[ELF][RISCV] Error on R_RISCV_PCREL_LO12_[IS] that point to absolute symbols

[MaskRay]

Summary

The referenced symbol is expected to point to an R_RISCV_*_HI20
relocation. An absolute symbol has no associated section, therefore
there cannot be a matching R_RISCV_*_HI20.

This fixes the crash reported by PR42038. For reference, ld.bfd errors:

(.init+0x4): dangerous relocation: %pcrel_lo missing matching %pcrel_hi

[MaskRay]

👀

[MaskRay]

🤡

[MaskRay]

@ruiu @jrtc27 👾

[jrtc27]

Seems like a good idea

[jrtc27]

ELF/InputSection.cpp

574 ↗

(On Diff #204542)

We should print the symbol name though like we do below

[MaskRay]

Print symbol name

lld/trunk/ELF/InputSection.cpp

 // For R_RISCV_PC_INDIRECT (R_RISCV_PCREL_LO12_{I,S}), the symbol actually
 // points the corresponding R_RISCV_PCREL_HI20 relocation, and the target VA
 // is calculated using PCREL_HI20's symbol.
 //
 // This function returns the R_RISCV_PCREL_HI20 relocation from
 // R_RISCV_PCREL_LO12's symbol and addend.
 static Relocation *getRISCVPCRelHi20(const Symbol *Sym, uint64_t Addend) {
   const Defined *D = cast<Defined>(Sym);
+  if (!D->Section) {
+    error("R_RISCV_PCREL_LO12 relocation points to an absolute symbol: " +
+          Sym->getName());
+    return nullptr;
+  }
   InputSection *IS = cast<InputSection>(D->Section);
 
   if (Addend != 0)
     warn("Non-zero addend in R_RISCV_PCREL_LO12 relocation to " +
          IS->getObjMsg(D->Value) + " is ignored");
 
   // Relocations are sorted by offset, so we can use std::equal_range to do
   // binary search.

lld/trunk/test/ELF/riscv-pcrel-hilo-error.s

+# REQUIRES: riscv
+# RUN: llvm-mc -filetype=obj -triple=riscv64 %s -o %t.o
+# RUN: not ld.lld %t.o --defsym external=0 2>&1 | FileCheck %s
+
+# CHECK: error: R_RISCV_PCREL_LO12 relocation points to an absolute symbol: external
+
+addi sp,sp,%pcrel_lo(external)