[v4,1/3] fuzz: adjust timeout to allow for longer inputs

Commit Message

Alexander Bulekov June 28, 2021, 5:23 a.m. UTC

Using a custom timeout is useful to continue fuzzing complex devices,
even after we run into some slow code-path. However, simply adding a
fixed timeout to each input effectively caps the maximum input
length/number of operations at some artificial value. There are two
major problems with this:
1. Some code might only be reachable through long IO sequences.
2. Longer inputs can actually be _better_ for performance. While the
   raw number of fuzzer executions decreases with larger inputs, the
   number of MMIO/PIO/DMA operation/second actually increases, since
   were are speding proportionately less time fork()ing.

With this change, we keep the custom-timeout, but we renew it, prior to
each MMIO/PIO/DMA operation. Thus, we time-out only when a specific
operation takes a long time.

Reviewed-by: Darren Kenny <[e-mail]>
Signed-off-by: Alexander Bulekov <[e-mail]>
---
 tests/qtest/fuzz/generic_fuzz.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

Patch

diff --git a/tests/qtest/fuzz/generic_fuzz.c b/tests/qtest/fuzz/generic_fuzz.c
index cea7d4058e..71d36e8f6f 100644
--- a/tests/qtest/fuzz/generic_fuzz.c
+++ b/tests/qtest/fuzz/generic_fuzz.c
@@ -661,15 +661,16 @@  static void generic_fuzz(QTestState *s, const unsigned char *Data, size_t Size)
     uint8_t op;
 
     if (fork() == 0) {
+        struct sigaction sact;
+        struct itimerval timer;
         /*
          * Sometimes the fuzzer will find inputs that take quite a long time to
          * process. Often times, these inputs do not result in new coverage.
          * Even if these inputs might be interesting, they can slow down the
-         * fuzzer, overall. Set a timeout to avoid hurting performance, too much
+         * fuzzer, overall. Set a timeout for each command to avoid hurting
+         * performance, too much
          */
         if (timeout) {
-            struct sigaction sact;
-            struct itimerval timer;
 
             sigemptyset(&sact.sa_mask);
             sact.sa_flags   = SA_NODEFER;
@@ -679,13 +680,17 @@  static void generic_fuzz(QTestState *s, const unsigned char *Data, size_t Size)
             memset(&timer, 0, sizeof(timer));
             timer.it_value.tv_sec = timeout / USEC_IN_SEC;
             timer.it_value.tv_usec = timeout % USEC_IN_SEC;
-            setitimer(ITIMER_VIRTUAL, &timer, NULL);
         }
 
         op_clear_dma_patterns(s, NULL, 0);
         pci_disabled = false;
 
         while (cmd && Size) {
+            /* Reset the timeout, each time we run a new command */
+            if (timeout) {
+                setitimer(ITIMER_VIRTUAL, &timer, NULL);
+            }
+
             /* Get the length until the next command or end of input */
             nextcmd = memmem(cmd, Size, SEPARATOR, strlen(SEPARATOR));
             cmd_len = nextcmd ? nextcmd - cmd : Size;