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

[grimar]

Summary

We produce broken output currently.
Code that writes .dynsym assumes output buffer is zero-filled,
though that is not always true. When --no-rosegment is given,
buffer can be filled with trap instructions. Patch fixes the issue.

It is relative with PR34705.

[grimar]

• Simplified testcase.

[ruiu]

LGTM

lld/trunk/ELF/SyntheticSections.cpp

 SymbolTableSection<ELFT>::SymbolTableSection(StringTableSection &StrTabSec)
     : SymbolTableBaseSection(StrTabSec) {
   this->Entsize = sizeof(Elf_Sym);
 }
 
 // Write the internal symbol table contents to the output symbol table.
 template <class ELFT> void SymbolTableSection<ELFT>::writeTo(uint8_t *Buf) {
   // The first entry is a null entry as per the ELF spec.
+  memset(Buf, 0, sizeof(Elf_Sym));
   Buf += sizeof(Elf_Sym);
 
   auto *ESym = reinterpret_cast<Elf_Sym *>(Buf);
 
   for (SymbolTableEntry &Ent : Symbols) {
     SymbolBody *Body = Ent.Symbol;
 
     // Set st_info and st_other.
+    ESym->st_other = 0;
     if (Body->isLocal()) {
       ESym->setBindingAndType(STB_LOCAL, Body->Type);
     } else {
       ESym->setBindingAndType(Body->symbol()->computeBinding(), Body->Type);
       ESym->setVisibility(Body->symbol()->Visibility);
     }
 
     ESym->st_name = Ent.StrTabOffset;
 
     // Set a section index.
     if (const OutputSection *OutSec = Body->getOutputSection())
       ESym->st_shndx = OutSec->SectionIndex;
     else if (isa<DefinedRegular>(Body))
       ESym->st_shndx = SHN_ABS;
     else if (isa<DefinedCommon>(Body))
       ESym->st_shndx = SHN_COMMON;
+    else
+      ESym->st_shndx = SHN_UNDEF;
 
     // Copy symbol size if it is a defined symbol. st_size is not significant
     // for undefined symbols, so whether copying it or not is up to us if that's
     // the case. We'll leave it as zero because by not setting a value, we can
     // get the exact same outputs for two sets of input files that differ only
     // in undefined symbol size in DSOs.
-    if (ESym->st_shndx != SHN_UNDEF)
+    if (ESym->st_shndx == SHN_UNDEF)
+      ESym->st_size = 0;
+    else
       ESym->st_size = Body->getSize<ELFT>();
 
     // st_value is usually an address of a symbol, but that has a
     // special meaining for uninstantiated common symbols (this can
     // occur if -r is given).
     if (!Config->DefineCommon && isa<DefinedCommon>(Body))
       ESym->st_value = cast<DefinedCommon>(Body)->Alignment;
     else

lld/trunk/test/ELF/dynsym-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 -o %t %t.o
+# RUN: llvm-readobj -dyn-symbols -s %t | FileCheck %s
+
+# CHECK:      DynamicSymbols [
+# CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name: @ (0)
+# CHECK-NEXT:     Value: 0x0
+# CHECK-NEXT:     Size: 0
+# CHECK-NEXT:     Binding: Local
+# CHECK-NEXT:     Type: None
+# CHECK-NEXT:     Other: 0
+# CHECK-NEXT:     Section: Undefined
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name: undef@
+# CHECK-NEXT:     Value: 0x0
+# CHECK-NEXT:     Size: 0
+# CHECK-NEXT:     Binding: Global
+# CHECK-NEXT:     Type: None
+# CHECK-NEXT:     Other: 0
+# CHECK-NEXT:     Section: Undefined
+# CHECK-NEXT:   }
+# CHECK-NEXT: ]
+
+callq undef@PLT