AddressSanitizer, What do these terms mean?

Question

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?)

Answer 1

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.