[ELF] Use pcrel format for eh_frame_ptr field encoding

[atanasyan]

Summary

The eh_frame_ptr field in the .eh_frame_hdr section contains an address of the .eh_frame section. Using an absolute 32-bit format for encoding of this field does not work for 64-bit targets. It is better to use a relative format because it covers both 32-bit and 64-bit cases. Sure this work if a distance between .eh_frame_hdr and .eh_frame sections is less than 4 Gb. I think it is a rather correct assumption.

[shankarke]

this fails if the access to eh frame using eh_frame_header is absolute ?

[atanasyan]

Current code fails if address of .eh_frame section is greater than 2^32 and does not fit into 4-byte. The alternative solution is to use DW_EH_PE_udata4 for 32-bit targets and DW_EH_PE_udata8 for 64-bit targets. But DW_EH_PE_pcrel covers both cases and it looks like gnu linkers use this format too.

[shankarke]

LGTM.

[ruiu]

LGTM

[atanasyan]

Thanks for review.

Closed by commit rL232414.

lib/ReaderWriter/ELF/SectionChunks.h

private:
   std::vector<uint32_t> _chains;
   const DynamicSymbolTable<ELFT> *_symbolTable;
 };
 
 template <class ELFT> class EHFrameHeader : public Section<ELFT> {
 public:
   EHFrameHeader(const ELFLinkingContext &context, StringRef name,
                 TargetLayout<ELFT> &layout, int32_t order)
-      : Section<ELFT>(context, name, "EHFrameHeader"), _layout(layout) {
+      : Section<ELFT>(context, name, "EHFrameHeader"), _ehFrameOffset(0),
+        _layout(layout) {
     this->setOrder(order);
     this->_entSize = 0;
     this->_type = SHT_PROGBITS;
     this->_flags = SHF_ALLOC;
     this->_alignment = ELFT::Is64Bits ? 8 : 4;
     // Minimum size for empty .eh_frame_hdr.
     this->_fsize = 1 + 1 + 1 + 1 + 4;
     this->_msize = this->_fsize;
   }
 
   void doPreFlight() override {
     // TODO: Generate a proper binary search table.
   }
 
   void finalize() override {
     OutputSection<ELFT> *s = _layout.findOutputSection(".eh_frame");
-    _ehFrameAddr = s ? s->virtualAddr() : 0;
+    OutputSection<ELFT> *h = _layout.findOutputSection(".eh_frame_hdr");
+    if (s && h)
+      _ehFrameOffset = s->virtualAddr() - (h->virtualAddr() + 4);
   }
 
-  virtual void write(ELFWriter *writer, TargetLayout<ELFT> &layout,
-                     llvm::FileOutputBuffer &buffer) override {
+  void write(ELFWriter *writer, TargetLayout<ELFT> &layout,
     uint8_t *chunkBuffer = buffer.getBufferStart();
     uint8_t *dest = chunkBuffer + this->fileOffset();
     int pos = 0;
     dest[pos++] = 1; // version
-    dest[pos++] = llvm::dwarf::DW_EH_PE_udata4; // eh_frame_ptr_enc
+    dest[pos++] = llvm::dwarf::DW_EH_PE_pcrel |
+                  llvm::dwarf::DW_EH_PE_sdata4; // eh_frame_ptr_enc
     dest[pos++] = llvm::dwarf::DW_EH_PE_omit; // fde_count_enc
     dest[pos++] = llvm::dwarf::DW_EH_PE_omit; // table_enc
-    *reinterpret_cast<typename llvm::object::ELFFile<ELFT>::Elf_Word *>(
-         dest + pos) = (uint32_t)_ehFrameAddr;
+    *reinterpret_cast<typename llvm::object::ELFFile<ELFT>::Elf_Sword *>(
+        dest + pos) = _ehFrameOffset;
   }
 
 private:
-  uint64_t _ehFrameAddr;
+  int32_t _ehFrameOffset;
   TargetLayout<ELFT> &_layout;
 };
 } // end namespace elf
 } // end namespace lld
 
 #endif

test/elf/eh_frame_hdr.test

 #RUN: yaml2obj -format=elf %s > %t
 #RUN: lld -flavor gnu -target x86_64-linux %t --noinhibit-exec \
 #RUN:   -o %t1
-#RUN: llvm-readobj -s %t1 | FileCheck %s
+#RUN: llvm-objdump -s %t1 | FileCheck %s
 
 !ELF
 FileHeader:
   Class: ELFCLASS64
   Data: ELFDATA2LSB
   Type: ET_REL
   Machine: EM_X86_64
 
 Sections:
 - Name: .eh_frame
   Type: SHT_PROGBITS
   Content: "00"
   AddressAlign: 8
   Flags: [SHF_ALLOC]
 
 Symbols:
   Local:
   - Name: .eh_frame
     Type: STT_SECTION
     Section: .eh_frame
 
-#CHECK: .eh_frame_hdr
+# CHECK:      Contents of section .eh_frame:
+# CHECK-NEXT:  4001e0 00
+# CHECK-NEXT: Contents of section .eh_frame_hdr:
+# CHECK-NEXT:  4001e8 011bffff f4ffffff
+#                              ^ 0x4001e0 - 0x4001e8 - 4 = 0xfffffff4