[ELF] - Do not produce broken .dynamic section with --no-rosegment

[grimar]

Summary

LLD produces broken .dynamic section when --no-rosegment and at least one
of following options is present:

1. -z rodynamic is given.
2. MIPS target.

That happens because code that writes .dynamic assumes target buffer is zero-filled,
what can be not true after LLD fills it with trap-instructions. With one of two options
above, .dynamic becomes SHF_ALLOC section, so can be affected.

[grimar]

• Simplified testcase.

[ruiu]

LGTM

ELF/SyntheticSections.cpp

1174

Look at line 1073 (it is always good to look around when you add code to an existing file). For consistency, 0ULL -> (uint64_t)0.

ELF/SyntheticSections.cpp

if (const SymbolBody *B = InX::MipsGot->getFirstGlobalEntry())
       add({DT_MIPS_GOTSYM, B->DynsymIndex});
     else
       add({DT_MIPS_GOTSYM, InX::DynSymTab->getNumSymbols()});
     add({DT_PLTGOT, InX::MipsGot});
     if (InX::MipsRldMap)
       add({DT_MIPS_RLD_MAP, InX::MipsRldMap});
   }
 
-  getParent()->Link = this->Link;
+  add({DT_NULL, 0ULL});
 
-  // +1 for DT_NULL
-  this->Size = (Entries.size() + 1) * this->Entsize;
+  getParent()->Link = this->Link;
+  this->Size = Entries.size() * this->Entsize;
 }
 
 template <class ELFT> void DynamicSection<ELFT>::writeTo(uint8_t *Buf) {
   auto *P = reinterpret_cast<Elf_Dyn *>(Buf);
 
   for (const Entry &E : Entries) {
     P->d_tag = E.Tag;
     switch (E.Kind) {

test/ELF/dynamic-no-rosegment.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: ld.lld -shared --no-rosegment -z rodynamic -o %t %t.o
+# RUN: llvm-readobj -dynamic-table -s %t | FileCheck %s
+
+# CHECK:      DynamicSection [ (10 entries)
+# CHECK-NEXT:   Tag                Type                 Name/Value
+# CHECK-NEXT:   0x0000000000000017 JMPREL               0x278
+# CHECK-NEXT:   0x0000000000000002 PLTRELSZ             24 (bytes)
+# CHECK-NEXT:   0x0000000000000003 PLTGOT               0x1000
+# CHECK-NEXT:   0x0000000000000014 PLTREL               RELA
+# CHECK-NEXT:   0x0000000000000006 SYMTAB               0x160
+# CHECK-NEXT:   0x000000000000000B SYMENT               24 (bytes)
+# CHECK-NEXT:   0x0000000000000005 STRTAB               0x268
+# CHECK-NEXT:   0x000000000000000A STRSZ                12 (bytes)
+# CHECK-NEXT:   0x0000000000000004 HASH                 0x1A8
+# CHECK-NEXT:   0x0000000000000000 NULL                 0x0
+# CHECK-NEXT: ]
+
+.globl _start
+_start:
+ callq foo@PLT