Fix memory errors when trim causes testcase growth (#881) (#903)

* Revert "fixed potential double free in custom trim (#881)"

This reverts commit e9d2f72382cab75832721d859c3e731da071435d.

* Revert "fix custom trim for increasing data"

This reverts commit 86a8ef168dda766d2f25f15c15c4d3ecf21d0667.

* Fix memory errors when trim causes testcase growth

Modify trim_case_custom to avoid writing into in_buf because
some custom mutators can cause the testcase to grow rather than
shrink.

Instead of modifying in_buf directly, we write the update out
to the disk when trimming is complete, and then the caller is
responsible for refreshing the in-memory buffer from the file.

This is still a bit sketchy because it does need to modify q->len in
order to notify the upper layers that something changed, and it could
end up telling upper layer code that the q->len is *bigger* than
the buffer (q->testcase_buf) that contains it, which is asking
for trouble down the line somewhere...

* Fix an unlikely situation

Put back some `unlikely()` calls that were in
the e9d2f72382cab75832721d859c3e731da071435d commit that was
reverted.

1 files changed, 2 insertions, 6 deletions

diff --git a/src/afl-fuzz-run.c b/src/afl-fuzz-run.c
index 397d62bf..6e5210b8 100644
--- a/src/afl-fuzz-run.c
+++ b/src/afl-fuzz-run.c
@@ -718,10 +718,7 @@ void sync_fuzzers(afl_state_t *afl) {
    trimmer uses power-of-two increments somewhere between 1/16 and 1/1024 of
    file size, to keep the stage short and sweet. */
 
-u8 trim_case(afl_state_t *afl, struct queue_entry *q, u8 **in_buf_p) {
-
-  // We need to pass pointers around, as growing testcases may need to realloc.
-  u8 *in_buf = *in_buf_p;
+u8 trim_case(afl_state_t *afl, struct queue_entry *q, u8 *in_buf) {
 
   u32 orig_len = q->len;
 
@@ -735,8 +732,7 @@ u8 trim_case(afl_state_t *afl, struct queue_entry *q, u8 **in_buf_p) {
 
       if (el->afl_custom_trim) {
 
-        trimmed_case = trim_case_custom(afl, q, in_buf_p, el);
-        in_buf = *in_buf_p;
+        trimmed_case = trim_case_custom(afl, q, in_buf, el);
         custom_trimmed = true;
 
       }