[wasm] Pass correct code section start in streaming compilation

The streaming decoder computed the code section start from the passed
"offset". That offset is computed from the module offset *after* the
number of functions has been read. Hence 1 is subtracted, with the
comment:
// The offset passed to {ProcessCodeSectionHeader} is an error offset and
// not the start offset of a buffer. Therefore we need the -1 here.

That subtraction of 1 worked when the number of functions was encoded in
a 1-byte LEB, otherwise it was off.

This CL fixes the immediate issue of passing the right code offset. The
usage of the previously existing offset also seems wrong, and I will try
to clean that up in a follow-up CL.

R=ahaas@chromium.org
CC=szuend@chromium.org

Bug: chromium:1150303
Change-Id: I64bb2ececeb4749b7ba2096cd148ccb4079eca4f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2562383
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71503}

src/wasm/module-compiler.cc

@@ -1752,6 +1752,7 @@ class AsyncStreamingProcessor final : public StreamingProcessor {
 
   bool ProcessCodeSectionHeader(int num_functions, uint32_t offset,
                                 std::shared_ptr<WireBytesStorage>,
+                                int code_section_start,
                                 int code_section_length) override;
 
   bool ProcessFunctionBody(Vector<const uint8_t> bytes,
@@ -2487,7 +2488,7 @@ bool AsyncStreamingProcessor::ProcessSection(SectionCode section_code,
 bool AsyncStreamingProcessor::ProcessCodeSectionHeader(
     int num_functions, uint32_t offset,
     std::shared_ptr<WireBytesStorage> wire_bytes_storage,
-    int code_section_length) {
+    int code_section_start, int code_section_length) {
   DCHECK_LE(0, code_section_length);
   before_code_section_ = false;
   TRACE_STREAMING("Start the code section with %d functions...\n",
@@ -2499,7 +2500,8 @@ bool AsyncStreamingProcessor::ProcessCodeSectionHeader(
     return false;
   }
 
-  decoder_.set_code_section(offset, static_cast<uint32_t>(code_section_length));
+  decoder_.set_code_section(code_section_start,
+                            static_cast<uint32_t>(code_section_length));
 
   prefix_hash_ = base::hash_combine(prefix_hash_,
                                     static_cast<uint32_t>(code_section_length));

src/wasm/streaming-decoder.cc

@@ -184,13 +184,13 @@ class V8_EXPORT_PRIVATE AsyncStreamingDecoder : public StreamingDecoder {
 
   void StartCodeSection(int num_functions,
                         std::shared_ptr<WireBytesStorage> wire_bytes_storage,
-                        int code_section_length) {
+                        int code_section_start, int code_section_length) {
     if (!ok()) return;
     // The offset passed to {ProcessCodeSectionHeader} is an error offset and
     // not the start offset of a buffer. Therefore we need the -1 here.
     if (!processor_->ProcessCodeSectionHeader(
             num_functions, module_offset() - 1, std::move(wire_bytes_storage),
-            code_section_length)) {
+            code_section_start, code_section_length)) {
       Fail();
     }
   }
@@ -638,12 +638,16 @@ AsyncStreamingDecoder::DecodeNumberOfFunctions::NextWithValue(
     return std::make_unique<DecodeSectionID>(streaming->module_offset());
   }
 
+  DCHECK_GE(kMaxInt, section_buffer_->module_offset() +
+                         section_buffer_->payload_offset());
+  int code_section_start = static_cast<int>(section_buffer_->module_offset() +
+                                            section_buffer_->payload_offset());
   DCHECK_GE(kMaxInt, payload_buf.length());
   int code_section_len = static_cast<int>(payload_buf.length());
   DCHECK_GE(kMaxInt, value_);
   streaming->StartCodeSection(static_cast<int>(value_),
                               streaming->section_buffers_.back(),
-                              code_section_len);
+                              code_section_start, code_section_len);
   if (!streaming->ok()) return nullptr;
   return std::make_unique<DecodeFunctionLength>(
       section_buffer_, section_buffer_->payload_offset() + bytes_consumed_,

src/wasm/streaming-decoder.h

@@ -39,6 +39,7 @@ class V8_EXPORT_PRIVATE StreamingProcessor {
   // finished successfully and the decoding should continue.
   virtual bool ProcessCodeSectionHeader(int num_functions, uint32_t offset,
                                         std::shared_ptr<WireBytesStorage>,
+                                        int code_section_start,
                                         int code_section_length) = 0;
 
   // Process a function body. Returns true if the processing finished

src/wasm/wasm-module.h

@@ -283,6 +283,8 @@ struct V8_EXPORT_PRIVATE WasmModule {
   uint32_t num_declared_functions = 0;  // excluding imported
   uint32_t num_exported_functions = 0;
   uint32_t num_declared_data_segments = 0;  // From the DataCount section.
+  // Position and size of the code section (payload only, i.e. without section
+  // ID and length).
   WireBytesRef code = {0, 0};
   WireBytesRef name = {0, 0};
   std::vector<TypeDefinition> types;  // by type index

test/inspector/debugger/wasm-script-code-offset-streaming-expected.txt

@@ -6,4 +6,4 @@ Wasm script parsed: ID 3, startColumn: 0, endColumn: 32, codeOffset: 0
 Wasm script parsed: ID 4, startColumn: 0, endColumn: 40, codeOffset: 36
 Wasm script parsed: ID 5, startColumn: 0, endColumn: 40, codeOffset: 36
 Wasm script parsed: ID 6, startColumn: 0, endColumn: 44, codeOffset: 36
-Wasm script parsed: ID 7, startColumn: 0, endColumn: 44, codeOffset: 40
+Wasm script parsed: ID 7, startColumn: 0, endColumn: 44, codeOffset: 36

test/unittests/wasm/streaming-decoder-unittest.cc

@@ -58,6 +58,7 @@ class MockStreamingProcessor : public StreamingProcessor {
 
   bool ProcessCodeSectionHeader(int num_functions, uint32_t offset,
                                 std::shared_ptr<WireBytesStorage>,
+                                int code_section_start,
                                 int code_section_length) override {
     return true;
   }