[llvm] [llvm-objcopy] Fix unaligned `p_offset` after copy. (PR #79889)

[James Henderson via llvm-commits]

https://github.com/jh7370 requested changes to this pull request.

I probably need to give some more thought, but I'm fairly confident the proposed fix is incorrect, as it would result in a segment that has a parent (i.e. one where there is some overlap with another segment) moving independently of its parent. Note that according to the comment immediately above the main change in this PR that states that a parent's offset should have already been assigned (and therefore will be correctly aligned), before this point. My suspicion (but I haven't checked) is that the algorithm for ordering and determining parent/child relationship is incorrect and/or the method to determine the alignment to use is incorrect. In particular, I suspect that the PT_INTERP segment in your original case is being treated as the parent of the second PT_LOAD and therefore its p_align value is the one being used to align the PT_INTERP, which in turn means the second PT_LOAD is also moved with PT_INTERP.

I think the correct fix would be to use the highest alignment in a set of overlapping segments when aligning the first segment in that list, adjusting its position from the "aligned" position according to how far off from the aligned value it was before copying. Alternatively, we treat the parent segment as the highest-alignment section, and adjust the other segments accordingly with it. There are some intricacies involved in both cases that we need to be careful of though.

https://github.com/llvm/llvm-project/pull/79889