[lld][ELF] Fix crash when relaxation pass encounters synthetic sections

In LoongArch and RISC-V, the relaxation pass iterates over input sections
within executable output sections. When a linker script places a synthetic
section (e.g., .got) into such an output section, the linker would crash
because synthetic sections do not have the relaxAux field initialized.

The relaxAux data structure is only allocated for non-synthetic sections
in initSymbolAnchors. This patch adds the necessary null checks in the
relaxation loops (relaxOnce and finalizeRelax) to skip sections that
do not require relaxation.

A null check is also added to elf::initSymbolAnchors to ensure the
subsequent sorting of anchors is safe.

Fixes: #184757

Reviewers: MaskRay

Pull Request: https://github.com/llvm/llvm-project/pull/184758

(cherry picked from commit 655d5e7f69)

lld/ELF/Arch/LoongArch.cpp

@@ -1579,7 +1579,8 @@ bool LoongArch::relaxOnce(int pass) const {
     if (!(osec->flags & SHF_EXECINSTR))
       continue;
     for (InputSection *sec : getInputSections(*osec, storage))
-      changed |= relax(ctx, *sec);
+      if (sec->relaxAux)
+        changed |= relax(ctx, *sec);
   }
   return changed;
 }
@@ -1591,6 +1592,8 @@ void LoongArch::finalizeRelax(int passes) const {
     if (!(osec->flags & SHF_EXECINSTR))
       continue;
     for (InputSection *sec : getInputSections(*osec, storage)) {
+      if (!sec->relaxAux)
+        continue;
       RelaxAux &aux = *sec->relaxAux;
       if (!aux.relocDeltas)
         continue;

lld/ELF/Arch/RISCV.cpp

@@ -706,6 +706,8 @@ void elf::initSymbolAnchors(Ctx &ctx) {
     if (!(osec->flags & SHF_EXECINSTR))
       continue;
     for (InputSection *sec : getInputSections(*osec, storage)) {
+      if (isa<SyntheticSection>(sec))
+        continue;
       sec->relaxAux = make<RelaxAux>();
       if (sec->relocs().size()) {
         sec->relaxAux->relocDeltas =
@@ -750,6 +752,8 @@ void elf::initSymbolAnchors(Ctx &ctx) {
     if (!(osec->flags & SHF_EXECINSTR))
       continue;
     for (InputSection *sec : getInputSections(*osec, storage)) {
+      if (!sec->relaxAux)
+        continue;
       llvm::sort(sec->relaxAux->anchors, [](auto &a, auto &b) {
         return std::make_pair(a.offset, a.end) <
                std::make_pair(b.offset, b.end);
@@ -980,7 +984,8 @@ bool RISCV::relaxOnce(int pass) const {
     if (!(osec->flags & SHF_EXECINSTR))
       continue;
     for (InputSection *sec : getInputSections(*osec, storage))
-      changed |= relax(ctx, pass, *sec);
+      if (sec->relaxAux)
+        changed |= relax(ctx, pass, *sec);
   }
   return changed;
 }
@@ -1104,6 +1109,8 @@ void RISCV::finalizeRelax(int passes) const {
     if (!(osec->flags & SHF_EXECINSTR))
       continue;
     for (InputSection *sec : getInputSections(*osec, storage)) {
+      if (!sec->relaxAux)
+        continue;
       RelaxAux &aux = *sec->relaxAux;
       if (!aux.relocDeltas)
         continue;

lld/test/ELF/loongarch-relax-synthetic-in-text.s

@@ -0,0 +1,31 @@
+# REQUIRES: loongarch
+# RUN: rm -rf %t && split-file %s %t
+# RUN: llvm-mc --filetype=obj -triple=loongarch64 -mattr=+relax %t/a.s -o %t/a.o
+
+## Do not crash when we encounter a synthetic section (like .got) that has
+## been placed inside an executable output section via a linker script.
+## Synthetic sections do not have relaxAux data structures initialized.
+
+# RUN: ld.lld -T %t/a.ld %t/a.o -o %t/a.out
+# RUN: llvm-objdump -s %t/a.out | FileCheck %s
+
+# CHECK:      Contents of section .text:
+# CHECK-NEXT: 0400001a 8440c002 10000000 00000000
+
+#--- a.s
+.global _start
+_start:
+  pcalau12i $a0, %got_pc_hi20(sym)
+  ld.d $a0, $a0, %got_pc_lo12(sym)
+
+.data
+sym:
+  .word 0
+
+#--- a.ld
+SECTIONS {
+  .text : {
+    *(.text)
+    *(.got)
+  }
+}

lld/test/ELF/riscv-relax-synthetic-in-text.s

@@ -0,0 +1,33 @@
+# REQUIRES: riscv
+# RUN: rm -rf %t && split-file %s %t
+# RUN: llvm-mc --filetype=obj -triple=riscv64 -mattr=+relax %t/a.s -o %t/a.o
+
+## Do not crash when we encounter a synthetic section (like .got) that has
+## been placed inside an executable output section via a linker script.
+## Synthetic sections do not have relaxAux data structures initialized.
+
+# RUN: ld.lld -T %t/a.ld %t/a.o -o %t/a.out
+# RUN: llvm-objdump -s %t/a.out | FileCheck %s
+
+# CHECK:      Contents of section .text:
+# CHECK-NEXT: 17050000 03350501 00000000 00000000
+# CHECK-NEXT: 18000000 00000000
+
+#--- a.s
+.global _start
+_start:
+1:
+  auipc a0, %got_pcrel_hi(sym)
+  ld a0, %pcrel_lo(1b)(a0)
+
+.data
+sym:
+  .word 0
+
+#--- a.ld
+SECTIONS {
+  .text : {
+    *(.text)
+    *(.got)
+  }
+}