[CFGuard] Add address-taken IAT tables and delay-load support

This patch adds support for creating Guard Address-Taken IAT Entry Tables (.giats$y sections) in object files, matching the behavior of MSVC. These contain lists of address-taken imported functions, which are used by the linker to create the final GIATS table.
Additionally, if any DLLs are delay-loaded, the linker must look through the .giats tables and add the respective load thunks of address-taken imports to the GFIDS table, as these are also valid call targets.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D87544

lld/COFF/DLL.cpp

@@ -19,6 +19,7 @@
 
 #include "DLL.h"
 #include "Chunks.h"
+#include "SymbolTable.h"
 #include "llvm/Object/COFF.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/Path.h"
@@ -653,9 +654,18 @@ void DelayLoadContents::create(Defined *h) {
         auto *c = make<HintNameChunk>(extName, 0);
         names.push_back(make<LookupChunk>(c));
         hintNames.push_back(c);
+        // Add a syntentic symbol for this load thunk, using the "__imp_load"
+        // prefix, in case this thunk needs to be added to the list of valid
+        // call targets for Control Flow Guard.
+        StringRef symName = saver.save("__imp_load_" + extName);
+        s->loadThunkSym =
+            cast<DefinedSynthetic>(symtab->addSynthetic(symName, t));
       }
     }
     thunks.push_back(tm);
+    StringRef tmName =
+        saver.save("__tailMerge_" + syms[0]->getDLLName().lower());
+    symtab->addSynthetic(tmName, tm);
     // Terminate with null values.
     addresses.push_back(make<NullChunk>(8));
     names.push_back(make<NullChunk>(8));

lld/COFF/ICF.cpp

@@ -131,7 +131,7 @@ bool ICF::assocEquals(const SectionChunk *a, const SectionChunk *b) {
   auto considerForICF = [](const SectionChunk &assoc) {
     StringRef Name = assoc.getSectionName();
     return !(Name.startswith(".debug") || Name == ".gfids$y" ||
-             Name == ".gljmp$y");
+             Name == ".giats$y" || Name == ".gljmp$y");
   };
   auto ra = make_filter_range(a->children(), considerForICF);
   auto rb = make_filter_range(b->children(), considerForICF);

lld/COFF/InputFiles.cpp

@@ -280,6 +280,8 @@ SectionChunk *ObjFile::readSection(uint32_t sectionNumber,
     debugChunks.push_back(c);
   else if (name == ".gfids$y")
     guardFidChunks.push_back(c);
+  else if (name == ".giats$y")
+    guardIATChunks.push_back(c);
   else if (name == ".gljmp$y")
     guardLJmpChunks.push_back(c);
   else if (name == ".sxdata")

lld/COFF/InputFiles.h

@@ -144,6 +144,7 @@ class ObjFile : public InputFile {
   ArrayRef<SectionChunk *> getDebugChunks() { return debugChunks; }
   ArrayRef<SectionChunk *> getSXDataChunks() { return sxDataChunks; }
   ArrayRef<SectionChunk *> getGuardFidChunks() { return guardFidChunks; }
+  ArrayRef<SectionChunk *> getGuardIATChunks() { return guardIATChunks; }
   ArrayRef<SectionChunk *> getGuardLJmpChunks() { return guardLJmpChunks; }
   ArrayRef<Symbol *> getSymbols() { return symbols; }
 
@@ -285,9 +286,11 @@ class ObjFile : public InputFile {
   // 32-bit x86.
   std::vector<SectionChunk *> sxDataChunks;
 
-  // Chunks containing symbol table indices of address taken symbols and longjmp
-  // targets.  These are not linked into the final binary when /guard:cf is set.
+  // Chunks containing symbol table indices of address taken symbols, address
+  // taken IAT entries, and longjmp targets. These are not linked into the
+  // final binary when /guard:cf is set.
   std::vector<SectionChunk *> guardFidChunks;
+  std::vector<SectionChunk *> guardIATChunks;
   std::vector<SectionChunk *> guardLJmpChunks;
 
   // This vector contains a list of all symbols defined or referenced by this

lld/COFF/Symbols.h

@@ -353,6 +353,13 @@ class DefinedImportData : public Defined {
   uint16_t getOrdinal() { return file->hdr->OrdinalHint; }
 
   ImportFile *file;
+
+  // This is a pointer to the synthetic symbol associated with the load thunk
+  // for this symbol that will be called if the DLL is delay-loaded. This is
+  // needed for Control Flow Guard because if this DefinedImportData symbol is a
+  // valid call target, the corresponding load thunk must also be marked as a
+  // valid call target.
+  DefinedSynthetic *loadThunkSym = nullptr;
 };
 
 // This class represents a symbol for a jump table entry which jumps

lld/COFF/Writer.cpp

@@ -227,6 +227,9 @@ class Writer {
   void markSymbolsForRVATable(ObjFile *file,
                               ArrayRef<SectionChunk *> symIdxChunks,
                               SymbolRVASet &tableSymbols);
+  void getSymbolsFromSections(ObjFile *file,
+                              ArrayRef<SectionChunk *> symIdxChunks,
+                              std::vector<Symbol *> &symbols);
   void maybeAddRVATable(SymbolRVASet tableSymbols, StringRef tableSym,
                         StringRef countSym);
   void setSectionPermissions();
@@ -607,8 +610,9 @@ void Writer::run() {
 
   createImportTables();
   createSections();
-  createMiscChunks();
   appendImportThunks();
+  // Import thunks must be added before the Control Flow Guard tables are added.
+  createMiscChunks();
   createExportTable();
   mergeSections();
   removeUnusedSections();
@@ -1628,6 +1632,8 @@ static void markSymbolsWithRelocations(ObjFile *file,
 // table.
 void Writer::createGuardCFTables() {
   SymbolRVASet addressTakenSyms;
+  SymbolRVASet giatsRVASet;
+  std::vector<Symbol *> giatsSymbols;
   SymbolRVASet longJmpTargets;
   for (ObjFile *file : ObjFile::instances) {
     // If the object was compiled with /guard:cf, the address taken symbols
@@ -1637,6 +1643,8 @@ void Writer::createGuardCFTables() {
     // possibly address-taken.
     if (file->hasGuardCF()) {
       markSymbolsForRVATable(file, file->getGuardFidChunks(), addressTakenSyms);
+      markSymbolsForRVATable(file, file->getGuardIATChunks(), giatsRVASet);
+      getSymbolsFromSections(file, file->getGuardIATChunks(), giatsSymbols);
       markSymbolsForRVATable(file, file->getGuardLJmpChunks(), longJmpTargets);
     } else {
       markSymbolsWithRelocations(file, addressTakenSyms);
@@ -1651,6 +1659,16 @@ void Writer::createGuardCFTables() {
   for (Export &e : config->exports)
     maybeAddAddressTakenFunction(addressTakenSyms, e.sym);
 
+  // For each entry in the .giats table, check if it has a corresponding load
+  // thunk (e.g. because the DLL that defines it will be delay-loaded) and, if
+  // so, add the load thunk to the address taken (.gfids) table.
+  for (Symbol *s : giatsSymbols) {
+    if (auto *di = dyn_cast<DefinedImportData>(s)) {
+      if (di->loadThunkSym)
+        addSymbolToRVASet(addressTakenSyms, di->loadThunkSym);
+    }
+  }
+
   // Ensure sections referenced in the gfid table are 16-byte aligned.
   for (const ChunkAndOffset &c : addressTakenSyms)
     if (c.inputChunk->getAlignment() < 16)
@@ -1659,6 +1677,10 @@ void Writer::createGuardCFTables() {
   maybeAddRVATable(std::move(addressTakenSyms), "__guard_fids_table",
                    "__guard_fids_count");
 
+  // Add the Guard Address Taken IAT Entry Table (.giats).
+  maybeAddRVATable(std::move(giatsRVASet), "__guard_iat_table",
+                   "__guard_iat_count");
+
   // Add the longjmp target table unless the user told us not to.
   if (config->guardCF == GuardCFLevel::Full)
     maybeAddRVATable(std::move(longJmpTargets), "__guard_longjmp_table",
@@ -1675,11 +1697,11 @@ void Writer::createGuardCFTables() {
 }
 
 // Take a list of input sections containing symbol table indices and add those
-// symbols to an RVA table. The challenge is that symbol RVAs are not known and
+// symbols to a vector. The challenge is that symbol RVAs are not known and
 // depend on the table size, so we can't directly build a set of integers.
-void Writer::markSymbolsForRVATable(ObjFile *file,
+void Writer::getSymbolsFromSections(ObjFile *file,
                                     ArrayRef<SectionChunk *> symIdxChunks,
-                                    SymbolRVASet &tableSymbols) {
+                                    std::vector<Symbol *> &symbols) {
   for (SectionChunk *c : symIdxChunks) {
     // Skip sections discarded by linker GC. This comes up when a .gfids section
     // is associated with something like a vtable and the vtable is discarded.
@@ -1697,7 +1719,7 @@ void Writer::markSymbolsForRVATable(ObjFile *file,
     }
 
     // Read each symbol table index and check if that symbol was included in the
-    // final link. If so, add it to the table symbol set.
+    // final link. If so, add it to the vector of symbols.
     ArrayRef<ulittle32_t> symIndices(
         reinterpret_cast<const ulittle32_t *>(data.data()), data.size() / 4);
     ArrayRef<Symbol *> objSymbols = file->getSymbols();
@@ -1709,12 +1731,24 @@ void Writer::markSymbolsForRVATable(ObjFile *file,
       }
       if (Symbol *s = objSymbols[symIndex]) {
         if (s->isLive())
-          addSymbolToRVASet(tableSymbols, cast<Defined>(s));
+          symbols.push_back(cast<Symbol>(s));
       }
     }
   }
 }
 
+// Take a list of input sections containing symbol table indices and add those
+// symbols to an RVA table.
+void Writer::markSymbolsForRVATable(ObjFile *file,
+                                    ArrayRef<SectionChunk *> symIdxChunks,
+                                    SymbolRVASet &tableSymbols) {
+  std::vector<Symbol *> syms;
+  getSymbolsFromSections(file, symIdxChunks, syms);
+
+  for (Symbol *s : syms)
+    addSymbolToRVASet(tableSymbols, cast<Defined>(s));
+}
+
 // Replace the absolute table symbol with a synthetic symbol pointing to
 // tableChunk so that we can emit base relocations for it and resolve section
 // relative relocations.

lld/test/COFF/giats.s

@@ -0,0 +1,117 @@
+# REQUIRES: x86
+
+# Make a DLL that exports exportfn1.
+# RUN: yaml2obj %p/Inputs/export.yaml -o %basename_t-exp.obj
+# RUN: lld-link /out:%basename_t-exp.dll /dll %basename_t-exp.obj /export:exportfn1 /implib:%basename_t-exp.lib
+
+# Make an object file that imports exportfn1.
+# RUN: llvm-mc -triple x86_64-windows-msvc %s -filetype=obj -o %basename_t.obj
+
+# Check that the Guard address-taken IAT entry tables are propagated to the final executable.
+# RUN: lld-link %basename_t.obj -guard:cf -entry:main -out:%basename_t-nodelay.exe %basename_t-exp.lib
+# RUN: llvm-readobj --file-headers --coff-load-config %basename_t-nodelay.exe | FileCheck %s --check-prefix CHECK
+
+# CHECK: ImageBase: 0x140000000
+# CHECK: LoadConfig [
+# CHECK:   GuardCFFunctionTable: 0x140002114
+# CHECK:   GuardCFFunctionCount: 1
+# CHECK:   GuardFlags: 0x10500
+# CHECK:   GuardAddressTakenIatEntryTable: 0x140002118
+# CHECK:   GuardAddressTakenIatEntryCount: 1
+# CHECK: ]
+# CHECK:      GuardFidTable [
+# CHECK-NEXT:   0x14000{{.*}}
+# CHECK-NEXT: ]
+# CHECK:      GuardIatTable [
+# CHECK-NEXT:   0x14000{{.*}}
+# CHECK-NEXT: ]
+
+
+# Check that the additional load thunk symbol is added to the GFIDs table.
+# RUN: lld-link %basename_t.obj -guard:cf -entry:main -out:%basename_t-delay.exe %basename_t-exp.lib -alternatename:__delayLoadHelper2=main -delayload:%basename_t-exp.dll
+# RUN: llvm-readobj --file-headers --coff-load-config %basename_t-delay.exe | FileCheck %s --check-prefix DELAY-CHECK
+
+# DELAY-CHECK: ImageBase: 0x140000000
+# DELAY-CHECK: LoadConfig [
+# DELAY-CHECK:   GuardCFFunctionTable: 0x140002114
+# DELAY-CHECK:   GuardCFFunctionCount: 2
+# DELAY-CHECK:   GuardFlags: 0x10500
+# DELAY-CHECK:   GuardAddressTakenIatEntryTable: 0x14000211C
+# DELAY-CHECK:   GuardAddressTakenIatEntryCount: 1
+# DELAY-CHECK: ]
+# DELAY-CHECK:      GuardFidTable [
+# DELAY-CHECK-NEXT:   0x14000{{.*}}
+# DELAY-CHECK-NEXT:   0x14000{{.*}}
+# DELAY-CHECK-NEXT: ]
+# DELAY-CHECK:      GuardIatTable [
+# DELAY-CHECK-NEXT:   0x14000{{.*}}
+# DELAY-CHECK-NEXT: ]
+
+
+# This assembly is reduced from C code like:
+# __declspec(noinline)
+# void IndirectCall(BOOL (func)(HANDLE)) {
+#   (*func)(NULL);
+# }
+# int main(int argc, char** argv) {
+#   IndirectCall(exportfn1);
+# }
+
+	.text
+	.def	 @feat.00;
+	.scl	3;
+	.type	0;
+	.endef
+	.globl	@feat.00
+.set @feat.00, 2048
+	.def	 IndirectCall;	.scl	2;	.type	32;	.endef
+	.globl	IndirectCall                    # -- Begin function IndirectCall
+	.p2align	4, 0x90
+IndirectCall:                           # @IndirectCall
+# %bb.0:
+	subq	$40, %rsp
+	movq	%rcx, 32(%rsp)
+	movq	32(%rsp), %rax
+	movq	%rax, %rdx        # This would otherwise be: movq __guard_dispatch_icall_fptr(%rip), %rdx
+	xorl	%ecx, %ecx
+	callq	*%rdx
+	nop
+	addq	$40, %rsp
+	retq
+                                        # -- End function
+	.def	 main;	.scl	2;	.type	32;	.endef
+	.globl	main                            # -- Begin function main
+	.p2align	4, 0x90
+main:                                   # @main
+# %bb.0:
+	subq	$56, %rsp
+	movq	__imp_exportfn1(%rip), %rax
+	movq	%rdx, 48(%rsp)
+	movl	%ecx, 44(%rsp)
+	movq	%rax, %rcx
+	callq	IndirectCall
+	xorl	%eax, %eax
+	addq	$56, %rsp
+	retq
+                                        # -- End function
+	.section	.gfids$y,"dr"
+	.section	.giats$y,"dr"
+	.symidx	__imp_exportfn1
+	.section	.gljmp$y,"dr"
+
+# Load configuration directory entry (winnt.h _IMAGE_LOAD_CONFIG_DIRECTORY64).
+# The linker will define the __guard_* symbols.
+        .section .rdata,"dr"
+.globl _load_config_used
+_load_config_used:
+        .long 256
+        .fill 124, 1, 0
+        .quad __guard_fids_table
+        .quad __guard_fids_count
+        .long __guard_flags
+        .fill 12, 1, 0
+        .quad __guard_iat_table
+        .quad __guard_iat_count
+        .quad __guard_longjmp_table
+        .quad __guard_fids_count
+        .fill 84, 1, 0

llvm/include/llvm/MC/MCObjectFileInfo.h

@@ -215,6 +215,7 @@ class MCObjectFileInfo {
   MCSection *XDataSection = nullptr;
   MCSection *SXDataSection = nullptr;
   MCSection *GFIDsSection = nullptr;
+  MCSection *GIATsSection = nullptr;
   MCSection *GLJMPSection = nullptr;
 
   // XCOFF specific sections
@@ -397,6 +398,7 @@ class MCObjectFileInfo {
   MCSection *getXDataSection() const { return XDataSection; }
   MCSection *getSXDataSection() const { return SXDataSection; }
   MCSection *getGFIDsSection() const { return GFIDsSection; }
+  MCSection *getGIATsSection() const { return GIATsSection; }
   MCSection *getGLJMPSection() const { return GLJMPSection; }
 
   // XCOFF specific sections