[LLD][RISCV] Error on PCREL_LO referencing other Section

The RISC-V psABI states that "The `R_RISCV_PCREL_LO12_I` or
`R_RISCV_PCREL_LO12_S` relocations contain a label pointing to an
instruction in the same section with an `R_RISCV_PCREL_HI20` relocation
entry that points to the target symbol."

In this case, GNU ld errors, but LLD does not -- I think because LLD is
doing the right thing, certainly in the testcase provided.

Nonetheless, I think an error is good here to bring LLD in line with
what GNU ld is doing in showing that the object they are using is not
following the psABI as written.

Fixes #107304

Author: lenary

lld/ELF/Arch/RISCV.cpp

@@ -671,6 +671,15 @@ void RISCV::relocateAlloc(InputSectionBase &sec, uint8_t *buf) const {
       errorOrWarn(sec.getLocation(rel.offset) +
                   ": R_RISCV_SET_ULEB128 not paired with R_RISCV_SUB_SET128");
       return;
+    case R_RISCV_PC_INDIRECT:
+      if (Defined *def = dyn_cast<Defined>(rel.sym);
+          def->section && def->section != &sec) {
+        errorOrWarn(sec.getLocation(rel.offset) +
+                    ": R_RISCV_PCREL_LO12 relocation points to a symbol '" +
+                    rel.sym->getName() + "' in a different section '" +
+                    def->section->name + "'");
+      }
+      break;
     default:
       break;
     }

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

@@ -0,0 +1,35 @@
+# REQUIRES: riscv
+
+# RUN: llvm-mc -filetype=obj -triple=riscv64 %s -o %t.o
+# RUN: not ld.lld %t.o 2>&1 | FileCheck %s
+
+# CHECK: error: {{.*}}:(.text.sec_one+0x0): R_RISCV_PCREL_LO12 relocation points to a symbol '.Lpcrel_hi0' in a different section '.text.sec_two'
+# CHECK: error: {{.*}}:(.text.sec_one+0x4): R_RISCV_PCREL_LO12 relocation points to a symbol '.Lpcrel_hi1' in a different section '.text.sec_two'
+# CHECK-NOT: R_RISCV_PCREL_LO12 relocation points to a symbol '.Lpcrel_hi2'
+
+## This test is checking that we warn the user when the relocations in their
+## object don't follow the RISC-V psABI. In particular, the psABI requires
+## that PCREL_LO12 relocations are in the same section as the pcrel_hi
+## instruction they point to.
+
+  .section .text.sec_one
+  .globl sec_one
+sec_one:
+  addi a0, a0, %pcrel_lo(.Lpcrel_hi0)
+  sw a0, %pcrel_lo(.Lpcrel_hi1)(a1)
+
+  .section .text.sec_two
+sec_two:
+.Lpcrel_hi0:
+  auipc a0, %pcrel_hi(a)
+.Lpcrel_hi1:
+  auipc a1, %pcrel_hi(a)
+
+.Lpcrel_hi2:
+  auipc a2, %pcrel_hi(a)
+  addi a2, a2, %pcrel_lo(.Lpcrel_hi2)
+
+  .data
+  .global a
+a:
+  .word 50