Re: Check pahole availibity and BPF support of toolchain before starting a Linux kernel build

[Sedat Dilek <sedat.dilek@gmail.com>]

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On Thu, Jan 14, 2021 at 3:50 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
>
> On Thu, Jan 14, 2021 at 3:29 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> >
> > On Thu, Jan 14, 2021 at 3:24 PM Jiri Olsa <jolsa@redhat.com> wrote:
> > >
> > > On Thu, Jan 14, 2021 at 03:13:18PM +0100, Sedat Dilek wrote:
> > > > On Mon, Jan 11, 2021 at 11:31 PM Jiri Olsa <jolsa@redhat.com> wrote:
> > > > >
> > > > > On Mon, Jan 11, 2021 at 10:30:22PM +0100, Sedat Dilek wrote:
> > > > >
> > > > > SNIP
> > > > >
> > > > > > > >
> > > > > > > > Building a new Linux-kernel...
> > > > > > > >
> > > > > > > > - Sedat -
> > > > > > > >
> > > > > > > > [1] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/
> > > > > > > > [2] https://github.com/ClangBuiltLinux/tc-build/issues/129#issuecomment-758026878
> > > > > > > > [3] https://github.com/ClangBuiltLinux/tc-build/issues/129#issuecomment-758056553
> > > > > > >
> > > > > > > There are no significant bug fixes between pahole 1.19 and master that
> > > > > > > would solve this problem, so let's try to repro this.
> > > > > > >
> > > > > >
> > > > > > You are right pahole fom latest Git does not solve the issue.
> > > > > >
> > > > > > + info BTFIDS vmlinux
> > > > > > + [  != silent_ ]
> > > > > > + printf   %-7s %s\n BTFIDS vmlinux
> > > > > >  BTFIDS  vmlinux
> > > > > > + ./tools/bpf/resolve_btfids/resolve_btfids vmlinux
> > > > > > FAILED: load BTF from vmlinux: Invalid argument
> > > > >
> > > > > hm, is there a .BTF section in vmlinux?
> > > > >
> > > > > is this working over vmlinux:
> > > > >  $ bpftool btf dump file ./vmlinux
> > > > >
> > > >
> > > > I switched to LLVM v12 from <apt.llvm.org> and saw the same FAILED line.
> > > >
> > > > The generated vmlinux file is cleaned on failure.
> > > >
> > > > + info BTFIDS vmlinux
> > > > + [  != silent_ ]
> > > > + printf   %-7s %s\n BTFIDS vmlinux
> > > >  BTFIDS  vmlinux
> > > > + ./tools/bpf/resolve_btfids/resolve_btfids vmlinux
> > > > FAILED: load BTF from vmlinux: Invalid argument
> > >
> > > did pahole generated the .BTF section? earlier in the log
> > >
> > > jirka
> > >
> >
> > Cannot say.
> >
> > I did:
> >
> > $ git diff
> > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> > index 2d0b28758aa5..c6e679d08bbe 100755
> > --- a/scripts/link-vmlinux.sh
> > +++ b/scripts/link-vmlinux.sh
> > @@ -401,12 +401,6 @@ fi
> >
> > vmlinux_link vmlinux "${kallsymso}" ${btf_vmlinux_bin_o}
> >
> > -# fill in BTF IDs
> > -if [ -n "${CONFIG_DEBUG_INFO_BTF}" -a -n "${CONFIG_BPF}" ]; then
> > -       info BTFIDS vmlinux
> > -       ${RESOLVE_BTFIDS} vmlinux
> > -fi
> > -
> > if [ -n "${CONFIG_BUILDTIME_TABLE_SORT}" ]; then
> >        info SORTTAB vmlinux
> >        if ! sorttable vmlinux; then
> >
>
> Before...
>
> Encountered error while encoding BTF.
> + llvm-objcopy --only-section=.BTF --set-section-flags
> .BTF=alloc,readonly --strip-all .tmp_vmlinux.btf .btf.vmlinux.bin.o
>
> ...I see lots of errors.
>
> I have cut the relevant stuff and placed it in the attachment.
>
> With freshly installed bpftool:
>
> $ /usr/sbin/bpftool btf dump file ./vmlinux
> Error: failed to load BTF from ./vmlinux: Invalid argument
>

This is with CONFIG_DEBUG_INFO_DWARF4=y.
( See attached patch with cover-letter - applied here on top of Linux
v5.11-rc3+).

Concerning this I am looking into [1]:

die__process_unit: DW_TAG_label (0xa) @ <0xa7> not handled!
die__process_unit: tag not supported 0xa (label)!

- Sedat -

[1] http://www.dwarfstd.org/doc/DWARF4.pdf > 3.5 LABEL ENTRIES

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Sender: "ndesaulniers via sendgmr" <ndesaulniers@ndesaulniers1.mtv.corp.google.com>
Date: Tue, 12 Jan 2021 16:32:32 -0800
Message-Id: <20210113003235.716547-1-ndesaulniers@google.com>
Subject: [PATCH v4 0/3] Kbuild: DWARF v5 support
From: Nick Desaulniers <ndesaulniers@google.com>
To: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nathan Chancellor <natechancellor@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Sedat Dilek <sedat.dilek@gmail.com>, linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com, linux-kbuild@vger.kernel.org, linux-arch@vger.kernel.org, Jakub Jelinek <jakub@redhat.com>, Fangrui Song <maskray@google.com>, Caroline Tice <cmtice@google.com>, Nick Clifton <nickc@redhat.com>, Nick Desaulniers <ndesaulniers@google.com>
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DWARF v5 is the latest standard of the DWARF debug info format.

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

Link: http://www.dwarfstd.org/doc/DWARF5.pdf

Patch 1 is a cleanup from Masahiro and isn't DWARF v5 specific.
Patch 2 is a cleanup that lays the ground work and isn't DWARF
v5 specific.
Patch 3 implements Kconfig and Kbuild support for DWARFv5.

Changes from v3:

Changes as per Arvind:
* only add -Wa,-gdwarf-5 for (LLVM=1|CC=clang)+LLVM_IAS=0 builds.
* add -gdwarf-5 to Kconfig shell script.
* only run Kconfig shell script for Clang.

Apologies to Sedat and Nathan; I appreciate previous testing/review, but
I did no carry forward your Tested-by and Reviewed-by tags, as the
patches have changed too much IMO.

Changes from v2:
* Drop two of the earlier patches that have been accepted already.
* Add measurements with GCC 10.2 to commit message.
* Update help text as per Arvind with help from Caroline.
* Improve case/wording between DWARF Versions as per Masahiro.

Changes from the RFC:
* split patch in 3 patch series, include Fangrui's patch, too.
* prefer `DWARF vX` format, as per Fangrui.
* use spaces between assignment in Makefile as per Masahiro.
* simplify setting dwarf-version-y as per Masahiro.
* indent `prompt` in Kconfig change as per Masahiro.
* remove explicit default in Kconfig as per Masahiro.
* add comments to test_dwarf5_support.sh.
* change echo in test_dwarf5_support.sh as per Masahiro.
* remove -u from test_dwarf5_support.sh as per Masahiro.
* add a -gdwarf-5 cc-option check to Kconfig as per Jakub.

Masahiro Yamada (1):
  Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO_DWARF4

Nick Desaulniers (2):
  Kbuild: make DWARF version a choice
  Kbuild: implement support for DWARF v5

 Makefile                          | 15 +++++++----
 include/asm-generic/vmlinux.lds.h |  6 ++++-
 lib/Kconfig.debug                 | 41 +++++++++++++++++++++++++------
 scripts/test_dwarf5_support.sh    |  9 +++++++
 4 files changed, 58 insertions(+), 13 deletions(-)
 create mode 100755 scripts/test_dwarf5_support.sh

-- 
2.30.0.284.gd98b1dd5eaa7-goog


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From MAILER-DAEMON Wed Jan 13 01:13:04 2021
Sender: "ndesaulniers via sendgmr" <ndesaulniers@ndesaulniers1.mtv.corp.google.com>
Date: Tue, 12 Jan 2021 16:32:33 -0800
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Subject: [PATCH v4 1/3] Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO_DWARF4
From: Nick Desaulniers <ndesaulniers@google.com>
To: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nathan Chancellor <natechancellor@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Sedat Dilek <sedat.dilek@gmail.com>, linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com, linux-kbuild@vger.kernel.org, linux-arch@vger.kernel.org, Jakub Jelinek <jakub@redhat.com>, Fangrui Song <maskray@google.com>, Caroline Tice <cmtice@google.com>, Nick Clifton <nickc@redhat.com>, Nick Desaulniers <ndesaulniers@google.com>
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From: Masahiro Yamada <masahiroy@kernel.org>

The -gdwarf-4 flag is supported by GCC 4.5+, and also by Clang.

You can see it at https://godbolt.org/z/6ed1oW

  For gcc 4.5.3 pane,    line 37:    .value 0x4
  For clang 10.0.1 pane, line 117:   .short 4

Given Documentation/process/changes.rst stating GCC 4.9 is the minimal
version, this cc-option is unneeded.

Note
----

CONFIG_DEBUG_INFO_DWARF4 controls the DWARF version only for C files.

As you can see in the top Makefile, -gdwarf-4 is only passed to CFLAGS.

  ifdef CONFIG_DEBUG_INFO_DWARF4
  DEBUG_CFLAGS    += -gdwarf-4
  endif

This flag is used when compiling *.c files.

On the other hand, the assembler is always given -gdwarf-2.

  KBUILD_AFLAGS   += -Wa,-gdwarf-2

Hence, the debug info that comes from *.S files is always DWARF v2.
This is simply because GAS supported only -gdwarf-2 for a long time.

Recently, GAS gained the support for --dwarf-[3|4|5] options. [1]
And, also we have Clang integrated assembler. So, the debug info
for *.S files might be improved if we want.

In my understanding, the current code is intentional, not a bug.

[1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=31bf18645d98b4d3d7357353be840e320649a67d

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
---
 lib/Kconfig.debug | 1 -
 1 file changed, 1 deletion(-)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 78361f0abe3a..dd7d8d35b2a5 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -258,7 +258,6 @@ config DEBUG_INFO_SPLIT
 
 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.
-- 
2.30.0.284.gd98b1dd5eaa7-goog

From MAILER-DAEMON Wed Jan 13 01:13:04 2021
Sender: "ndesaulniers via sendgmr" <ndesaulniers@ndesaulniers1.mtv.corp.google.com>
Date: Tue, 12 Jan 2021 16:32:34 -0800
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Subject: [PATCH v4 2/3] Kbuild: make DWARF version a choice
From: Nick Desaulniers <ndesaulniers@google.com>
To: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nathan Chancellor <natechancellor@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Sedat Dilek <sedat.dilek@gmail.com>, linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com, linux-kbuild@vger.kernel.org, linux-arch@vger.kernel.org, Jakub Jelinek <jakub@redhat.com>, Fangrui Song <maskray@google.com>, Caroline Tice <cmtice@google.com>, Nick Clifton <nickc@redhat.com>, Nick Desaulniers <ndesaulniers@google.com>
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Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice. Adds an
explicit CONFIG_DEBUG_INFO_DWARF2, which is the default. Does so in a
way that's forward compatible with existing configs, and makes adding
future versions more straightforward.

Suggested-by: Fangrui Song <maskray@google.com>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
 Makefile          | 14 +++++++++-----
 lib/Kconfig.debug | 21 ++++++++++++++++-----
 2 files changed, 25 insertions(+), 10 deletions(-)

diff --git a/Makefile b/Makefile
index d49c3f39ceb4..656fff17b331 100644
--- a/Makefile
+++ b/Makefile
@@ -826,12 +826,16 @@ else
 DEBUG_CFLAGS	+= -g
 endif
 
-ifneq ($(LLVM_IAS),1)
-KBUILD_AFLAGS	+= -Wa,-gdwarf-2
+dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
+dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
+DEBUG_CFLAGS	+= -gdwarf-$(dwarf-version-y)
+ifneq ($(dwarf-version-y)$(LLVM_IAS),21)
+# Binutils 2.35+ required for -gdwarf-4+ support.
+dwarf-aflag	:= $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y))
+ifdef CONFIG_CC_IS_CLANG
+DEBUG_CFLAGS	+= $(dwarf-aflag)
 endif
-
-ifdef CONFIG_DEBUG_INFO_DWARF4
-DEBUG_CFLAGS	+= -gdwarf-4
+KBUILD_AFLAGS	+= $(dwarf-aflag)
 endif
 
 ifdef CONFIG_DEBUG_INFO_REDUCED
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index dd7d8d35b2a5..e80770fac4f0 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -256,13 +256,24 @@ config DEBUG_INFO_SPLIT
 	  to know about the .dwo files and include them.
 	  Incompatible with older versions of ccache.
 
+choice
+	prompt "DWARF version"
+	help
+	  Which version of DWARF debug info to emit.
+
+config DEBUG_INFO_DWARF2
+	bool "Generate DWARF Version 2 debuginfo"
+	help
+	  Generate DWARF v2 debug info.
+
 config DEBUG_INFO_DWARF4
-	bool "Generate dwarf4 debuginfo"
+	bool "Generate DWARF Version 4 debuginfo"
 	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 DWARF v4 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.
+
+endchoice # "DWARF version"
 
 config DEBUG_INFO_BTF
 	bool "Generate BTF typeinfo"
-- 
2.30.0.284.gd98b1dd5eaa7-goog

From MAILER-DAEMON Wed Jan 13 01:13:04 2021
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Subject: [PATCH v4 3/3] Kbuild: implement support for DWARF v5
From: Nick Desaulniers <ndesaulniers@google.com>
To: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nathan Chancellor <natechancellor@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Sedat Dilek <sedat.dilek@gmail.com>, linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com, linux-kbuild@vger.kernel.org, linux-arch@vger.kernel.org, Jakub Jelinek <jakub@redhat.com>, Fangrui Song <maskray@google.com>, Caroline Tice <cmtice@google.com>, Nick Clifton <nickc@redhat.com>, Nick Desaulniers <ndesaulniers@google.com>, Arvind Sankar <nivedita@alum.mit.edu>
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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 4 | 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

515M    vmlinux.gcc10.2.dwarf5.compressed
599M    vmlinux.gcc10.2.dwarf4.compressed
624M    vmlinux.gcc10.2.dwarf2.compressed
630M    vmlinux.gcc10.2.dwarf5
765M    vmlinux.gcc10.2.dwarf4
809M    vmlinux.gcc10.2.dwarf2

Though the quality of debug info is harder to quantify; size is not a
proxy for quality.

Jakub notes:
  All [GCC] 5.1 - 6.x did was start accepting -gdwarf-5 as experimental
  option that enabled some small DWARF subset (initially only a few
  DW_LANG_* codes newly added to DWARF5 drafts).  Only GCC 7 (released
  after DWARF 5 has been finalized) started emitting DWARF5 section
  headers and got most of the DWARF5 changes in...

Version check GCC so that we don't need to worry about the difference in
command line args between GNU readelf and llvm-readelf/llvm-dwarfdump to
validate the DWARF Version in the assembler feature detection script.

Link: http://www.dwarfstd.org/doc/DWARF5.pdf
Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
Suggested-by: Jakub Jelinek <jakub@redhat.com>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Suggested-by: Fangrui Song <maskray@google.com>
Suggested-by: Caroline Tice <cmtice@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
 Makefile                          |  1 +
 include/asm-generic/vmlinux.lds.h |  6 +++++-
 lib/Kconfig.debug                 | 17 +++++++++++++++++
 scripts/test_dwarf5_support.sh    |  9 +++++++++
 4 files changed, 32 insertions(+), 1 deletion(-)
 create mode 100755 scripts/test_dwarf5_support.sh

diff --git a/Makefile b/Makefile
index 656fff17b331..1067cfd98249 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 49944f00d2b3..37dc4110875e 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -843,7 +843,11 @@
 		.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 e80770fac4f0..60a4f5e27ada 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -273,6 +273,23 @@ 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 DWARF Version 5 debuginfo"
+	depends on GCC_VERSION >= 50000 || CC_IS_CLANG
+	depends on CC_IS_GCC || $(success,$(srctree)/scripts/test_dwarf5_support.sh $(CC) $(CLANG_FLAGS))
+	help
+	  Generate DWARF v5 debug info. Requires binutils 2.35, gcc 5.0+ (gcc
+	  5.0+ accepts the -gdwarf-5 flag but only had partial support for some
+	  draft features until 7.0), and gdb 8.0+.
+
+	  Changes to the structure of debug info in Version 5 allow for around
+	  15-18% savings in resulting image and debug info section sizes as
+	  compared to DWARF Version 4. DWARF Version 5 standardizes previous
+	  extensions such as accelerators for symbol indexing and the format
+	  for fission (.dwo/.dwp) files. Users may not want to select this
+	  config if they rely on tooling that has not yet been updated to
+	  support DWARF Version 5.
+
 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..142a1b5c7fa2
--- /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"' | $* -gdwarf-5 -Wa,-gdwarf-5 -c -x assembler -o /dev/null -
-- 
2.30.0.284.gd98b1dd5eaa7-goog