[llvm-readobj] - Don't crash when checking the number of dynamic symbols.

[grimar]

Summary

When we deriving the number of symbols from the DT_HASH table, we can crash when
calculate the number of symbols in the symbol table when SHT_DYNSYM
has sh_entsize == 0.

The patch fixes the issue.

[jhenderson]

Couple of nits, otherwise LGTM.

llvm/test/tools/llvm-readobj/ELF/dyn-symbols-size-from-hash-table.test

163

null -> zero

Also below in file names.

251

I'd change "is no-op" to "is the standard entsize value"

llvm/test/tools/llvm-readobj/ELF/dyn-symbols-size-from-hash-table.test

 # RUN: yaml2obj --docnum=2 %s -o %t2-larger -DCHAIN="[1, 2, 3, 4]"
 # RUN: llvm-readobj --dyn-symbols %t2-larger 2>&1 | \
 # RUN:   FileCheck %s --check-prefixes=LLVM2,LLVM2-MORE,LLVM2-ALL,WARN \
 # RUN:                --implicit-check-not=warning: -DNCHAIN=4
 # RUN: llvm-readelf --dyn-symbols %t2-larger 2>&1 | \
 # RUN:   FileCheck %s --check-prefixes=GNU2,GNU2-MORE,GNU2-ALL,WARN \
 # RUN:                --implicit-check-not=warning: -DNCHAIN=4
 
+# WARN: warning: '{{.*}}2-{{.*}}': hash table nchain ([[NCHAIN]]) differs from symbol count derived from SHT_DYNSYM section header (3)
+
+## Show we report a warning when the sh_entsize of the SHT_DYNSYM section is null and therefore we are unable
+## to derive the number of dynamic symbols from SHT_DYNSYM section header.
+# RUN: yaml2obj --docnum=2 %s -o %t2-null-entsize -DCHAIN="[1, 2, 3, 4]" -DENTSIZE=0
+# RUN: llvm-readobj --dyn-symbols %t2-null-entsize 2>&1 | \
+# RUN:   FileCheck %s -DFILE=%t2-null-entsize --check-prefixes=LLVM2,LLVM2-MORE,LLVM2-ALL,WARN-ENTSIZE \
+# RUN:                --implicit-check-not=warning:
+# RUN: llvm-readelf --dyn-symbols %t2-null-entsize 2>&1 | \
+# RUN:   FileCheck %s -DFILE=%t2-null-entsize --check-prefixes=GNU2,GNU2-MORE,GNU2-ALL,WARN-ENTSIZE \
+# RUN:                --implicit-check-not=warning: -DNCHAIN=4
+
+## WARN-ENTSIZE: warning: '[[FILE]]': SHT_DYNSYM section has sh_entsize == 0
+
 ## Show there's no warning if the sizes match
 # RUN: yaml2obj --docnum=2 %s -o %t2-same -DCHAIN="[1, 2, 3]"
 # RUN: llvm-readobj --dyn-symbols %t2-same 2>&1 | \
 # RUN:   FileCheck %s --check-prefixes=LLVM2,LLVM2-MORE --implicit-check-not=warning:
 # RUN: llvm-readelf --dyn-symbols %t2-same 2>&1 | \
 # RUN:   FileCheck %s --check-prefixes=GNU2,GNU2-MORE \
 # RUN:                --implicit-check-not=warning: -DNCHAIN=3
 
-# WARN: warning: '{{.*}}2-{{.*}}': hash table nchain ([[NCHAIN]]) differs from symbol count derived from SHT_DYNSYM section header (3)
-
 # LLVM2:           DynamicSymbols [
 # LLVM2-NEXT:        Symbol {
 # LLVM2-NEXT:          Name:  (0)
 # LLVM2-NEXT:          Value: 0x0
 # LLVM2-NEXT:          Size: 0
 # LLVM2-NEXT:          Binding: Local (0x0)
 # LLVM2-NEXT:          Type: None (0x0)
 # LLVM2-NEXT:          Other: 0
Sections:
   - Name:         .data
     Type:         SHT_PROGBITS
   - Name:         .dynsym
     Type:         SHT_DYNSYM
     Flags:        [ SHF_ALLOC ]
     ShSize:       0x48
     Address:      0x400
     AddressAlign: 0x400
+## 0x18 is no-op.
+    EntSize:      [[ENTSIZE=0x18]]
   - Name:         .hash
     Type:         SHT_HASH
     Flags:        [ SHF_ALLOC ]
     Address:      0x600
     AddressAlign: 0x200
     Bucket:       [ 1 ]
     Chain:        [[CHAIN]]
   - Name:         .dynstr

llvm/tools/llvm-readobj/ELFDumper.cpp

if (DynSymFromTable && DynSymFromTable->Addr != DynSymRegion->Addr)
       reportUniqueWarning(
           createError("SHT_DYNSYM section header and DT_SYMTAB disagree about "
                       "the location of the dynamic symbol table"));
 
     // According to the ELF gABI: "The number of symbol table entries should
     // equal nchain". Check to see if the DT_HASH hash table nchain value
     // conflicts with the number of symbols in the dynamic symbol table
     // according to the section header.
-    if (HashTable &&
-        HashTable->nchain != DynSymRegion->Size / DynSymRegion->EntSize)
+    if (HashTable) {
+      if (DynSymRegion->EntSize == 0)
+        reportUniqueWarning(
+            createError("SHT_DYNSYM section has sh_entsize == 0"));
+      else if (HashTable->nchain != DynSymRegion->Size / DynSymRegion->EntSize)
         reportUniqueWarning(createError(
             "hash table nchain (" + Twine(HashTable->nchain) +
             ") differs from symbol count derived from SHT_DYNSYM section "
             "header (" +
             Twine(DynSymRegion->Size / DynSymRegion->EntSize) + ")"));
     }
+  }
 
   // Delay the creation of the actual dynamic symbol table until now, so that
   // checks can always be made against the section header-based properties,
   // without worrying about tag order.
   if (DynSymFromTable) {
     if (!DynSymRegion) {
       DynSymRegion = DynSymFromTable;
     } else {