fix(dotnet): accept legacy small exception section sizes

Apply CoreCLR-compatible clause counting while validating and advancing by the physical section extent. Preserve malformed, reserved, chained and truncation checks.

Validation: managed decoder suite passes; no native compiler or runtime verification claimed.

Fixes #3279

src/dotnet/ManagedCilDecoding.test.ts

@@ -7,6 +7,7 @@ import {
   buildManagedPeFixture,
   managedPeFixtureTarget,
 } from "./ManagedPe.fixture.js";
+import { parseExceptionRegions } from "./ManagedMemberInstructionDecoder.js";
 
 const managedBodyWithSections = (...sections: Buffer[]) => {
   const header = Buffer.alloc(12);
@@ -666,6 +667,59 @@ describe("managed exception region flags and filters", () => {
 });
 
 describe("managed exception section decoding", () => {
+  it("reads legacy small EH DataSize without counting its header", () => {
+    const section = Buffer.alloc(16);
+    section[0] = 0x01;
+    section[1] = 12;
+
+    expect(managedBodyWithSections(section)).toMatchObject({
+      status: "present",
+      issue: null,
+      exception_regions: [expect.any(Object)],
+      normalized_il_sha256: expect.any(String),
+    });
+  });
+
+  it("advances a chained legacy section by its physical size", () => {
+    const legacy = Buffer.alloc(16);
+    legacy[0] = 0x81;
+    legacy[1] = 12;
+    const conformant = Buffer.alloc(16);
+    conformant[0] = 0x01;
+    conformant[1] = 16;
+
+    expect(managedBodyWithSections(legacy, conformant)).toMatchObject({
+      status: "present",
+      issue: null,
+      exception_regions: [expect.any(Object), expect.any(Object)],
+      normalized_il_sha256: expect.any(String),
+    });
+  });
+
+  it("rejects a legacy-sized clause whose physical bytes leave the artifact", () => {
+    const section = Buffer.alloc(12);
+    section[0] = 0x01;
+    section[1] = 12;
+
+    expect(parseExceptionRegions(section, 0, section.length)).toMatchObject({
+      status: "malformed",
+      regions: [],
+      issue: "Exception section size leaves artifact",
+    });
+  });
+
+  it("still rejects a small EH DataSize that matches neither layout", () => {
+    const section = Buffer.alloc(17);
+    section[0] = 0x01;
+    section[1] = 13;
+
+    expect(managedBodyWithSections(section)).toMatchObject({
+      status: "malformed",
+      exception_regions: [],
+      issue: "Exception section size does not contain whole clauses",
+    });
+  });
+
   it("marks an exception section with a partial clause as malformed", () => {
     const section = Buffer.alloc(17);
     section[0] = 0x01;
@@ -695,6 +749,19 @@ describe("managed exception section decoding", () => {
     });
   });
 
+  it("keeps small section reserved bytes invalid", () => {
+    const section = Buffer.alloc(16);
+    section[0] = 0x01;
+    section[1] = 16;
+    section[2] = 1;
+
+    expect(managedBodyWithSections(section)).toMatchObject({
+      status: "malformed",
+      exception_regions: [],
+      issue: "Small exception section reserved bytes are nonzero",
+    });
+  });
+
   it("preserves decoded clauses when a chained section has an unsupported kind", () => {
     const first = Buffer.alloc(16);
     first[0] = 0x81;

src/dotnet/ManagedMemberInstructionDecoder.ts

@@ -543,11 +543,36 @@ export const parseExceptionRegions = (
         "Exception section size leaves artifact",
         regions,
       );
-    if ((size - 4) % clauseSize !== 0)
+    let count: number;
+    let sectionSize = size;
+    if (fat) {
+      if ((size - 4) % clauseSize !== 0)
+        return malformedExceptionRegions(
+          "Exception section size does not contain whole clauses",
+          regions,
+        );
+      count = (size - 4) / clauseSize;
+    } else if ((size - 4) % clauseSize === 0) {
+      count = (size - 4) / clauseSize;
+    } else if (size >= clauseSize && size % clauseSize === 0) {
+      // CoreCLR accommodates legacy VB/MC++ sections whose DataSize omits the
+      // four-byte section header. DataSize is still not the physical extent.
+      count = size / clauseSize;
+      sectionSize = 4 + count * clauseSize;
+    } else {
       return malformedExceptionRegions(
         "Exception section size does not contain whole clauses",
         regions,
       );
+    }
+    if (
+      sectionOffset > methodEnd - sectionSize ||
+      sectionOffset > bytes.length - sectionSize
+    )
+      return malformedExceptionRegions(
+        "Exception section size leaves artifact",
+        regions,
+      );
     if (
       !fat &&
       (bytes[sectionOffset + 2] !== 0 || bytes[sectionOffset + 3] !== 0)
@@ -557,7 +582,6 @@ export const parseExceptionRegions = (
         regions,
       );
 
-    const count = (size - 4) / clauseSize;
     // Accumulates with the method IL bound; the body reserves what it retains.
     budget?.admitTransient(count * 256, sectionOffset);
     for (const region of readExceptionClauses(
@@ -569,7 +593,7 @@ export const parseExceptionRegions = (
       regions.push(region);
 
     hasMoreSections = (kind & 0x80) !== 0;
-    sectionOffset += size;
+    sectionOffset += sectionSize;
   }
   return { status: "complete", regions };
 };