Support split debug symbols without build-id (gentoo-style)

[Chilledheart]

Under Gentoo distribution (Linux), if a library is built with splitdebug feature where debug symbols are splited from the elf binaries into one specific location under /usr/lib/debug directory. However, somehow the tool used to split debug symbols only creates debuglink without introducing build-id. It is different from Debian-style and Fedora-style. Could we add support loading symbols from debuglink?

Both of gdb and lldb can load symbols from debuglink without extra setups.

(gdb) b main
Breakpoint 1 at 0xd81d7: file /home/hky/yass/src/cli/cli.cpp, line 37.
(gdb) r
Starting program: /home/hky/yass/build-release/yass_cli
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Breakpoint 1, main (argc=1, argv=0x7fffffffd948) at /home/hky/yass/src/cli/cli.cpp:37
37        if (!SetUTF8Locale()) {
(gdb) info shared
From                To                  Syms Read   Shared Object Library
0x00007ffff7fcb060  0x00007ffff7ff00c1  Yes         /lib64/ld-linux-x86-64.so.2
0x00007ffff7ee11f0  0x00007ffff7f4fb69  Yes         /lib64/libm.so.6
0x00007ffff7eb7330  0x00007ffff7ecccdd  Yes         /usr/lib/gcc/x86_64-pc-linux-gnu/12/libgcc_s.so.1
0x00007ffff7d03380  0x00007ffff7e52bc9  Yes         /lib64/libc.so.6

Split debug files are located at /usr/lib/debug directory (at least, gentoo).

➜  ~ find /usr/lib/debug -name "libc.so.6*"
/usr/lib/debug/lib64/libc.so.6.debug
/usr/lib/debug/lib/libc.so.6.debug

Using llvm-readobj to dump debuglink section:

➜  build-release git:(develop) llvm-readobj --sections --section-data /lib64/libc.so.6|grep -A 15 gnu_debuglink
    Name: .gnu_debuglink (1031)
    Type: SHT_PROGBITS (0x1)
    Flags [ (0x0)
    ]
    Address: 0x0
    Offset: 0x1CAFF4
    Size: 20
    Link: 0
    Info: 0
    AddressAlignment: 4
    EntrySize: 0
    SectionData (
      0000: 6C696263 2E736F2E 362E6465 62756700  |libc.so.6.debug.|
      0010: 74B07A00                             |t.z.|
    )
  }

[Chilledheart]

Errors showed by orbit in the same machine:

Could not load debug symbols for "/lib64/libc.so.6": Failed to find symbols for module "/lib64/libc.so.6" with build_id="":
- Unable to find local symbols for module "/lib64/libc.so.6": build id is empty.
- Not able to search for symbols on the instance.

Also: Could not load fallback symbols for "/lib64/libc.so.6": 
  * Could not find module file itself in cache: Unable to find object file in cache for module "/lib64/libc.so.6"
  * Could not find module file itself locally.
  * Could not search for module file itself on the instance.

Using readelf tool, it shows there is no build-id section:

➜  build-release git:(develop) readelf -n /lib64/libc.so.6

Displaying notes found in: .note.gnu.property
  Owner                Data size        Description
  GNU                  0x00000020       NT_GNU_PROPERTY_TYPE_0
      Properties: x86 feature used: x86, x87, XMM, YMM, ZMM, MASK
        x86 ISA used: x86-64-baseline, x86-64-v2, x86-64-v3, x86-64-v4

Displaying notes found in: .note.ABI-tag
  Owner                Data size        Description
  GNU                  0x00000010       NT_GNU_ABI_TAG (ABI version tag)
    OS: Linux, ABI: 3.2.0

[Chilledheart]

It shows /usr/lib/debug is a hard coded path in lldb (debugger)'s source code lldb/source/Symbol/LocateSymbolFile.cpp

    // Add /usr/lib/debug directory.
    {
      FileSpec file_spec("/usr/lib/debug");
      FileSystem::Instance().Resolve(file_spec);
      debug_file_search_paths.AppendIfUnique(file_spec);
    }