[llvm][llvm-objcopy] When outputting to binary don't output segments that cover no sections

[jakehehrlich]

Summary

Sometimes LLD will produce a PT_LOAD segment that only covers the headers (and covers no sections). GNU objcopy does not output the segment contents for these sections. In particular this is an issue in building magenta because the final link step for the kernel would produce just such a PT_LOAD segment. This change is to support this case and to match what GNU objcopy does in this case.

[jhenderson]

Looks fine to me, but could we have a test, please?

tools/llvm-objcopy/Object.cpp

358

Not sure if there's a typical way of doing this, but I have a preference for an explicit comparison against nullptr (i.e. Segment->firstSection() != nullptr). I'm happy either way though.

378–379

Ditto.

[jakehehrlich]

The binary being uploaded can be downloaded here: https://drive.google.com/file/d/0B3gtIAmiMwZXNlZzckpOcEJfYzA/view?usp=sharing

It was compiled by clang (which I think just uses llvm-mc) and linked using ld.lld. It has a sectionless segment that covers a PT_PHDR segment. I plan on using this binary for multiple tests involving this strange case.

The test ensures that exactly the .text and .data sections worth of data is copied. If the extra PT_LOAD segment was copied it would cause the size to increase by at least a page.

[jhenderson]

LGTM

test/tools/llvm-objcopy/sectionless-segment.test

+# RUN: llvm-objcopy -O binary %p/Inputs/pt-phdr.elf %t
+# RUN: wc -c < %t | FileCheck %s
+
+# CHECK: 4110

tools/llvm-objcopy/Object.cpp

 
 template <class ELFT> size_t BinaryObject<ELFT>::totalSize() const {
   return TotalSize;
 }
 
 template <class ELFT>
 void BinaryObject<ELFT>::write(FileOutputBuffer &Out) const {
   for (auto &Segment : this->Segments) {
-    if (Segment->Type == llvm::ELF::PT_LOAD) {
+    // GNU objcopy does not output segments that do not cover a section. Such
+    // segments can sometimes be produced by LLD due to how LLD handles PT_PHDR.
+    if (Segment->Type == llvm::ELF::PT_LOAD &&
+        Segment->firstSection() != nullptr) {
       Segment->writeSegment(Out);
     }
   }
 }
 
 template <class ELFT> void BinaryObject<ELFT>::finalize() {
   for (auto &Segment : this->Segments)
     Segment->finalize();
 
   // Put all segments in offset order.
   auto CompareSegments = [](const SegPtr &A, const SegPtr &B) {
     return A->Offset < B->Offset;
   };
   std::sort(std::begin(this->Segments), std::end(this->Segments),
             CompareSegments);
 
   uint64_t Offset = 0;
   for (auto &Segment : this->Segments) {
-    if (Segment->Type == llvm::ELF::PT_LOAD) {
+    if (Segment->Type == llvm::ELF::PT_LOAD &&
+        Segment->firstSection() != nullptr) {
       Offset = alignTo(Offset, Segment->Align);
       Segment->Offset = Offset;
       Offset += Segment->FileSize;
     }
   }
   TotalSize = Offset;
 }