: support compressed debug info")
  commit 7b16994437c7 ("Makefile: Improve compressed debug info support detection")
  commit 695afd3d7d58 ("kbuild: Simplify DEBUG_INFO Kconfig handling")]
---
 Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Makefile b/Makefile
index 71e55c5cd74a..4c6ac3c6995f 100644
--- a/Makefile
+++ b/Makefile
@@ -745,8 +745,11 @@ KBUILD_CFLAGS   += $(call cc-option, -gsplit-dwarf, -g)
 else
 KBUILD_CFLAGS	+= -g
 endif
+ifneq ($(LLVM_IAS),1)
 KBUILD_AFLAGS	+= -Wa,-gdwarf-2
 endif
+endif
+
 ifdef CONFIG_DEBUG_INFO_DWARF4
 KBUILD_CFLAGS	+= $(call cc-option, -gdwarf-4,)
 endif
-- 
2.29.2.576.ga3fc446d84-goog