[lld] [lld][ELF] Demote symbols in discarded sections to Undefined. (PR #68777)

[Bevin Hansson via llvm-commits]

bevin-hansson wrote:

> Does GNU ld permit this, or does it result in dangling relocations? If GNU ld does permit this then there's a chance that this might break some programs where the dangling relocations are benign. I guess that we may need to add a command line option to suppress the errors, although maybe not initially.

When I tested a while back, GNU ld seems to keep the section if there is still a reference to it, even though it was marked /DISCARD/. I don't think this is possible in lld, since the relocation processing is done after the linker script handling, and we've already made the call to keep or not keep the section at that point.

> How does this work with uses of /DISCARD/ in a ld -r relocatable link? I guess a follow up question is what is it supposed to do for ld -r relocatable links.

I haven't tested -r. I would assume that discarding something in an -r invocation today does not trigger these types of errors, so they wouldn't with this patch either.

> It would still be user error to /DISCARD/ a section where some other section had relocations to that section, but I don't think undefined symbols are checked for.

Hm, would it? Could you not discard a section that contains something which is referenced from the current file, and have that reference be resolved from some other object at the final link?

> A possible alternative implementation could be to test each relocation in scanOne to see if the symbol is defined in a discarded section. You could then make a specific error message, or do the work to make the symbol undefined at that point. Not sure how well that would work out, but could be worth a prototype.

Yeah, it doesn't have to go through this logic. It's just that the logic for emitting these errors already existed (and only worked for Undefined), so I went that path.

https://github.com/llvm/llvm-project/pull/68777