So I'm using the AddressSanitizer. But it uses some dense terms when describing the problem.
Shadow bytes around the buggy address:
0x0c067fff7fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c067fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c067fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c067fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c067fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c067fff8000: fa fa 00 00 00 00[fa]fa 00 00 00 fa fa fa 00 00
0x0c067fff8010: 00 fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c067fff8020: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c067fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c067fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c067fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Freed heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
Container overflow: fc
Array cookie: ac
Intra object redzone: bb
ASan internal: fe
Left alloca redzone: ca
Right alloca redzone: cb
==7320==ABORTING
What does Heap left redzone mean? (and the others but I'm mostly interested in the fa as there is one [fa] which indicates the problem probably?)
What does Heap left redzone mean?
When AddressSanitizer heap interposer allocates heap memory in response to something like:
char *p = malloc(5);
it allocates more memory than you asked for. Let's say it allocates 32 bytes at address q. It then would mark the first 16 bytes (region [q, q+15]) as inaccessible heap left red zone (fa), the next 5 bytes as addressable (0), and the next 11 bytes as heap right red zone (fb).
Finally it would return the q+16 to the application (assigned to p).
Now if the application attempts to read or write from p-1, p-2, ... p-15, all such attempts would be detected because they will all land on the left red zone. This is heap underflow.
Similarly, attempts to access p+5, p+6, ... p+10 (heap overflow) would be detected because they will all land on the right red zone.
Why would an application ever have heap underflow? Consider this code:
int idx = get_valid_index(...); // return -1 on failure
...
if (p[idx] == ...) { // BUG: forgot to check idx!=-1
This actually happens more often that you'd think, and appears to have happened to you.