[llvm-objcopy]Preserve data in segments not covered by sections

llvm-objcopy previously knew nothing about data in segments that wasn't
covered by section headers, meaning that it wrote zeroes instead of what
was there. As it is possible for this data to be useful to the loader,
this patch causes llvm-objcopy to start preserving this data. Data in
sections that are explicitly removed continues to be written as zeroes.

This fixes https://bugs.llvm.org/show_bug.cgi?id=41005.

Reviewed by: jakehehrlich, rupprecht

Differential Revision: https://reviews.llvm.org/D59483

llvm-svn: 356919

1 files changed, 35 insertions, 3 deletions

diff --git a/llvm/tools/llvm-objcopy/ELF/Object.cpp b/llvm/tools/llvm-objcopy/ELF/Object.cpp
index 4639d905..7cceb70 100644
--- a/llvm/tools/llvm-objcopy/ELF/Object.cpp
+++ b/llvm/tools/llvm-objcopy/ELF/Object.cpp
@@ -906,7 +906,9 @@ template <class ELFT> void ELFBuilder<ELFT>::setParentSegment(Segment &Child) {
 template <class ELFT> void ELFBuilder<ELFT>::readProgramHeaders() {
   uint32_t Index = 0;
   for (const auto &Phdr : unwrapOrError(ElfFile.program_headers())) {
-    Segment &Seg = Obj.addSegment();
+    ArrayRef<uint8_t> Data{ElfFile.base() + Phdr.p_offset,
+                           (size_t)Phdr.p_filesz};
+    Segment &Seg = Obj.addSegment(Data);
     Seg.Type = Phdr.p_type;
     Seg.Flags = Phdr.p_flags;
     Seg.OriginalOffset = Phdr.p_offset;
@@ -1350,7 +1352,31 @@ template <class ELFT> void ELFWriter<ELFT>::writeShdrs() {
 
 template <class ELFT> void ELFWriter<ELFT>::writeSectionData() {
   for (auto &Sec : Obj.sections())
-    Sec.accept(*SecWriter);
+    // Segments are responsible for writing their contents, so only write the
+    // section data if the section is not in a segment. Note that this renders
+    // sections in segments effectively immutable.
+    if (Sec.ParentSegment == nullptr)
+      Sec.accept(*SecWriter);
+}
+
+template <class ELFT> void ELFWriter<ELFT>::writeSegmentData() {
+  for (Segment &Seg : Obj.segments()) {
+    uint8_t *B = Buf.getBufferStart() + Seg.Offset;
+    assert(Seg.FileSize == Seg.getContents().size() &&
+           "Segment size must match contents size");
+    std::memcpy(B, Seg.getContents().data(), Seg.FileSize);
+  }
+
+  // Iterate over removed sections and overwrite their old data with zeroes.
+  for (auto &Sec : Obj.removedSections()) {
+    Segment *Parent = Sec.ParentSegment;
+    if (Parent == nullptr || Sec.Type == SHT_NOBITS || Sec.Size == 0)
+      continue;
+    uint64_t Offset =
+        Sec.OriginalOffset - Parent->OriginalOffset + Parent->Offset;
+    uint8_t *B = Buf.getBufferStart();
+    std::memset(B + Offset, 0, Sec.Size);
+  }
 }
 
 Error Object::removeSections(
@@ -1396,7 +1422,10 @@ Error Object::removeSections(
       return E;
   }
 
-  // Now finally get rid of them all togethor.
+  // Transfer removed sections into the Object RemovedSections container for use
+  // later.
+  std::move(Iter, Sections.end(), std::back_inserter(RemovedSections));
+  // Now finally get rid of them all together.
   Sections.erase(Iter, std::end(Sections));
   return Error::success();
 }
@@ -1542,6 +1571,9 @@ template <class ELFT> size_t ELFWriter<ELFT>::totalSize() const {
 }
 
 template <class ELFT> Error ELFWriter<ELFT>::write() {
+  // Segment data must be written first, so that the ELF header and program
+  // header tables can overwrite it, if covered by a segment.
+  writeSegmentData();
   writeEhdr();
   writePhdrs();
   writeSectionData();