[ELF] Fix SysV hash tables with --no-rosegment

[smeenai]

Summary

When setting up the chain, we copy over the bucket's previous symbol
index, assuming that this index will be 0 (STN_UNDEF) for an unused
bucket (marking the end of the chain). When linking with --no-rosegment,
however, unused buckets will in fact contain the padding value, and so
the hash table will end up containing invalid chains. Explicitly
zero-initialize the buckets to fix this problem. Also explicitly
zero-initialize the chain entry for STN_UNDEF; I don't think this is
actually necessary, but it doesn't hurt, and should make the hash table
identical both with and without --no-rosegment.

[ruiu]

ELF/SyntheticSections.cpp

1842

I'd do memset(Buf, 0, Size) here just like we did at beginning of GnuHashTableSection::writeTo(). That's less error-prone and easy to understand than filling holes with zeros. Zero-clearing a contiguous chunk of memory is very fast, so we don't have to minimize the number of bytes written to memory in this case.

[smeenai]

Rui's comment

[ruiu]

LGTM

ELF/SyntheticSections.cpp

1839–1841

If you copy-n-paste this comment, update the other comment to refer this one, so that we won't end up having the identical comment in two functions.

[smeenai]

Change comment to refer to existing one

ELF/SyntheticSections.cpp

void HashTableSection::finalizeContents() {
   NumEntries += InX::DynSymTab->getNumSymbols(); // The chain entries.
 
   // Create as many buckets as there are symbols.
   NumEntries += InX::DynSymTab->getNumSymbols();
   this->Size = NumEntries * 4;
 }
 
 void HashTableSection::writeTo(uint8_t *Buf) {
+  // See comment in GnuHashTableSection::writeTo.
+  memset(Buf, 0, Size);
+
   unsigned NumSymbols = InX::DynSymTab->getNumSymbols();
 
   uint32_t *P = reinterpret_cast<uint32_t *>(Buf);
   write32(P++, NumSymbols); // nbucket
   write32(P++, NumSymbols); // nchain
 
   uint32_t *Buckets = P;
   uint32_t *Chains = P + NumSymbols;
 

test/ELF/sysv-hash-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 -hash-table %t | FileCheck %s
+
+# CHECK:      HashTable {
+# CHECK-NEXT:   Num Buckets: 2
+# CHECK-NEXT:   Num Chains: 2
+# CHECK-NEXT:   Buckets: [1, 0]
+# CHECK-NEXT:   Chains: [0, 0]
+# CHECK-NEXT: }
+
+callq undef@PLT