fuzz: add a crossover function to generic-fuzzer

Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20201023150746.107063-10-alxndr@bu.edu>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/qtest/fuzz/generic_fuzz.c

@@ -810,6 +810,91 @@ static void generic_pre_fuzz(QTestState *s)
     counter_shm_init();
 }
 
+/*
+ * When libfuzzer gives us two inputs to combine, return a new input with the
+ * following structure:
+ *
+ * Input 1 (data1)
+ * SEPARATOR
+ * Clear out the DMA Patterns
+ * SEPARATOR
+ * Disable the pci_read/write instructions
+ * SEPARATOR
+ * Input 2 (data2)
+ *
+ * The idea is to collate the core behaviors of the two inputs.
+ * For example:
+ * Input 1: maps a device's BARs, sets up three DMA patterns, and triggers
+ *          device functionality A
+ * Input 2: maps a device's BARs, sets up one DMA pattern, and triggers device
+ *          functionality B
+ *
+ * This function attempts to produce an input that:
+ * Ouptut: maps a device's BARs, set up three DMA patterns, triggers
+ *          functionality A device, replaces the DMA patterns with a single
+ *          patten, and triggers device functionality B.
+ */
+static size_t generic_fuzz_crossover(const uint8_t *data1, size_t size1, const
+                                     uint8_t *data2, size_t size2, uint8_t *out,
+                                     size_t max_out_size, unsigned int seed)
+{
+    size_t copy_len = 0, size = 0;
+
+    /* Check that we have enough space for data1 and at least part of data2 */
+    if (max_out_size <= size1 + strlen(SEPARATOR) * 3 + 2) {
+        return 0;
+    }
+
+    /* Copy_Len in the first input */
+    copy_len = size1;
+    memcpy(out + size, data1, copy_len);
+    size += copy_len;
+    max_out_size -= copy_len;
+
+    /* Append a separator */
+    copy_len = strlen(SEPARATOR);
+    memcpy(out + size, SEPARATOR, copy_len);
+    size += copy_len;
+    max_out_size -= copy_len;
+
+    /* Clear out the DMA Patterns */
+    copy_len = 1;
+    if (copy_len) {
+        out[size] = OP_CLEAR_DMA_PATTERNS;
+    }
+    size += copy_len;
+    max_out_size -= copy_len;
+
+    /* Append a separator */
+    copy_len = strlen(SEPARATOR);
+    memcpy(out + size, SEPARATOR, copy_len);
+    size += copy_len;
+    max_out_size -= copy_len;
+
+    /* Disable PCI ops. Assume data1 took care of setting up PCI */
+    copy_len = 1;
+    if (copy_len) {
+        out[size] = OP_DISABLE_PCI;
+    }
+    size += copy_len;
+    max_out_size -= copy_len;
+
+    /* Append a separator */
+    copy_len = strlen(SEPARATOR);
+    memcpy(out + size, SEPARATOR, copy_len);
+    size += copy_len;
+    max_out_size -= copy_len;
+
+    /* Copy_Len over the second input */
+    copy_len = MIN(size2, max_out_size);
+    memcpy(out + size, data2, copy_len);
+    size += copy_len;
+    max_out_size -= copy_len;
+
+    return  size;
+}
+
+
 static GString *generic_fuzz_cmdline(FuzzTarget *t)
 {
     GString *cmd_line = g_string_new(TARGET_NAME);
@@ -830,6 +915,7 @@ static void register_generic_fuzz_targets(void)
             .get_init_cmdline = generic_fuzz_cmdline,
             .pre_fuzz = generic_pre_fuzz,
             .fuzz = generic_fuzz,
+            .crossover = generic_fuzz_crossover
     });
 }