dwarf_loader: Check .notes section for LTO build info
In [1], linux introduced a note with type BUILD_COMPILER_LTO_INFO. If
this note type exist, there is no need to scan .debug_abbrev section for
cross-cu reference. With a kernel built with [1], the cus__merging_cu()
overhead is about 3us for either LTO or non-LTO vmlinux.
[1] https://lore.kernel.org/bpf/20210401012406.1800957-1-yhs@fb.com/
Committer testing:
Using a thin-LTO built vmlinux that doesn't have the
LINUX_ELFNOTE_BUILD_LTO note:
$ readelf --notes vmlinux.clang.thin.LTO
Displaying notes found in: .notes
Owner Data size Description
Xen 0x00000006 Unknown note type: (0x00000006)
description data: 6c 69 6e 75 78 00
Xen 0x00000004 Unknown note type: (0x00000007)
description data: 32 2e 36 00
Xen 0x00000008 Unknown note type: (0x00000005)
description data: 78 65 6e 2d 33 2e 30 00
Xen 0x00000008 Unknown note type: (0x00000003)
description data: 00 00 00 80 ff ff ff ff
Xen 0x00000008 Unknown note type: (0x0000000f)
description data: 00 00 00 00 80 00 00 00
Xen 0x00000008 NT_VERSION (version)
description data: c0 e1 33 83 ff ff ff ff
Xen 0x00000008 NT_ARCH (architecture)
description data: 00 20 00 81 ff ff ff ff
Xen 0x00000029 Unknown note type: (0x0000000a)
description data: 21 77 72 69 74 61 62 6c 65 5f 70 61 67 65 5f 74 61 62 6c 65 73 7c 70 61 65 5f 70 67 64 69 72 5f 61 62 6f 76 65 5f 34 67 62
Xen 0x00000004 Unknown note type: (0x00000011)
description data: 01 88 00 00
Xen 0x00000004 Unknown note type: (0x00000009)
description data: 79 65 73 00
Xen 0x00000008 Unknown note type: (0x00000008)
description data: 67 65 6e 65 72 69 63 00
Xen 0x00000010 Unknown note type: (0x0000000d)
description data: 01 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00
Xen 0x00000004 Unknown note type: (0x0000000e)
description data: 01 00 00 00
Xen 0x00000004 Unknown note type: (0x00000010)
description data: 01 00 00 00
Xen 0x00000008 Unknown note type: (0x0000000c)
description data: 00 00 00 00 00 80 ff ff
Xen 0x00000008 Unknown note type: (0x00000004)
description data: 00 00 00 00 00 00 00 00
Xen 0x00000008 Unknown note type: (0x00000012)
description data: e0 02 00 01 00 00 00 00
Linux 0x00000017 OPEN
description data: 34 2e 31 39 2e 34 2d 33 30 30 2e 66 63 32 39 2e 78 38 36 5f 36 34 00
GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
Build ID: 354f81317b1b3c35f3f81f8d9f04d0c8caccb09a
$
Then with one with the new ELF note stating that this binary was built
with LTO:
[acme@five pahole]$ readelf --notes vmlinux.clang.thin.LTO+ELF_note
Displaying notes found in: .notes
Owner Data size Description
Xen 0x00000006 Unknown note type: (0x00000006)
description data: 6c 69 6e 75 78 00
Xen 0x00000004 Unknown note type: (0x00000007)
description data: 32 2e 36 00
Xen 0x00000008 Unknown note type: (0x00000005)
description data: 78 65 6e 2d 33 2e 30 00
Xen 0x00000008 Unknown note type: (0x00000003)
description data: 00 00 00 80 ff ff ff ff
Xen 0x00000008 Unknown note type: (0x0000000f)
description data: 00 00 00 00 80 00 00 00
Xen 0x00000008 NT_VERSION (version)
description data: c0 e1 33 83 ff ff ff ff
Xen 0x00000008 NT_ARCH (architecture)
description data: 00 20 00 81 ff ff ff ff
Xen 0x00000029 Unknown note type: (0x0000000a)
description data: 21 77 72 69 74 61 62 6c 65 5f 70 61 67 65 5f 74 61 62 6c 65 73 7c 70 61 65 5f 70 67 64 69 72 5f 61 62 6f 76 65 5f 34 67 62
Xen 0x00000004 Unknown note type: (0x00000011)
description data: 01 88 00 00
Xen 0x00000004 Unknown note type: (0x00000009)
description data: 79 65 73 00
Xen 0x00000008 Unknown note type: (0x00000008)
description data: 67 65 6e 65 72 69 63 00
Xen 0x00000010 Unknown note type: (0x0000000d)
description data: 01 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00
Xen 0x00000004 Unknown note type: (0x0000000e)
description data: 01 00 00 00
Xen 0x00000004 Unknown note type: (0x00000010)
description data: 01 00 00 00
Xen 0x00000008 Unknown note type: (0x0000000c)
description data: 00 00 00 00 00 80 ff ff
Xen 0x00000008 Unknown note type: (0x00000004)
description data: 00 00 00 00 00 00 00 00
Xen 0x00000008 Unknown note type: (0x00000012)
description data: e0 02 00 01 00 00 00 00
Linux 0x00000017 OPEN
description data: 34 2e 31 39 2e 34 2d 33 30 30 2e 66 63 32 39 2e 78 38 36 5f 36 34 00
Linux 0x00000004 func
description data: 01 00 00 00
GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
Build ID: aeba9ffc929acd3cd573b4d1afc8df9af4f3694d
$
Now to see the diff:
$ readelf --notes vmlinux.clang.thin.LTO+ELF_note > with-note
$ readelf --notes vmlinux.clang.thin.LTO > without-note
$ diff -u without-note with-note
--- without-note 2021-04-02 10:23:57.545349084 -0300
+++ with-note 2021-04-02 10:23:50.690196102 -0300
@@ -37,5 +37,7 @@
description data: e0 02 00 01 00 00 00 00
Linux 0x00000017 OPEN
description data: 34 2e 31 39 2e 34 2d 33 30 30 2e 66 63 32 39 2e 78 38 36 5f 36 34 00
+ Linux 0x00000004 func
+ description data: 01 00 00 00
GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
- Build ID: 354f81317b1b3c35f3f81f8d9f04d0c8caccb09a
+ Build ID: aeba9ffc929acd3cd573b4d1afc8df9af4f3694d
$
Signed-off-by: Yonghong Song <yhs@fb.com>
Suggested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Bill Wendling <morbo@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: David Blaikie <dblaikie@gmail.com>
Cc: Fāng-ruì Sòng <maskray@google.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: bpf@vger.kernel.org
Cc: dwarves@vger.kernel.org
Cc: kernel-team@fb.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
1 file changed
