Add standalone C++ reproducer

reproducer.cpp

@@ -0,0 +1,74 @@
+// Minimal reproducer for Gemma3 integer division-by-zero
+// Mirrors the vulnerable code in src/models/gemma3.cpp:32
+// and src/llama-model.cpp:1147-1171
+//
+// Compile: g++ -o reproducer reproducer.cpp -fsanitize=undefined -fno-sanitize-recover=all
+// Run:     ./reproducer
+
+#include <cmath>
+#include <cstdint>
+#include <cstdio>
+#include <cstring>
+#include <array>
+#include <stdexcept>
+#include <string>
+
+#define LLAMA_MAX_LAYERS 512
+
+// Mirrors llama_hparams (simplified)
+struct llama_hparams {
+    uint32_t n_embd        = 3072;   // from gemma3.embedding_length in GGUF
+    uint32_t n_layer_all   = 62;     // from gemma3.block_count = 62 → LLM_TYPE_27B
+    uint32_t n_embd_head_k_full = 0;
+
+    std::array<uint32_t, LLAMA_MAX_LAYERS> n_head_arr = {};  // all zeros — key missing from GGUF
+
+    uint32_t n_head(uint32_t il = 0) const {
+        return n_head_arr[il];  // returns 0 when key is absent
+    }
+};
+
+enum llm_type { LLM_TYPE_UNKNOWN, LLM_TYPE_1B, LLM_TYPE_4B, LLM_TYPE_8B, LLM_TYPE_12B, LLM_TYPE_27B };
+
+int main() {
+    llama_hparams hparams;
+
+    // --- Mirrors llama-model.cpp:1147 (general hparams loader) ---
+    // When n_head() == 0, n_embd_head_k_full is set to 0
+    if (hparams.n_head() > 0) {
+        hparams.n_embd_head_k_full = hparams.n_embd / hparams.n_head();
+    } else {
+        hparams.n_embd_head_k_full = 0;
+    }
+
+    // --- Mirrors gemma3.cpp:20-32 (load_arch_hparams) ---
+    llm_type type = LLM_TYPE_UNKNOWN;
+    switch (hparams.n_layer_all) {
+        case 18: type = LLM_TYPE_UNKNOWN; break;  // 270M
+        case 26: type = LLM_TYPE_1B;      break;
+        case 32: type = LLM_TYPE_8B;      break;
+        case 34: type = LLM_TYPE_4B;      break;
+        case 48: type = LLM_TYPE_12B;     break;
+        case 62: type = LLM_TYPE_27B;     break;  // <-- block_count=62 triggers this
+        default: type = LLM_TYPE_UNKNOWN; break;
+    }
+
+    printf("block_count = %u → type = %s\n", hparams.n_layer_all,
+           type == LLM_TYPE_27B ? "LLM_TYPE_27B" : "other");
+    printf("n_head(0)   = %u  (key absent from GGUF → stays 0)\n", hparams.n_head(0));
+    printf("n_embd      = %u\n", hparams.n_embd);
+    printf("\nExecuting vulnerable line (gemma3.cpp:32):\n");
+    printf("  hparams.n_embd / hparams.n_head(0) = %u / %u\n",
+           hparams.n_embd, hparams.n_head(0));
+
+    // THE VULNERABLE COMPUTATION — mirrors gemma3.cpp:32 exactly
+    // On x86_64: SIGFPE (exit 136)
+    // On ARM64:  silent UB (SDIV returns 0), UBSan aborts with "division by zero"
+    float f_attention_scale = (type == LLM_TYPE_27B)
+        ? 1.0f / std::sqrt(float(hparams.n_embd / hparams.n_head(0)))  // INTEGER DIV BY ZERO
+        : 1.0f / std::sqrt(float(hparams.n_embd_head_k_full));
+
+    // Should never reach here on x86_64
+    printf("f_attention_scale = %f  (should not reach here on x86_64)\n", f_attention_scale);
+    return 0;
+}