[llvm-branch-commits] ELF: Only rewrite non-preemptible IFUNCs to IPLT functions if a non-IRELATIVE relocation is needed. (PR #133531)

[Peter Collingbourne via llvm-branch-commits]

pcc wrote:

> This patch has a few problems
> 
> * The check in `processAux` was moved to the wrong place (it will never get hit because this is after we deal with all symbols not defined locally). This patch moves it to approximately the right place but will need some logic changes to `isStaticLinkTimeConstant` to recognize ifuncs as not being static link time constants:
> 
> ```
> diff --git a/lld/ELF/Relocations.cpp b/lld/ELF/Relocations.cpp
> index 1439fd859f51..969d71461cb1 100644
> --- a/lld/ELF/Relocations.cpp
> +++ b/lld/ELF/Relocations.cpp
> @@ -1127,6 +1127,8 @@ void RelocationScanner::processAux(RelExpr expr, RelType type, uint64_t offset,
>    // handling of GOT-generating relocations.
>    if (isStaticLinkTimeConstant(expr, type, sym, offset) ||
>        (!ctx.arg.isPic && sym.isUndefWeak())) {
> +    if (LLVM_UNLIKELY(isIfunc && !needsGot(expr) && !needsPlt(expr)))
> +      sym.setFlags(HAS_DIRECT_RELOC);
>      sec->addReloc({expr, type, offset, addend, &sym});
>      return;
>    }
> @@ -1194,9 +1196,6 @@ void RelocationScanner::processAux(RelExpr expr, RelType type, uint64_t offset,
>      }
>    }
>  
> -  if (LLVM_UNLIKELY(isIfunc && !needsGot(expr) && !needsPlt(expr)))
> -    sym.setFlags(HAS_DIRECT_RELOC);
> -
>    // When producing an executable, we can perform copy relocations (for
>    // STT_OBJECT) and canonical PLT (for STT_FUNC) if sym is defined by a DSO.
>    // Copy relocations/canonical PLT entries are unsupported for
> ```
> 
> * Need to adjust how IRELATIVE relocations get emitted. Currently we emit relocations with the wrong type (assuming they will be demoted) and fix up the type in `RelocationBaseSection::partitionRels` if they turn out to not be demoted. But this isn't good enough for PDEs which need them to be static relocations. Here's what I think we should do instead:
>   
>   * Always emit ifunc relocations as IRELATIVE during `processAux`
>   * In `handleNonPreemptibleIfunc`, when we demote an ifunc to an iplt entry, loop over all irelatives pointing to the symbol and rewrite them as either relative (for PIE/shared) or static relocations (for PDE). It's fine to loop over the irelatives every time because they are rare.

All of this is done in the latest change (except that I decided to handle the demotion in a separate loop instead of `handleNonPreemptibleIfunc`, PTAL

https://github.com/llvm/llvm-project/pull/133531