[llvm-objcopy] Route section copies through writeSectionContents

[fzakaria]

Motivation

The ELF section writers currently copy section contents directly into a WritableMemoryBuffer. Before introducing an alternative output implementation, these writes need to go through a common function.

Change

This change adds a virtual writeSectionContents helper and routes the section byte-copy paths through it. The default implementation continues to write into the existing memory buffer, so this does not change the output strategy by itself.

While routing SHT_SYMTAB_SHNDX through the helper, the indexes are materialized using the target Elf_Word type. This ensures that the entries use the output ELF endianness when converting between formats. The big-endian test case exercises this behavior.

This change was developed with assistance from OpenAI Codex and includes a trailer following the AI policy.

[llvmorg-github-actions]

@llvm/pr-subscribers-llvm-binary-utilities

Author: Farid Zakaria (fzakaria)

Changes

Motivation

The ELF section writers currently copy section contents directly into a WritableMemoryBuffer. Before introducing an alternative output implementation, these writes need to go through a common function.

Change

This change adds a virtual writeSectionContents helper and routes the section byte-copy paths through it. The default implementation continues to write into the existing memory buffer, so this does not change the output strategy by itself.

While routing SHT_SYMTAB_SHNDX through the helper, the indexes are materialized using the target Elf_Word type. This ensures that the entries use the output ELF endianness when converting between formats. The big-endian test case exercises this behavior.

This change was developed with assistance from OpenAI Codex and includes a trailer following the AI policy.

---

Full diff: https://github.com/llvm/llvm-project/pull/217974.diff

3 Files Affected:

• (modified) llvm/lib/ObjCopy/ELF/ELFObject.cpp (+26-15)
• (modified) llvm/lib/ObjCopy/ELF/ELFObject.h (+1)
• (modified) llvm/test/tools/llvm-objcopy/ELF/many-sections.test (+4)

diff --git a/llvm/lib/ObjCopy/ELF/ELFObject.cpp b/llvm/lib/ObjCopy/ELF/ELFObject.cpp
index ac818343f3f8f..dace67fdca44d 100644
--- a/llvm/lib/ObjCopy/ELF/ELFObject.cpp
+++ b/llvm/lib/ObjCopy/ELF/ELFObject.cpp
@@ -183,9 +183,14 @@ Error BinarySectionWriter::visit(const GroupSection &Sec) {
                            "cannot write '" + Sec.Name + "' out to binary");
 }
 
+void SectionWriter::writeSectionContents(ArrayRef<uint8_t> Data,
+                                         uint64_t Offset) {
+  llvm::copy(Data, Out.getBufferStart() + Offset);
+}
+
 Error SectionWriter::visit(const Section &Sec) {
   if (Sec.Type != SHT_NOBITS)
-    llvm::copy(Sec.Contents, Out.getBufferStart() + Sec.Offset);
+    writeSectionContents(Sec.Contents, Sec.Offset);
 
   return Error::success();
 }
@@ -453,7 +458,7 @@ void Section::restoreSymTabLink(SymbolTableSection &SymTab) {
 }
 
 Error SectionWriter::visit(const OwnedDataSection &Sec) {
-  llvm::copy(Sec.Data, Out.getBufferStart() + Sec.Offset);
+  writeSectionContents(Sec.Data, Sec.Offset);
   return Error::success();
 }
 
@@ -487,9 +492,7 @@ Error ELFSectionWriter<ELFT>::visit(const DecompressedSection &Sec) {
                              "failed to decompress section '" + Sec.Name +
                                  "': " + toString(std::move(E)));
 
-  uint8_t *Buf = reinterpret_cast<uint8_t *>(Out.getBufferStart()) + Sec.Offset;
-  llvm::copy(Decompressed, Buf);
-
+  writeSectionContents(Decompressed, Sec.Offset);
   return Error::success();
 }
 
@@ -532,11 +535,10 @@ Error BinarySectionWriter::visit(const CompressedSection &Sec) {
 
 template <class ELFT>
 Error ELFSectionWriter<ELFT>::visit(const CompressedSection &Sec) {
-  uint8_t *Buf = reinterpret_cast<uint8_t *>(Out.getBufferStart()) + Sec.Offset;
   Elf_Chdr_Impl<ELFT> Chdr = {};
   switch (Sec.CompressionType) {
   case DebugCompressionType::None:
-    llvm::copy(Sec.OriginalData, Buf);
+    writeSectionContents(Sec.OriginalData, Sec.Offset);
     return Error::success();
   case DebugCompressionType::Zlib:
     Chdr.ch_type = ELF::ELFCOMPRESS_ZLIB;
@@ -547,10 +549,10 @@ Error ELFSectionWriter<ELFT>::visit(const CompressedSection &Sec) {
   }
   Chdr.ch_size = Sec.DecompressedSize;
   Chdr.ch_addralign = Sec.DecompressedAlign;
-  memcpy(Buf, &Chdr, sizeof(Chdr));
-  Buf += sizeof(Chdr);
-
-  llvm::copy(Sec.CompressedData, Buf);
+  writeSectionContents(
+      ArrayRef(reinterpret_cast<const uint8_t *>(&Chdr), sizeof(Chdr)),
+      Sec.Offset);
+  writeSectionContents(Sec.CompressedData, Sec.Offset + sizeof(Chdr));
   return Error::success();
 }
 
@@ -613,8 +615,14 @@ Error StringTableSection::accept(MutableSectionVisitor &Visitor) {
 
 template <class ELFT>
 Error ELFSectionWriter<ELFT>::visit(const SectionIndexSection &Sec) {
-  uint8_t *Buf = reinterpret_cast<uint8_t *>(Out.getBufferStart()) + Sec.Offset;
-  llvm::copy(Sec.Indexes, reinterpret_cast<Elf_Word *>(Buf));
+  SmallVector<Elf_Word, 0> Indexes;
+  Indexes.reserve(Sec.Indexes.size());
+  for (uint32_t Index : Sec.Indexes)
+    Indexes.emplace_back(Index);
+  writeSectionContents(
+      ArrayRef(reinterpret_cast<const uint8_t *>(Indexes.data()),
+               Indexes.size() * sizeof(Elf_Word)),
+      Sec.Offset);
   return Error::success();
 }
 
@@ -1029,7 +1037,7 @@ void RelocationSection::replaceSectionReferences(
 }
 
 Error SectionWriter::visit(const DynamicRelocationSection &Sec) {
-  llvm::copy(Sec.Contents, Out.getBufferStart() + Sec.Offset);
+  writeSectionContents(Sec.Contents, Sec.Offset);
   return Error::success();
 }
 
@@ -1182,7 +1190,10 @@ Error ELFSectionWriter<ELFT>::visit(const GnuDebugLinkSection &Sec) {
   Elf_Word *CRC =
       reinterpret_cast<Elf_Word *>(Buf + Sec.Size - sizeof(Elf_Word));
   *CRC = Sec.CRC32;
-  llvm::copy(Sec.FileName, Buf);
+  writeSectionContents(
+      ArrayRef(reinterpret_cast<const uint8_t *>(Sec.FileName.data()),
+               Sec.FileName.size()),
+      Sec.Offset);
   return Error::success();
 }
 
diff --git a/llvm/lib/ObjCopy/ELF/ELFObject.h b/llvm/lib/ObjCopy/ELF/ELFObject.h
index 2783ef27ac9de..9df62d8642917 100644
--- a/llvm/lib/ObjCopy/ELF/ELFObject.h
+++ b/llvm/lib/ObjCopy/ELF/ELFObject.h
@@ -107,6 +107,7 @@ class MutableSectionVisitor {
 class SectionWriter : public SectionVisitor {
 protected:
   WritableMemoryBuffer &Out;
+  virtual void writeSectionContents(ArrayRef<uint8_t> Data, uint64_t Offset);
 
 public:
   ~SectionWriter() override = default;
diff --git a/llvm/test/tools/llvm-objcopy/ELF/many-sections.test b/llvm/test/tools/llvm-objcopy/ELF/many-sections.test
index 6622db237026f..64ee1e4b4b48b 100644
--- a/llvm/test/tools/llvm-objcopy/ELF/many-sections.test
+++ b/llvm/test/tools/llvm-objcopy/ELF/many-sections.test
@@ -5,6 +5,10 @@ RUN: llvm-objcopy %t %t2
 RUN: llvm-readobj --file-headers --sections --symbols %t2 | FileCheck %s
 RUN: llvm-readelf --symbols %t2 | FileCheck --check-prefix=SYMS %s
 
+## Check that SHT_SYMTAB_SHNDX entries use the output endianness.
+RUN: llvm-objcopy --output-target=elf64-s390 %t %t.be
+RUN: llvm-readobj --file-headers --sections --symbols %t.be | FileCheck %s
+
 ## The ELF header should have e_shnum == 0 and e_shstrndx == SHN_XINDEX.
 # CHECK:        SectionHeaderCount: 0
 # CHECK-NEXT:   StringTableSectionIndex: 65535