Fix incorrect ASSERT in vertex validation.

Also adds a test that would have fired the ASSERT.

BUG=dawn:80
BUG=dawn:107

Change-Id: I56cdbc91956465c8941b45bb5e9da4c27da301ae
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/7840
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Yunchao He <yunchao.he@intel.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/dawn_native/RenderPipeline.cpp

@@ -34,14 +34,22 @@ namespace dawn_native {
             if (attribute->shaderLocation >= kMaxVertexAttributes) {
                 return DAWN_VALIDATION_ERROR("Setting attribute out of bounds");
             }
+
+            // No underflow is possible because the max vertex format size is smaller than
+            // kMaxVertexAttributeEnd.
             ASSERT(kMaxVertexAttributeEnd >= VertexFormatSize(attribute->format));
-            ASSERT(vertexBufferStride == 0 ||
+            if (attribute->offset > kMaxVertexAttributeEnd - VertexFormatSize(attribute->format)) {
-                   vertexBufferStride >= VertexFormatSize(attribute->format));
-            if (attribute->offset > kMaxVertexAttributeEnd - VertexFormatSize(attribute->format) ||
-                (vertexBufferStride > 0 &&
-                 attribute->offset + VertexFormatSize(attribute->format) > vertexBufferStride)) {
                 return DAWN_VALIDATION_ERROR("Setting attribute offset out of bounds");
             }
+
+            // No overflow is possible because the offset is already validated to be less
+            // than kMaxVertexAttributeEnd.
+            ASSERT(attribute->offset < kMaxVertexAttributeEnd);
+            if (vertexBufferStride > 0 &&
+                attribute->offset + VertexFormatSize(attribute->format) > vertexBufferStride) {
+                return DAWN_VALIDATION_ERROR("Setting attribute offset out of bounds");
+            }
+
             if ((*attributesSetMask)[attribute->shaderLocation]) {
                 return DAWN_VALIDATION_ERROR("Setting already set attribute");
             }

src/tests/unittests/validation/VertexInputValidationTests.cpp

@@ -363,3 +363,18 @@ TEST_F(VertexInputTest, SetAttributeOffsetOverflow) {
         }
     )");
 }
+
+// Check for some potential underflow in the vertex input validation
+TEST_F(VertexInputTest, VertexFormatLargerThanNonZeroStride) {
+    utils::ComboVertexInputDescriptor state;
+    state.bufferCount = 1;
+    state.cBuffers[0].stride = 4;
+    state.cBuffers[0].attributeCount = 1;
+    state.cAttributes[0].format = dawn::VertexFormat::Float4;
+    CreatePipeline(false, state, R"(
+        #version 450
+        void main() {
+            gl_Position = vec4(0.0);
+        }
+    )");
+}