Re: [PATCH dwarves 0/2] dwarf_loader: improve cus__merging_cu()

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On 4/2/21 6:27 AM, Arnaldo Carvalho de Melo wrote:
Em Thu, Apr 01, 2021 at 12:35:45PM -0700, Bill Wendling escreveu:
On Wed, Mar 31, 2021 at 7:58 PM Yonghong Song <yhs@xxxxxx> wrote:

Function cus__merging_cu() is introduced in Commit 39227909db3c
("dwarf_loader: Permit merging all DWARF CU's for clang LTO built
binary") to test whether cross-cu references may happen.
The original implementation anticipates compilation flags
in dwarf, but later some concerns about binary size surfaced
and the decision is to scan .debug_abbrev as a faster way
to check cross-cu references. Also putting a note in vmlinux
to indicate whether lto is enabled for built or not can
provide a much faster way.

This patch set implemented this two approaches, first
checking the note (in Patch #2), if not found, then
check .debug_abbrev (in Patch #1).

Yonghong Song (2):
   dwarf_loader: check .debug_abbrev for cross-cu references
   dwarf_loader: check .notes section for lto build info

  dwarf_loader.c | 76 ++++++++++++++++++++++++++++++++++++--------------
  1 file changed, 55 insertions(+), 21 deletions(-)

With this series of patches, the compilation passes for me with
ThinLTO. You may add this if you like:

Tested-by: Bill Wendling <morbo@xxxxxxxxxx>

Thanks, added, and also this "Committer testing" section:

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@xxxxxx>
Suggested-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Tested-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Tested-by: Bill Wendling <morbo@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: David Blaikie <dblaikie@xxxxxxxxx>
Cc: Fāng-ruì Sòng <maskray@xxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: bpf@xxxxxxxxxxxxxxx
Cc: dwarves@xxxxxxxxxxxxxxx
Cc: kernel-team@xxxxxx
Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>

Looks great! Thanks a lot for detailed testing log and Suggested-by/Tested-by/Cc list!



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