HomePhabricator

[ELF] Assign file offsets of non-SHF_ALLOC after SHF_ALLOC and set sh_addr=0 to…

Authored by MaskRay on Aug 13 2020, 9:00 AM.

Description

[ELF] Assign file offsets of non-SHF_ALLOC after SHF_ALLOC and set sh_addr=0 to non-SHF_ALLOC

  • GNU ld places non-SHF_ALLOC sections after SHF_ALLOC sections. This has the advantage that the file offsets of a non-SHF_ALLOC cannot be contained in a PT_LOAD. This patch matches the behavior.
  • For non-SHF_ALLOC non-orphan sections, GNU ld may assign non-zero sh_addr and treat them similar to SHT_NOBITS (not advance location counter). This is an alternative approach to what we have done in D85100. By placing non-SHF_ALLOC sections at the end, we can drop special cases in createSection and findOrphanPos added by D85100.

    Different from GNU ld, we set sh_addr to 0 for non-SHF_ALLOC sections. 0 arguably is better because non-SHF_ALLOC sections don't appear in the memory image.

ELF spec says:

sh_addr - If the section will appear in the memory image of a process, this
member gives the address at which the section's first byte should
reside. Otherwise, the member contains 0.

D85100 appeared to take a detour. If we take a combined view on D85100 and this
patch, the overall complexity slightly increases (one more 3-line loop) and
compatibility with GNU ld improves.

The behavior we don't want to match is the special treatment of .symtab
.shstrtab .strtab: they can be matched in LLD but not in GNU ld.

Reviewed By: jhenderson, psmith

Differential Revision: https://reviews.llvm.org/D85867