implement function to calculate proper patch size for future jmp hooks

include/rtdisasm.h

@@ -18,4 +18,11 @@ int rtdisasm_analyze_single(const uint8_t* code, unsigned limit, const instructi
 // and non-zero integer is offset from "code"
 int rtdisasm_find_target(const uint8_t* code, unsigned limit, unsigned rt_target);
 
+// will analyze all instructions until "wanted" size of bytes is reached
+// within instruction boundaries, therefore making that address suitable
+// for patching and not break instruction fetching. on success returns
+// non-zero value - patch size, within instructions aligned,
+// returns 0 if unknown instruction met and -1 if "limit" is reached.
+int rtdisasm_estimate_patch(const uint8_t* code, unsigned limit, int wanted);
+
 #endif

src/rtdisasm.c

@@ -394,3 +394,32 @@ int rtdisasm_find_target(const uint8_t* code, unsigned limit, unsigned rt_target
         if (cur >= end) return -1;
     } while (1);
 }
+
+int rtdisasm_estimate_patch(const uint8_t* code, unsigned limit, int wanted)
+{
+    const uint8_t* cur = code;
+    const uint8_t* const end = code + limit;
+    unsigned remaining = limit;
+    unsigned patch = 0;
+    if (cur == end) return -1;
+
+    do {
+        const instruction_t* ins;
+        int len = rtdisasm_analyze_single(cur, remaining, &ins);
+        TRACE("rtdisasm_analyze_single len %d\n", len);
+        if (len < 1) return len;
+
+        // advance past instruction and check if we hit limit
+        // cuz we don't want patch on the limit
+        cur += len;
+        remaining -= len;
+        if (cur >= end) return -1;
+
+        // we're good, let's go until we have enough bytes for patch
+        patch += len;
+    } while(patch < wanted);
+
+    // at this point it should contain patch size
+    // aligned with instruction boundaries
+    return patch;
+}

src/rtdisasm_test.c

@@ -37,6 +37,7 @@ static void test_3()
     __m256 odds = _mm256_set_ps(1.0, 3.0, 5.0, 7.0, 9.0, 11.0, 13.0, 15.0);
 
     __m256 result = _mm256_sub_ps(evens, odds);
+    (void)result;
     __asm__("nop"); // TARGET
 }
 static void test_3_end() {}
@@ -69,5 +70,11 @@ int main()
 
     printf("test3 %d\n", rtdisasm_find_target((const uint8_t*)test_3, size, RT_TARGET_NOP));
 
+    printf("\n== TEST 4 ==\n");
+    printf("test1 patch %d\n", rtdisasm_estimate_patch((const uint8_t*)test_1, 16, 8));
+    printf("test2 patch %d\n", rtdisasm_estimate_patch((const uint8_t*)test_2, 16, 8));
+    printf("test3 patch %d\n", rtdisasm_estimate_patch((const uint8_t*)test_3, 16, 8));
+
+
     return 0;
 }