Manually checking build id in elf files

Manually checking build id in elf files

[Alex Bennee]

Hi,

I'm trying to track down a weird failure in oprofile due to bad CRC's
in debug files [1]. As has
been noted in the bug report gdb should complain. However I'm able to
load the symbol-file
with nary a harsh word from gdb. The manual says gdb uses one of two
methods to verify
a symbol file is the correct one. The second method mentions "Build
Id" and the ld man page
seems to imply I should see a "note" section in the library to
indicate the build id.

I'd like to manually check it but readelf doesn't show any such notes:

vnms@vnms:~$ readelf -n /usr/lib/libcairo.so.2.17.3
vnms@vnms:~$ readelf -n /usr/lib/debug/usr/lib/libcairo.so.2.17.3
vnms@vnms:~$

Am I missing something? Is there a way to use readelf to read this information?

[1] https://bugs.launchpad.net/ubuntu/+source/cairo/+bug/415424
-- 
Alex, homepage: http://www.bennee.com/~alex/
http://www.half-llama.co.uk

Re: Manually checking build id in elf files

[Jan Kratochvil]

On Thu, 03 Sep 2009 15:03:05 +0200, Alex Bennee wrote:
> I'd like to manually check it but readelf doesn't show any such notes:
> 
> vnms@vnms:~$ readelf -n /usr/lib/libcairo.so.2.17.3
> vnms@vnms:~$ readelf -n /usr/lib/debug/usr/lib/libcairo.so.2.17.3

Try elfutils:

# eu-readelf -n /lib64/libc.so.6
Note section [ 1] '.note.gnu.build-id' of 36 bytes at offset 0x270:
  Owner          Data size  Type
  GNU                   20  GNU_BUILD_ID
    Build ID: ec8dd400904ddfcac8b1c343263a790f977159dc
[...]

# eu-unstrip -n -e /lib64/libc.so.6
0x3979600000+0x36d868 ec8dd400904ddfcac8b1c343263a790f977159dc@0x3979600280 /lib64/libc.so.6 /usr/lib/debug/lib64/libc-2.10.1.so.debug 


> Am I missing something? Is there a way to use readelf to read this information?

From binutils you can use objdump:

# objdump -s -j .note.gnu.build-id /lib64/libc.so.6 
[...]
Contents of section .note.gnu.build-id:
 3979600270 04000000 14000000 03000000 474e5500  ............GNU.
----------->
 3979600280 ec8dd400 904ddfca c8b1c343 263a790f  .....M.....C&:y.
 3979600290 977159dc                             .qY.    


Regards,
Jan

Re: Manually checking build id in elf files

[Alex Bennee]

2009/9/3 Jan Kratochvil <jan.kratochvil@redhat.com>:
> On Thu, 03 Sep 2009 15:03:05 +0200, Alex Bennee wrote:
>> I'd like to manually check it but readelf doesn't show any such notes:
>>
>> vnms@vnms:~$ readelf -n /usr/lib/libcairo.so.2.17.3
>> vnms@vnms:~$ readelf -n /usr/lib/debug/usr/lib/libcairo.so.2.17.3
>
> Try elfutils:

heh, there was me assuming readelf was part of elfutils :-)
>
> # eu-readelf -n /lib64/libc.so.6
> Note section [ 1] '.note.gnu.build-id' of 36 bytes at offset 0x270:
>  Owner          Data size  Type
>  GNU                   20  GNU_BUILD_ID
>    Build ID: ec8dd400904ddfcac8b1c343263a790f977159dc
> [...]
>
> # eu-unstrip -n -e /lib64/libc.so.6
> 0x3979600000+0x36d868 ec8dd400904ddfcac8b1c343263a790f977159dc@0x3979600280 /lib64/libc.so.6 /usr/lib/debug/lib64/libc-2.10.1.so.debug

I do seem to be seeing different output:

vnms@vnms:~$ eu-unstrip -n -e /lib/libc-2.7.so
0+0x361278 - /lib/libc-2.7.so /usr/lib/debug/lib/libc-2.7.so
vnms@vnms:~$ eu-readelf -n /lib/libc-2.7.so

Note section [ 1] '.note.ABI-tag' of 32 bytes at offset 0x270:
  Owner          Data size  Type
  GNU                   16  VERSION
    OS: Linux, ABI: 2.6.8
vnms@vnms:~$ objdump -s -j .note.gnu.build-id /lib/libc.so.6

/lib/libc.so.6:     file format elf64-x86-64

vnms@vnms:~$ eu-unstrip -n -e /lib/libc-2.7.so
0+0x361278 - /lib/libc-2.7.so /usr/lib/debug/lib/libc-2.7.so
vnms@vnms:~$ eu-readelf -n /usr/lib/libcairo.so.2.17.3
vnms@vnms:~$ objdump -s -j .note.gnu.build-id /usr/lib/libcairo.so.2.17.3

/usr/lib/libcairo.so.2.17.3:     file format elf64-x86-64

My version is:
$ eu-readelf --version
readelf (Red Hat elfutils) 0.131

Which makes me wonder why gdb is allowing the load of the symbol file given:

1. There seems to be no build ID
2. The CRC in the library doesn't match the CRC of the library.debug

The rabbit hole gets deeper :-/

-- 
Alex, homepage: http://www.bennee.com/~alex/
http://www.half-llama.co.uk

Re: Manually checking build id in elf files

[Jan Kratochvil]

On Thu, 03 Sep 2009 15:45:32 +0200, Alex Bennee wrote:
> My version is:
> $ eu-readelf --version
> readelf (Red Hat elfutils) 0.131

This version should already have the build-id support.

> Which makes me wonder why gdb is allowing the load of the symbol file given:

Explicit `symbol-file' command will load it as it can load arbitrary file for
the symbols.  Automatic finding of the associated separate debug info files
(find_separate_debug_file) ignores any files with non-matching build-id or
CRC32.

> 1. There seems to be no build ID

Yes, I think your mail has nothing to do with build-id.

> 2. The CRC in the library doesn't match the CRC of the library.debug

In the Bug it is shown libc has correct CRC32 but libcairo does not.


Regards,
Jan

Re: Manually checking build id in elf files

[Andreas Schwab]

Alex Bennee <kernel-hacker@bennee.com> writes:

> Which makes me wonder why gdb is allowing the load of the symbol file given:
>
> 1. There seems to be no build ID
> 2. The CRC in the library doesn't match the CRC of the library.debug

The CRC is stored in the .gnu_debuglink section.

Andreas.

-- 
Andreas Schwab, schwab@redhat.com
GPG Key fingerprint = D4E8 DBE3 3813 BB5D FA84  5EC7 45C6 250E 6F00 984E
"And now for something completely different."