[ELF] Explicitly write null bytes in string tables

[jhenderson]

Summary

Following rL309829, if a string table appears in an executable segment, the strings will not be null terminated. This is a problem, for example, for the .dynstr section when using -no-rosegment. The strings end up being terminated with 0xcc because prior to this patch, LLD did not explicitly write the null terminators. This change fixes that by always writing the null terminators.

[jhenderson]

@rafael accepted this on the mailing list.

[ruiu]

LGTM

ELF/SyntheticSections.cpp

unsigned StringTableSection::addString(StringRef S, bool HashIt) {
   this->Size = this->Size + S.size() + 1;
   Strings.push_back(S);
   return Ret;
 }
 
 void StringTableSection::writeTo(uint8_t *Buf) {
   for (StringRef S : Strings) {
     memcpy(Buf, S.data(), S.size());
+    Buf[S.size()] = '\0';
     Buf += S.size() + 1;
   }
 }
 
 // Returns the number of version definition entries. Because the first entry
 // is for the version definition itself, it is the number of versioned symbols
 // plus one. Note that we don't support multiple versions yet.
 static unsigned getVerDefNum() { return Config->VersionDefinitions.size() + 1; }

test/ELF/dynstr-no-rosegment.s

+# Verify that a .dynstr in the .text segment has null byte separators
+
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: ld.lld %t.o -no-rosegment -o %t.so -shared
+# RUN: llvm-objdump %t.so -s -j .dynstr | FileCheck %s
+
+# CHECK: 00666f6f 00 .foo.
+
+.globl foo
+foo:
+    ret