[builtins] Remove indirection for pointers to code and data blobs

Mark the labels to the code and data global, which removes the need
for having separate pointers to these labels in the .data section.

This means that `v8_Default_embedded_blob_code_` and
`v8_Default_embedded_blob_data_` can now actually be read-only when
RELRO is enabled.

The actual contents of `v8_Default_embedded_blob_code_` remain
potentially non-readable for platforms where code is marked as
execute-only, but these changes do not attempt to read them.

Bug: v8:12850

Change-Id: Ic1bc8e68fe44a9ce45380c83b0be5fa94e7da267
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3616510
Commit-Queue: Georgia Kouveli <georgia.kouveli@arm.com>
Reviewed-by: Jakob Linke <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80331}

src/execution/isolate.cc

@@ -141,9 +141,9 @@
 #include "src/execution/simulator-base.h"
 #endif
 
-extern "C" const uint8_t* v8_Default_embedded_blob_code_;
+extern "C" const uint8_t v8_Default_embedded_blob_code_[];
 extern "C" uint32_t v8_Default_embedded_blob_code_size_;
-extern "C" const uint8_t* v8_Default_embedded_blob_data_;
+extern "C" const uint8_t v8_Default_embedded_blob_data_[];
 extern "C" uint32_t v8_Default_embedded_blob_data_size_;
 
 namespace v8 {
@@ -3696,8 +3696,8 @@ void Isolate::InitializeDefaultEmbeddedBlob() {
     }
   }
 
-  if (code == nullptr) {
-    CHECK_EQ(0, code_size);
+  if (code_size == 0) {
+    CHECK_EQ(0, data_size);
   } else {
     SetEmbeddedBlob(code, code_size, data, data_size);
   }

src/snapshot/embedded/embedded-empty.cc

@@ -8,12 +8,12 @@
 
 #include "src/base/macros.h"
 
-extern "C" const uint8_t* v8_Default_embedded_blob_code_;
+extern "C" const uint8_t v8_Default_embedded_blob_code_[];
 extern "C" uint32_t v8_Default_embedded_blob_code_size_;
-extern "C" const uint8_t* v8_Default_embedded_blob_data_;
+extern "C" const uint8_t v8_Default_embedded_blob_data_[];
 extern "C" uint32_t v8_Default_embedded_blob_data_size_;
 
-const uint8_t* v8_Default_embedded_blob_code_ = nullptr;
+const uint8_t v8_Default_embedded_blob_code_[1] = {0};
 uint32_t v8_Default_embedded_blob_code_size_ = 0;
-const uint8_t* v8_Default_embedded_blob_data_ = nullptr;
+const uint8_t v8_Default_embedded_blob_data_[1] = {0};
 uint32_t v8_Default_embedded_blob_data_size_ = 0;

src/snapshot/embedded/embedded-file-writer.cc

@@ -157,7 +157,8 @@ void EmbeddedFileWriter::WriteCodeSection(PlatformEmbeddedFileWriterBase* w,
 #endif
 
   w->AlignToCodeAlignment();
-  w->DeclareLabel(EmbeddedBlobCodeDataSymbol().c_str());
+  w->DeclareSymbolGlobal(EmbeddedBlobCodeSymbol().c_str());
+  w->DeclareLabel(EmbeddedBlobCodeSymbol().c_str());
 
   STATIC_ASSERT(Builtins::kAllBuiltinsAreIsolateIndependent);
   for (Builtin builtin = Builtins::kFirst; builtin <= Builtins::kLast;
@@ -170,31 +171,6 @@ void EmbeddedFileWriter::WriteCodeSection(PlatformEmbeddedFileWriterBase* w,
 
 void EmbeddedFileWriter::WriteFileEpilogue(PlatformEmbeddedFileWriterBase* w,
                                            const i::EmbeddedData* blob) const {
-  {
-    base::EmbeddedVector<char, kTemporaryStringLength>
-        embedded_blob_code_symbol;
-    base::SNPrintF(embedded_blob_code_symbol, "v8_%s_embedded_blob_code_",
-                   embedded_variant_);
-
-    w->Comment("Pointer to the beginning of the embedded blob code.");
-    w->SectionData();
-    w->AlignToDataAlignment();
-    w->DeclarePointerToSymbol(embedded_blob_code_symbol.begin(),
-                              EmbeddedBlobCodeDataSymbol().c_str());
-    w->Newline();
-
-    base::EmbeddedVector<char, kTemporaryStringLength>
-        embedded_blob_data_symbol;
-    base::SNPrintF(embedded_blob_data_symbol, "v8_%s_embedded_blob_data_",
-                   embedded_variant_);
-
-    w->Comment("Pointer to the beginning of the embedded blob data section.");
-    w->AlignToDataAlignment();
-    w->DeclarePointerToSymbol(embedded_blob_data_symbol.begin(),
-                              EmbeddedBlobDataDataSymbol().c_str());
-    w->Newline();
-  }
-
   {
     base::EmbeddedVector<char, kTemporaryStringLength>
         embedded_blob_code_size_symbol;
@@ -224,7 +200,7 @@ void EmbeddedFileWriter::WriteFileEpilogue(PlatformEmbeddedFileWriterBase* w,
                    embedded_variant_);
 
     w->MaybeEmitUnwindData(unwind_info_symbol.begin(),
-                           EmbeddedBlobCodeDataSymbol().c_str(), blob,
+                           EmbeddedBlobCodeSymbol().c_str(), blob,
                            reinterpret_cast<const void*>(&unwind_infos_[0]));
   }
 #endif  // V8_OS_WIN64

src/snapshot/embedded/embedded-file-writer.h

@@ -125,20 +125,20 @@ class EmbeddedFileWriter : public EmbeddedFileWriterInterface {
   // Fairly arbitrary but should fit all symbol names.
   static constexpr int kTemporaryStringLength = 256;
 
-  std::string EmbeddedBlobCodeDataSymbol() const {
+  std::string EmbeddedBlobCodeSymbol() const {
     base::EmbeddedVector<char, kTemporaryStringLength>
-        embedded_blob_code_data_symbol;
-    base::SNPrintF(embedded_blob_code_data_symbol,
-                   "v8_%s_embedded_blob_code_data_", embedded_variant_);
-    return std::string{embedded_blob_code_data_symbol.begin()};
+        embedded_blob_code_symbol;
+    base::SNPrintF(embedded_blob_code_symbol, "v8_%s_embedded_blob_code_",
+                   embedded_variant_);
+    return std::string{embedded_blob_code_symbol.begin()};
   }
 
-  std::string EmbeddedBlobDataDataSymbol() const {
+  std::string EmbeddedBlobDataSymbol() const {
     base::EmbeddedVector<char, kTemporaryStringLength>
-        embedded_blob_data_data_symbol;
-    base::SNPrintF(embedded_blob_data_data_symbol,
-                   "v8_%s_embedded_blob_data_data_", embedded_variant_);
-    return std::string{embedded_blob_data_data_symbol.begin()};
+        embedded_blob_data_symbol;
+    base::SNPrintF(embedded_blob_data_symbol, "v8_%s_embedded_blob_data_",
+                   embedded_variant_);
+    return std::string{embedded_blob_data_symbol.begin()};
   }
 
   void WriteDataSection(PlatformEmbeddedFileWriterBase* w,
@@ -146,7 +146,8 @@ class EmbeddedFileWriter : public EmbeddedFileWriterInterface {
     w->Comment("The embedded blob data section starts here.");
     w->SectionRoData();
     w->AlignToDataAlignment();
-    w->DeclareLabel(EmbeddedBlobDataDataSymbol().c_str());
+    w->DeclareSymbolGlobal(EmbeddedBlobDataSymbol().c_str());
+    w->DeclareLabel(EmbeddedBlobDataSymbol().c_str());
 
     WriteBinaryContentsAsInlineAssembly(w, blob->data(), blob->data_size());
   }

src/snapshot/embedded/platform-embedded-file-writer-aix.cc

@@ -32,10 +32,6 @@ void PlatformEmbeddedFileWriterAIX::SectionText() {
   fprintf(fp_, ".csect [GL], 6\n");
 }
 
-void PlatformEmbeddedFileWriterAIX::SectionData() {
-  fprintf(fp_, ".csect .data[RW]\n");
-}
-
 void PlatformEmbeddedFileWriterAIX::SectionRoData() {
   fprintf(fp_, ".csect[RO]\n");
 }
@@ -50,14 +46,6 @@ void PlatformEmbeddedFileWriterAIX::DeclareUint32(const char* name,
   Newline();
 }
 
-void PlatformEmbeddedFileWriterAIX::DeclarePointerToSymbol(const char* name,
-                                                           const char* target) {
-  AlignToCodeAlignment();
-  DeclareLabel(name);
-  fprintf(fp_, "  %s %s\n", DirectiveAsString(PointerSizeDirective()), target);
-  Newline();
-}
-
 void PlatformEmbeddedFileWriterAIX::DeclareSymbolGlobal(const char* name) {
   // These symbols are not visible outside of the final binary, this allows for
   // reduced binary size, and less work for the dynamic linker.

src/snapshot/embedded/platform-embedded-file-writer-aix.h

@@ -22,14 +22,12 @@ class PlatformEmbeddedFileWriterAIX : public PlatformEmbeddedFileWriterBase {
   }
 
   void SectionText() override;
-  void SectionData() override;
   void SectionRoData() override;
 
   void AlignToCodeAlignment() override;
   void AlignToDataAlignment() override;
 
   void DeclareUint32(const char* name, uint32_t value) override;
-  void DeclarePointerToSymbol(const char* name, const char* target) override;
 
   void DeclareSymbolGlobal(const char* name) override;
   void DeclareLabel(const char* name) override;

src/snapshot/embedded/platform-embedded-file-writer-base.h

@@ -54,7 +54,6 @@ class PlatformEmbeddedFileWriterBase {
   FILE* fp() const { return fp_; }
 
   virtual void SectionText() = 0;
-  virtual void SectionData() = 0;
   virtual void SectionRoData() = 0;
 
   virtual void AlignToCodeAlignment() = 0;
@@ -62,7 +61,6 @@ class PlatformEmbeddedFileWriterBase {
   virtual void AlignToDataAlignment() = 0;
 
   virtual void DeclareUint32(const char* name, uint32_t value) = 0;
-  virtual void DeclarePointerToSymbol(const char* name, const char* target) = 0;
 
   virtual void DeclareSymbolGlobal(const char* name) = 0;
   virtual void DeclareLabel(const char* name) = 0;

src/snapshot/embedded/platform-embedded-file-writer-generic.cc

@@ -41,10 +41,6 @@ void PlatformEmbeddedFileWriterGeneric::SectionText() {
   }
 }
 
-void PlatformEmbeddedFileWriterGeneric::SectionData() {
-  fprintf(fp_, ".section .data\n");
-}
-
 void PlatformEmbeddedFileWriterGeneric::SectionRoData() {
   fprintf(fp_, ".section .rodata\n");
 }
@@ -58,14 +54,6 @@ void PlatformEmbeddedFileWriterGeneric::DeclareUint32(const char* name,
   Newline();
 }
 
-void PlatformEmbeddedFileWriterGeneric::DeclarePointerToSymbol(
-    const char* name, const char* target) {
-  DeclareSymbolGlobal(name);
-  DeclareLabel(name);
-  fprintf(fp_, "  %s %s%s\n", DirectiveAsString(PointerSizeDirective()),
-          SYMBOL_PREFIX, target);
-}
-
 void PlatformEmbeddedFileWriterGeneric::DeclareSymbolGlobal(const char* name) {
   fprintf(fp_, ".global %s%s\n", SYMBOL_PREFIX, name);
   // These symbols are not visible outside of the final binary, this allows for

src/snapshot/embedded/platform-embedded-file-writer-generic.h

@@ -24,7 +24,6 @@ class PlatformEmbeddedFileWriterGeneric
   }
 
   void SectionText() override;
-  void SectionData() override;
   void SectionRoData() override;
 
   void AlignToCodeAlignment() override;
@@ -32,7 +31,6 @@ class PlatformEmbeddedFileWriterGeneric
   void AlignToDataAlignment() override;
 
   void DeclareUint32(const char* name, uint32_t value) override;
-  void DeclarePointerToSymbol(const char* name, const char* target) override;
 
   void DeclareSymbolGlobal(const char* name) override;
   void DeclareLabel(const char* name) override;

src/snapshot/embedded/platform-embedded-file-writer-mac.cc

@@ -29,8 +29,6 @@ const char* DirectiveAsString(DataDirective directive) {
 
 void PlatformEmbeddedFileWriterMac::SectionText() { fprintf(fp_, ".text\n"); }
 
-void PlatformEmbeddedFileWriterMac::SectionData() { fprintf(fp_, ".data\n"); }
-
 void PlatformEmbeddedFileWriterMac::SectionRoData() {
   fprintf(fp_, ".const_data\n");
 }
@@ -44,13 +42,6 @@ void PlatformEmbeddedFileWriterMac::DeclareUint32(const char* name,
   Newline();
 }
 
-void PlatformEmbeddedFileWriterMac::DeclarePointerToSymbol(const char* name,
-                                                           const char* target) {
-  DeclareSymbolGlobal(name);
-  DeclareLabel(name);
-  fprintf(fp_, "  %s _%s\n", DirectiveAsString(PointerSizeDirective()), target);
-}
-
 void PlatformEmbeddedFileWriterMac::DeclareSymbolGlobal(const char* name) {
   // TODO(jgruber): Investigate switching to .globl. Using .private_extern
   // prevents something along the compilation chain from messing with the

src/snapshot/embedded/platform-embedded-file-writer-mac.h

@@ -22,7 +22,6 @@ class PlatformEmbeddedFileWriterMac : public PlatformEmbeddedFileWriterBase {
   }
 
   void SectionText() override;
-  void SectionData() override;
   void SectionRoData() override;
 
   void AlignToCodeAlignment() override;
@@ -30,7 +29,6 @@ class PlatformEmbeddedFileWriterMac : public PlatformEmbeddedFileWriterBase {
   void AlignToDataAlignment() override;
 
   void DeclareUint32(const char* name, uint32_t value) override;
-  void DeclarePointerToSymbol(const char* name, const char* target) override;
 
   void DeclareSymbolGlobal(const char* name) override;
   void DeclareLabel(const char* name) override;

src/snapshot/embedded/platform-embedded-file-writer-win.cc

@@ -364,15 +364,6 @@ void PlatformEmbeddedFileWriterWin::SectionText() {
   }
 }
 
-void PlatformEmbeddedFileWriterWin::SectionData() {
-  if (target_arch_ == EmbeddedTargetArch::kArm64) {
-    fprintf(fp_, "  AREA |.data|, DATA, ALIGN=%d, READWRITE\n",
-            ARM64_DATA_ALIGNMENT_POWER);
-  } else {
-    fprintf(fp_, ".DATA\n");
-  }
-}
-
 void PlatformEmbeddedFileWriterWin::SectionRoData() {
   if (target_arch_ == EmbeddedTargetArch::kArm64) {
     fprintf(fp_, "  AREA |.rodata|, DATA, ALIGN=%d, READONLY\n",
@@ -389,13 +380,6 @@ void PlatformEmbeddedFileWriterWin::DeclareUint32(const char* name,
           value);
 }
 
-void PlatformEmbeddedFileWriterWin::DeclarePointerToSymbol(const char* name,
-                                                           const char* target) {
-  DeclareSymbolGlobal(name);
-  fprintf(fp_, "%s%s %s %s%s\n", SYMBOL_PREFIX, name,
-          DirectiveAsString(PointerSizeDirective()), SYMBOL_PREFIX, target);
-}
-
 void PlatformEmbeddedFileWriterWin::StartPdataSection() {
   if (target_arch_ == EmbeddedTargetArch::kArm64) {
     fprintf(fp_, "  AREA |.pdata|, DATA, ALIGN=%d, READONLY\n",
@@ -584,10 +568,6 @@ void PlatformEmbeddedFileWriterWin::SectionText() {
   fprintf(fp_, ".section .text$hot,\"xr\"\n");
 }
 
-void PlatformEmbeddedFileWriterWin::SectionData() {
-  fprintf(fp_, ".section .data\n");
-}
-
 void PlatformEmbeddedFileWriterWin::SectionRoData() {
   fprintf(fp_, ".section .rdata\n");
 }
@@ -601,14 +581,6 @@ void PlatformEmbeddedFileWriterWin::DeclareUint32(const char* name,
   Newline();
 }
 
-void PlatformEmbeddedFileWriterWin::DeclarePointerToSymbol(const char* name,
-                                                           const char* target) {
-  DeclareSymbolGlobal(name);
-  DeclareLabel(name);
-  fprintf(fp_, "  %s %s%s\n", DirectiveAsString(PointerSizeDirective()),
-          SYMBOL_PREFIX, target);
-}
-
 void PlatformEmbeddedFileWriterWin::StartPdataSection() {
   fprintf(fp_, ".section .pdata\n");
 }

src/snapshot/embedded/platform-embedded-file-writer-win.h

@@ -21,14 +21,12 @@ class PlatformEmbeddedFileWriterWin : public PlatformEmbeddedFileWriterBase {
   }
 
   void SectionText() override;
-  void SectionData() override;
   void SectionRoData() override;
 
   void AlignToCodeAlignment() override;
   void AlignToDataAlignment() override;
 
   void DeclareUint32(const char* name, uint32_t value) override;
-  void DeclarePointerToSymbol(const char* name, const char* target) override;
 
   void DeclareSymbolGlobal(const char* name) override;
   void DeclareLabel(const char* name) override;