[ELF] -r: Synthesize R_RISCV_ALIGN at input section start

Without linker relaxation enabled for a particular relocatable file or
section (e.g., using .option norelax), the assembler will not generate
R_RISCV_ALIGN relocations for alignment directives. This becomes
problematic in a two-stage linking process:

```
ld -r a.o b.o -o ab.o
// b.o is norelax. Its alignment information is lost in ab.o.
ld ab.o -o ab
```

When ab.o is linked into an executable, the preceding relaxed section
(a.o's content) might shrink. Since there's no R_RISCV_ALIGN relocation
in b.o for the linker to act upon, the `.word 0x3a393837` data in b.o
may end up unaligned in the final executable.

To address the issue, this patch inserts NOP bytes and synthesizes an
R_RISCV_ALIGN relocation at the beginning of a text section when the
alignment >= 4.

For simplicity, when RVC is disabled, we synthesize an ALIGN relocation
(addend: 2) for a 4-byte aligned section, allowing the linker to trim
the excess 2 bytes.

See also https://sourceware.org/bugzilla/show_bug.cgi?id=33236

Pull Request: #151639

Author: MaskRay

lld/ELF/Arch/RISCV.cpp

@@ -45,7 +45,21 @@ class RISCV final : public TargetInfo {
                 uint64_t val) const override;
   void relocateAlloc(InputSectionBase &sec, uint8_t *buf) const override;
   bool relaxOnce(int pass) const override;
+  template <class ELFT, class RelTy>
+  bool synthesizeAlignForInput(uint64_t &dot, InputSection *sec,
+                               Relocs<RelTy> rels);
+  template <class ELFT, class RelTy>
+  void finalizeSynthesizeAligns(uint64_t &dot, InputSection *sec,
+                                Relocs<RelTy> rels);
+  template <class ELFT>
+  bool synthesizeAlignAux(uint64_t &dot, InputSection *sec);
+  bool synthesizeAlign(uint64_t &dot, InputSection *sec) override;
   void finalizeRelax(int passes) const override;
+
+  // The following two variables are used by synthesized ALIGN relocations.
+  InputSection *baseSec = nullptr;
+  // r_offset and r_addend pairs.
+  SmallVector<std::pair<uint64_t, uint64_t>, 0> synthesizedAligns;
 };
 
 } // end anonymous namespace
@@ -956,6 +970,116 @@ bool RISCV::relaxOnce(int pass) const {
   return changed;
 }
 
+// If the section alignment is >= 4, advance `dot` to insert NOPs and synthesize
+// an ALIGN relocation. Otherwise, return false to use default handling.
+template <class ELFT, class RelTy>
+bool RISCV::synthesizeAlignForInput(uint64_t &dot, InputSection *sec,
+                                    Relocs<RelTy> rels) {
+  if (!baseSec) {
+    // Record the first input section with RELAX relocations. We will synthesize
+    // ALIGN relocations here.
+    for (auto rel : rels) {
+      if (rel.getType(false) == R_RISCV_RELAX) {
+        baseSec = sec;
+        break;
+      }
+    }
+  } else if (sec->addralign >= 4) {
+    // If the alignment is >= 4 and the section does not start with an ALIGN
+    // relocation, synthesize one.
+    bool hasAlignRel = llvm::any_of(rels, [](const RelTy &rel) {
+      return rel.r_offset == 0 && rel.getType(false) == R_RISCV_ALIGN;
+    });
+    if (!hasAlignRel) {
+      synthesizedAligns.emplace_back(dot - baseSec->getVA(),
+                                     sec->addralign - 2);
+      dot += sec->addralign - 2;
+      return true;
+    }
+  }
+  return false;
+}
+
+// Finalize the relocation section by appending synthesized ALIGN relocations
+// after processing all input sections.
+template <class ELFT, class RelTy>
+void RISCV::finalizeSynthesizeAligns(uint64_t &dot, InputSection *sec,
+                                     Relocs<RelTy> rels) {
+  auto *f = cast<ObjFile<ELFT>>(baseSec->file);
+  auto shdr = f->template getELFShdrs<ELFT>()[baseSec->relSecIdx];
+  // Create a copy of InputSection.
+  sec = make<InputSection>(*f, shdr, baseSec->name);
+  auto *baseRelSec = cast<InputSection>(f->getSections()[baseSec->relSecIdx]);
+  *sec = *baseRelSec;
+  baseSec = nullptr;
+
+  // Allocate buffer for original and synthesized relocations in RELA format.
+  // If CREL is used, OutputSection::finalizeNonAllocCrel will convert RELA to
+  // CREL.
+  auto newSize = rels.size() + synthesizedAligns.size();
+  auto *relas = makeThreadLocalN<typename ELFT::Rela>(newSize);
+  sec->size = newSize * sizeof(typename ELFT::Rela);
+  sec->content_ = reinterpret_cast<uint8_t *>(relas);
+  sec->type = SHT_RELA;
+  // Copy original relocations to the new buffer, potentially converting CREL to
+  // RELA.
+  for (auto [i, r] : llvm::enumerate(rels)) {
+    relas[i].r_offset = r.r_offset;
+    relas[i].setSymbolAndType(r.getSymbol(0), r.getType(0), false);
+    if constexpr (RelTy::HasAddend)
+      relas[i].r_addend = r.r_addend;
+  }
+  // Append synthesized ALIGN relocations to the buffer.
+  for (auto [i, r] : llvm::enumerate(synthesizedAligns)) {
+    auto &rela = relas[rels.size() + i];
+    rela.r_offset = r.first;
+    rela.setSymbolAndType(0, R_RISCV_ALIGN, false);
+    rela.r_addend = r.second;
+  }
+  // Replace the old relocation section with the new one in the output section.
+  // addOrphanSections ensures that the output relocation section is processed
+  // after osec.
+  for (SectionCommand *cmd : sec->getParent()->commands) {
+    auto *isd = dyn_cast<InputSectionDescription>(cmd);
+    if (!isd)
+      continue;
+    for (auto *&isec : isd->sections)
+      if (isec == baseRelSec)
+        isec = sec;
+  }
+}
+
+template <class ELFT>
+bool RISCV::synthesizeAlignAux(uint64_t &dot, InputSection *sec) {
+  bool ret = false;
+  if (sec) {
+    invokeOnRelocs(*sec, ret = synthesizeAlignForInput<ELFT>, dot, sec);
+  } else if (baseSec) {
+    invokeOnRelocs(*baseSec, finalizeSynthesizeAligns<ELFT>, dot, sec);
+  }
+  return ret;
+}
+
+// Without linker relaxation enabled for a particular relocatable file or
+// section, the assembler will not generate R_RISCV_ALIGN relocations for
+// alignment directives. This becomes problematic in a two-stage linking
+// process: ld -r a.o b.o -o ab.o; ld ab.o -o ab. This function synthesizes an
+// R_RISCV_ALIGN relocation at section start when needed.
+//
+// When called with an input section (`sec` is not null): If the section
+// alignment is >= 4, advance `dot` to insert NOPs and synthesize an ALIGN
+// relocation.
+//
+// When called after all input sections are processed (`sec` is null): The
+// output relocation section is updated with all the newly synthesized ALIGN
+// relocations.
+bool RISCV::synthesizeAlign(uint64_t &dot, InputSection *sec) {
+  assert(ctx.arg.relocatable);
+  if (ctx.arg.is64)
+    return synthesizeAlignAux<ELF64LE>(dot, sec);
+  return synthesizeAlignAux<ELF32LE>(dot, sec);
+}
+
 void RISCV::finalizeRelax(int passes) const {
   llvm::TimeTraceScope timeScope("Finalize RISC-V relaxation");
   Log(ctx) << "relaxation passes: " << passes;

lld/ELF/LinkerScript.cpp

@@ -1230,6 +1230,9 @@ bool LinkerScript::assignOffsets(OutputSection *sec) {
   if (sec->firstInOverlay)
     state->overlaySize = 0;
 
+  bool synthesizeAlign = ctx.arg.relocatable && ctx.arg.relax &&
+                         (sec->flags & SHF_EXECINSTR) &&
+                         ctx.arg.emachine == EM_RISCV;
   // We visited SectionsCommands from processSectionCommands to
   // layout sections. Now, we visit SectionsCommands again to fix
   // section offsets.
@@ -1260,7 +1263,10 @@ bool LinkerScript::assignOffsets(OutputSection *sec) {
       if (isa<PotentialSpillSection>(isec))
         continue;
       const uint64_t pos = dot;
-      dot = alignToPowerOf2(dot, isec->addralign);
+      // If synthesized ALIGN may be needed, call maybeSynthesizeAlign and
+      // disable the default handling if the return value is true.
+      if (!(synthesizeAlign && ctx.target->synthesizeAlign(dot, isec)))
+        dot = alignToPowerOf2(dot, isec->addralign);
       isec->outSecOff = dot - sec->addr;
       dot += isec->getSize();
 
@@ -1276,6 +1282,12 @@ bool LinkerScript::assignOffsets(OutputSection *sec) {
   if (ctx.in.relroPadding && sec == ctx.in.relroPadding->getParent())
     expandOutputSection(alignToPowerOf2(dot, ctx.arg.commonPageSize) - dot);
 
+  if (synthesizeAlign) {
+    const uint64_t pos = dot;
+    ctx.target->synthesizeAlign(dot, nullptr);
+    expandOutputSection(dot - pos);
+  }
+
   // Non-SHF_ALLOC sections do not affect the addresses of other OutputSections
   // as they are not part of the process image.
   if (!(sec->flags & SHF_ALLOC)) {

lld/ELF/OutputSections.cpp

@@ -889,9 +889,17 @@ void OutputSection::sortInitFini() {
 std::array<uint8_t, 4> OutputSection::getFiller(Ctx &ctx) {
   if (filler)
     return *filler;
-  if (flags & SHF_EXECINSTR)
-    return ctx.target->trapInstr;
-  return {0, 0, 0, 0};
+  if (!(flags & SHF_EXECINSTR))
+    return {0, 0, 0, 0};
+  if (ctx.arg.relocatable && ctx.arg.emachine == EM_RISCV) {
+    // See RISCV::maybeSynthesizeAlign: Synthesized NOP bytes and ALIGN
+    // relocations might be needed between two input sections. Use a NOP for the
+    // filler.
+    if (ctx.arg.eflags & EF_RISCV_RVC)
+      return {1, 0, 1, 0};
+    return {0x13, 0, 0, 0};
+  }
+  return ctx.target->trapInstr;
 }
 
 void OutputSection::checkDynRelAddends(Ctx &ctx) {

lld/ELF/Target.h

@@ -96,6 +96,9 @@ class TargetInfo {
 
   // Do a linker relaxation pass and return true if we changed something.
   virtual bool relaxOnce(int pass) const { return false; }
+  virtual bool synthesizeAlign(uint64_t &dot, InputSection *sec) {
+    return false;
+  }
   // Do finalize relaxation after collecting relaxation infos.
   virtual void finalizeRelax(int passes) const {}
 

lld/test/ELF/riscv-relocatable-align.s

@@ -0,0 +1,130 @@
+# RUN: rm -rf %t && split-file %s %t && cd %t
+# RUN: llvm-mc -filetype=obj -triple=riscv64 -mattr=+c,+relax a.s -o ac.o
+# RUN: llvm-mc -filetype=obj -triple=riscv64 -mattr=+c,+relax b.s -o bc.o
+# RUN: llvm-mc -filetype=obj -triple=riscv64 -mattr=+c,+relax b1.s -o b1c.o
+# RUN: llvm-mc -filetype=obj -triple=riscv64 -mattr=+c,+relax c.s -o cc.o
+# RUN: llvm-mc -filetype=obj -triple=riscv64 -mattr=+c d.s -o dc.o
+
+## No RELAX. Don't synthesize ALIGN.
+# RUN: ld.lld -r bc.o dc.o -o bd.ro
+# RUN: llvm-readelf -r bd.ro | FileCheck %s --check-prefix=NOREL
+
+# NOREL: no relocations
+
+# RUN: ld.lld -r bc.o bc.o ac.o bc.o b1c.o cc.o dc.o -o out.ro
+# RUN: llvm-objdump -dr -M no-aliases out.ro | FileCheck %s
+
+# RUN: llvm-mc -filetype=obj -triple=riscv32 -mattr=+relax a.s -o a.o
+# RUN: llvm-mc -filetype=obj -triple=riscv32 -mattr=+relax b.s -o b.o
+# RUN: llvm-mc -filetype=obj -triple=riscv32 -mattr=+relax d.s -o d.o
+# RUN: ld.lld -r a.o b.o d.o -o out0.ro
+# RUN: ld.lld -Ttext=0x10000 out0.ro -o out0
+# RUN: llvm-objdump -dr -M no-aliases out0 | FileCheck %s --check-prefix=CHECK1
+
+# CHECK:      <b0>:
+# CHECK-NEXT:   0: 00158513             addi    a0, a1, 0x1
+# CHECK-NEXT:   4: 0001                 c.nop
+# CHECK-NEXT:   6: 0001                 c.nop
+# CHECK-EMPTY:
+# CHECK-NEXT: <b0>:
+# CHECK-NEXT:   8: 00158513             addi    a0, a1, 0x1
+# CHECK-EMPTY:
+# CHECK-NEXT: <_start>:
+# CHECK-NEXT:   c: 00000097             auipc   ra, 0x0
+# CHECK-NEXT:           000000000000000c:  R_RISCV_CALL_PLT     foo
+# CHECK-NEXT:           000000000000000c:  R_RISCV_RELAX        *ABS*
+# CHECK-NEXT:  10: 000080e7             jalr    ra, 0x0(ra) <_start>
+# CHECK-NEXT:  14: 0001                 c.nop
+# CHECK-NEXT:           0000000000000014:  R_RISCV_ALIGN        *ABS*+0x6
+# CHECK-NEXT:  16: 0001                 c.nop
+# CHECK-NEXT:  18: 0001                 c.nop
+# CHECK-EMPTY:
+# CHECK-NEXT: <b0>:
+# CHECK-NEXT:  1a: 00158513             addi    a0, a1, 0x1
+# CHECK-NEXT:  1e: 0001                 c.nop
+# CHECK-NEXT:  20: 0001                 c.nop
+# CHECK-NEXT:           0000000000000020:  R_RISCV_ALIGN        *ABS*+0x6
+# CHECK-NEXT:  22: 0001                 c.nop
+# CHECK-NEXT:  24: 00000013             addi    zero, zero, 0x0
+# CHECK-EMPTY:
+# CHECK-NEXT: <b0>:
+# CHECK-NEXT:  28: 00158513             addi    a0, a1, 0x1
+# CHECK-EMPTY:
+# CHECK-NEXT: <c0>:
+# CHECK-NEXT:  2c: 00000097             auipc   ra, 0x0
+# CHECK-NEXT:           000000000000002c:  R_RISCV_CALL_PLT     foo
+# CHECK-NEXT:           000000000000002c:  R_RISCV_RELAX        *ABS*
+# CHECK-NEXT:  30: 000080e7             jalr    ra, 0x0(ra) <c0>
+# CHECK-NEXT:  34: 0001                 c.nop
+# CHECK-NEXT:           0000000000000034:  R_RISCV_ALIGN        *ABS*+0x2
+# CHECK-EMPTY:
+# CHECK-NEXT: <d0>:
+# CHECK-NEXT:  36: 00258513             addi    a0, a1, 0x2
+
+# CHECK1:      <_start>:
+# CHECK1-NEXT:    010000ef      jal     ra, 0x10010 <foo>
+# CHECK1-NEXT:    00000013      addi zero, zero, 0x0
+# CHECK1-EMPTY:
+# CHECK1-NEXT: <b0>:
+# CHECK1-NEXT:    00158513      addi    a0, a1, 0x1
+# CHECK1-EMPTY:
+# CHECK1-NEXT: <d0>:
+# CHECK1-NEXT:    00258513      addi    a0, a1, 0x2
+
+## Test CREL.
+# RUN: llvm-mc -filetype=obj -triple=riscv64 -mattr=+c,+relax --crel a.s -o acrel.o
+# RUN: ld.lld -r acrel.o bc.o -o out1.ro
+# RUN: llvm-objdump -dr -M no-aliases out1.ro | FileCheck %s --check-prefix=CHECK2
+
+# CHECK2:      <_start>:
+# CHECK2-NEXT:   0: 00000097             auipc   ra, 0x0
+# CHECK2-NEXT:           0000000000000000:  R_RISCV_CALL_PLT     foo
+# CHECK2-NEXT:           0000000000000000:  R_RISCV_RELAX        *ABS*
+# CHECK2-NEXT:   4: 000080e7             jalr    ra, 0x0(ra) <_start>
+# CHECK2-NEXT:   8: 0001                 c.nop
+# CHECK2-NEXT:           0000000000000008:  R_RISCV_ALIGN        *ABS*+0x6
+# CHECK2-NEXT:   a: 0001                 c.nop
+# CHECK2-NEXT:   c: 0001                 c.nop
+# CHECK2-EMPTY:
+# CHECK2-NEXT: <b0>:
+# CHECK2-NEXT:   e: 00158513             addi    a0, a1, 0x1
+
+#--- a.s
+.globl _start
+_start:
+  call foo
+
+.section .text1,"ax"
+.globl foo
+foo:
+
+#--- b.s
+## Needs synthesized ALIGN
+.option push
+.option norelax
+.balign 8
+b0:
+  addi a0, a1, 1
+.option pop
+
+#--- b1.s
+.option push
+.option norelax
+  .reloc ., R_RISCV_ALIGN, 6
+  addi x0, x0, 0
+  c.nop
+.balign 8
+b0:
+  addi a0, a1, 1
+.option pop
+
+#--- c.s
+.balign 2
+c0:
+  call foo
+
+#--- d.s
+## Needs synthesized ALIGN
+.balign 4
+d0:
+  addi a0, a1, 2