[ELF] Emit DT_TEXTREL dynamic table flag.

If one or more dynamic relocation might modify a read-only section,
dynamic table should contain DT_TEXTREL tag.

The patch introduces new `RelocationTable::canModifyReadonlySection()`
method. This method checks through the relocations to see if any modifies
a read-only section. The DynamicTable class calls this method and emits
the DT_TEXTREL tag if necessary.

The patch reviewed by Rui Ueyama and Shankar Easwaran.

llvm-svn: 208670

lld/lib/ReaderWriter/ELF/SectionChunks.h

@@ -954,6 +954,16 @@ public:
     _symbolTable = symbolTable;
   }
 
+  /// \brief Check if any relocation modifies a read-only section.
+  bool canModifyReadonlySection() const {
+    for (const auto &rel : _relocs) {
+      const DefinedAtom *atom = rel.first;
+      if ((atom->permissions() & DefinedAtom::permRW_) != DefinedAtom::permRW_)
+        return true;
+    }
+    return false;
+  }
+
   virtual void finalize() {
     this->_link = _symbolTable ? _symbolTable->ordinal() : 0;
     if (this->_parent)
@@ -1080,6 +1090,11 @@ public:
       _dt_relasz = addEntry(dyn);
       dyn.d_tag = isRela ? DT_RELAENT : DT_RELENT;
       _dt_relaent = addEntry(dyn);
+
+      if (_layout.getDynamicRelocationTable()->canModifyReadonlySection()) {
+        dyn.d_tag = DT_TEXTREL;
+        _dt_textrel = addEntry(dyn);
+      }
     }
     if (_layout.hasPLTRelocationTable()) {
       dyn.d_tag = DT_PLTRELSZ;
@@ -1165,6 +1180,7 @@ private:
   std::size_t _dt_jmprel;
   std::size_t _dt_fini_array;
   std::size_t _dt_fini_arraysz;
+  std::size_t _dt_textrel;
   TargetLayout<ELFT> &_layout;
   DynamicSymbolTable<ELFT> *_dynamicSymbolTable;
   HashSection<ELFT> *_hashTable;

lld/test/elf/Mips/dt-textrel.test

@@ -0,0 +1,47 @@
+# Check that if a dynamic relocation modify a read-only section,
+# .dynamic section contains the DT_TEXTREL tag.
+
+# RUN: yaml2obj -format=elf %S/Inputs/pic-obj.yaml > %t-so-obj
+# RUN: lld -flavor gnu -target mipsel -shared -o %t-so %t-so-obj
+# RUN: yaml2obj -format=elf %s > %t-obj
+# RUN: lld -flavor gnu -target mipsel -e T0 -o %t-exe %t-obj %t-so
+# RUN: llvm-readobj -dynamic-table %t-exe | FileCheck %s
+
+# CHECK: 0x{{[0-9A-F]+}} TEXTREL
+
+FileHeader:
+  Class:           ELFCLASS32
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_MIPS
+  Flags:           [ EF_MIPS_NOREORDER, EF_MIPS_PIC, EF_MIPS_CPIC,
+                     EF_MIPS_ABI_O32, EF_MIPS_ARCH_32 ]
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    AddressAlign:    0x04
+    Content:         '00000000'
+
+  - Name:            .rel.text
+    Type:            SHT_REL
+    Link:            .symtab
+    Info:            .text
+    AddressAlign:    0x04
+    Relocations:
+      - Offset:      0
+        Symbol:      T1
+        Type:        R_MIPS_32
+
+Symbols:
+  Local:
+    - Name:            .text
+      Type:            STT_SECTION
+      Section:         .text
+
+  Global:
+    - Name:            T0
+      Type:            STT_FUNC
+      Section:         .text
+      Size:            0x04
+    - Name:            T1