[PATCH 04/10] fuzz/generic-fuzz: add a limit on DMA bytes written

[Alexander Bulekov]

As we have repplaced fork-based fuzzing, with reboots - we can no longer
use a timeout+exit() to avoid slow inputs. Libfuzzer has its own timer
that it uses to catch slow inputs, however these timeouts are usually
seconds-minutes long: more than enough to bog-down the fuzzing process.
However, I found that slow inputs often attempt to fill overly large DMA
requests. Thus, we can mitigate most timeouts by setting a cap on the
total number of DMA bytes written by an input.

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
---
 tests/qtest/fuzz/generic_fuzz.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tests/qtest/fuzz/generic_fuzz.c b/tests/qtest/fuzz/generic_fuzz.c
index c2e5642150..eab92cbc23 100644
--- a/tests/qtest/fuzz/generic_fuzz.c
+++ b/tests/qtest/fuzz/generic_fuzz.c
@@ -52,6 +52,7 @@ enum cmds {
 #define USEC_IN_SEC 1000000000
 
 #define MAX_DMA_FILL_SIZE 0x10000
+#define MAX_TOTAL_DMA_SIZE 0x10000000
 
 #define PCI_HOST_BRIDGE_CFG 0xcf8
 #define PCI_HOST_BRIDGE_DATA 0xcfc
@@ -64,6 +65,7 @@ typedef struct {
 static useconds_t timeout = DEFAULT_TIMEOUT_US;
 
 static bool qtest_log_enabled;
+size_t dma_bytes_written;
 
 MemoryRegion *sparse_mem_mr;
 
@@ -197,6 +199,7 @@ void fuzz_dma_read_cb(size_t addr, size_t len, MemoryRegion 
*mr)
      */
     if (dma_patterns->len == 0
         || len == 0
+        || dma_bytes_written > MAX_TOTAL_DMA_SIZE
         || (mr != current_machine->ram && mr != sparse_mem_mr)) {
         return;
     }
@@ -269,6 +272,7 @@ void fuzz_dma_read_cb(size_t addr, size_t len, MemoryRegion 
*mr)
                 fflush(stderr);
             }
             qtest_memwrite(qts_global, addr, buf, l);
+            dma_bytes_written += l;
         }
         len -= l;
         buf += l;
@@ -648,6 +652,7 @@ static void generic_fuzz(QTestState *s, const unsigned char 
*Data, size_t Size)
 
     op_clear_dma_patterns(s, NULL, 0);
     pci_disabled = false;
+    dma_bytes_written = 0;
 
     QPCIBus *pcibus = qpci_new_pc(s, NULL);
     g_ptr_array_foreach(fuzzable_pci_devices, pci_enum, pcibus);
-- 
2.39.0