libdwarf: Add symbol value when processing .rela relocations

[emaste]

Summary

In practice .rela debug relocations often use the the .debug_* sections as the symbol, which has the value 0:

Relocation section '.rela.debug_info' at offset 0xd18 contains 205 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000000006  001a0000000a R_X86_64_32       0000000000000000 .debug_abbrev + 0
00000000000c  001e0000000a R_X86_64_32       0000000000000000 .debug_str + 0
000000000012  001e0000000a R_X86_64_32       0000000000000000 .debug_str + 45

so in those cases there is no difference if the symbol value is used in the relocation or not.

It is not always the case though:

000000000039  002700000001 R_X86_64_64       0000000000000010 xen_pci_driver + 0

[emaste]

See also D1819

[kib]

It looks fine, but definitely is extremely subtle. The dwarf_get_reloc_size() returns zero for non-abs relocations, so dwarf_write_{m.l}sb() does nothing for other relocation types.

This at least deserves explanation in a comment.

[rpaulo]

I agree with kib that this deserves a comment. And perhaps add another comment in D1819 explaining why it isn't needed for .rel relocations.

[emaste]

explicitly skip _dwarf_write_* if size==0 and add comment explaining that means unknown or non-absolute relocation

[emaste]

Closed by commit rS278593 (authored by @emaste).