[PATCH] D74755: [llvm-objcopy] Attribute an empty section to a segment ending at its address

[James Henderson via Phabricator via llvm-commits]

jhenderson added a comment.

> In the case of nested segments, e.g. PT_LOAD⊇PT_GNU_RELRO⊇PT_TLS. We want ParentSegment to refer
>  to PT_LOAD so that layoutSectionsForOnlyKeepDebug() will advance sh_offset to make sh_offset≡sh_addr
>  (mod p_align), this is naturally satisfied because PT_LOAD comes before PT_TLS (GNU ld/gold/lld).
> 
> (If PT_LOAD comes after PT_TLS, the rule will break, but this is really impractical.)

I still don't understand what is meant by "really impractical". It is not hard to write a linker script where this is the case and produce runnable output on my Linux VM, and whilst I wouldn't generally encourage people to write linker scripts, there are plenty of people out there who do for various perfectly valid reasons.

I've been thinking about alternatives that would a) get rid of the size 1, and b) solve the problem, without changing the existing behaviour in bad ways for non-empty segments/sections.

The problem as I understand it is that an empty section is not allocated to an empty segment at the same location. How about we just change the `sectionWithinSegment` check for empty sections to simply say something like:

  if (Section.Size == 0 && Segment.FileSize == 0)
    return Segment.OriginalOffset == Section.OriginalOffset;
  // Possibly add equivalent case for SHT_NOBITS using addresses.

This ensures empty sections are considered to be within empty segments if they share the same offset (I think this makes a reasonable amount of sense, although I could imagine some weird issues if you end up with multiple empty sections and segments at the same location with different alignments). We might want similar logic added to `segmentOverlapsSegment`, although I'm not sure.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D74755/new/

https://reviews.llvm.org/D74755