[PATCH] Object/COFF: Support large relocation table.

[Rui Ueyama]

Hi majnemer,

NumberOfRelocations field in COFF section table is only 16-bit wide. If an
object has more than 65535 relocations, the number of relocations is stored
to VirtualAddress field in the first relocation field, and a special flag
(IMAGE_SCN_LNK_NRELOC_OVFL) is set to Characteristics field.

In test we cheated a bit. I made up a test file so that it has
IMAGE_SCN_LNK_NRELOC_OVFL flag but the number of relocations is much smaller
than 65535. This is to avoid checking in a large test file just to test a
file with many relocations.

http://llvm-reviews.chandlerc.com/D3139

Files:
  lib/Object/COFFObjectFile.cpp
  test/tools/llvm-objdump/Inputs/many-relocs.obj-i386
  test/tools/llvm-objdump/coff-many-relocs.test

Index: lib/Object/COFFObjectFile.cpp
===================================================================
--- lib/Object/COFFObjectFile.cpp
+++ lib/Object/COFFObjectFile.cpp
@@ -367,25 +367,48 @@
 relocation_iterator COFFObjectFile::section_rel_begin(DataRefImpl Ref) const {
   const coff_section *Sec = toSec(Ref);
   DataRefImpl Ret;
-  if (Sec->NumberOfRelocations == 0)
+  if (Sec->NumberOfRelocations == 0) {
     Ret.p = 0;
-  else
-    Ret.p = reinterpret_cast<uintptr_t>(base() + Sec->PointerToRelocations);
-
+  } else {
+    auto begin = reinterpret_cast<const coff_relocation*>(
+        base() + Sec->PointerToRelocations);
+    if (Sec->Characteristics & COFF::IMAGE_SCN_LNK_NRELOC_OVFL &&
+        Sec->NumberOfRelocations == UINT16_MAX) {
+      // Skip the first relocation entry repurposed to store the number of
+      // relocations.
+      begin++;
+    }
+    Ret.p = reinterpret_cast<uintptr_t>(begin);
+  }
   return relocation_iterator(RelocationRef(Ret, this));
 }
 
+static uint32_t getNumberOfRelocations(const coff_section *Sec,
+                                       const uint8_t *base) {
+  // The field for the number of relocations in COFF section table is only
+  // 16-bit wide. If a section has more than 65535 relocations, 0xFFFF is set to
+  // NumberOfRelocations field, and the actual relocation count is stored in the
+  // VirtualAddress field in the first relocation entry.
+  if (Sec->Characteristics & COFF::IMAGE_SCN_LNK_NRELOC_OVFL &&
+      Sec->NumberOfRelocations == UINT16_MAX) {
+    auto *FirstReloc = reinterpret_cast<const coff_relocation*>(
+        base + Sec->PointerToRelocations);
+    return FirstReloc->VirtualAddress;
+  }
+  return Sec->NumberOfRelocations;
+}
+
 relocation_iterator COFFObjectFile::section_rel_end(DataRefImpl Ref) const {
   const coff_section *Sec = toSec(Ref);
   DataRefImpl Ret;
-  if (Sec->NumberOfRelocations == 0)
+  if (Sec->NumberOfRelocations == 0) {
     Ret.p = 0;
-  else
-    Ret.p = reinterpret_cast<uintptr_t>(
-              reinterpret_cast<const coff_relocation*>(
-                base() + Sec->PointerToRelocations)
-              + Sec->NumberOfRelocations);
-
+  } else {
+    auto begin = reinterpret_cast<const coff_relocation*>(
+        base() + Sec->PointerToRelocations);
+    uint32_t NumReloc = getNumberOfRelocations(Sec, base());
+    Ret.p = reinterpret_cast<uintptr_t>(begin + NumReloc);
+  }
   return relocation_iterator(RelocationRef(Ret, this));
 }
 
Index: test/tools/llvm-objdump/coff-many-relocs.test
===================================================================
--- /dev/null
+++ test/tools/llvm-objdump/coff-many-relocs.test
@@ -0,0 +1,14 @@
+// Test that llvm-objdump can handle IMAGE_SCN_LNK_NRELOC_OVFL.
+// RUN: llvm-objdump -r %p/Inputs/many-relocs.obj-i386 | FileCheck %s
+
+CHECK:      RELOCATION RECORDS FOR [.text]:
+CHECK-NEXT: 10 IMAGE_REL_I386_DIR16 foo
+CHECK-NEXT: 17 IMAGE_REL_I386_REL16 foo
+CHECK-NEXT: 24 IMAGE_REL_I386_DIR32 foo
+CHECK-NEXT: 31 IMAGE_REL_I386_DIR32NB foo
+CHECK-NEXT: 38 IMAGE_REL_I386_SEG12 foo
+CHECK-NEXT: 45 IMAGE_REL_I386_SECTION foo
+CHECK-NEXT: 52 IMAGE_REL_I386_SECREL foo
+CHECK-NEXT: 59 IMAGE_REL_I386_TOKEN foo
+CHECK-NEXT: 66 IMAGE_REL_I386_SECREL7 foo
+CHECK-NEXT: 73 IMAGE_REL_I386_REL32 foo
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