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fuzz: adjust timeout to allow for longer inputs

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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 <darren.kenny@oracle.com>
Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
This commit is contained in:
Alexander Bulekov 2021-07-13 11:00:35 -04:00
parent af16990a1b
commit 993f52f4d4
1 changed files with 9 additions and 4 deletions
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13
tests/qtest/fuzz/generic_fuzz.c
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@ -668,15 +668,16 @@ 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;
@ -686,13 +687,17 @@ 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;