[Bug sanitizer/107908] New: A null pointer dereference bug was missed by UBsan at -O0

[Bug sanitizer/107908] New: A null pointer dereference bug was missed by UBsan at -O0

[shaohua.li at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107908

            Bug ID: 107908
           Summary: A null pointer dereference bug was missed by UBsan at
                    -O0
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: shaohua.li at inf dot ethz.ch
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org, marxin at gcc dot gnu.org
  Target Milestone: ---

`gcc-trunk -O0 -fsanitize=undefined -fno-sanitize-recover=all` misses the
NULL-pointer-dereference, while `gcc-trunk -Ox -fsanitize=address
-fno-sanitize-recover=all` (x=1, 2, 3, or s) can detect it.

Compiler explorer: https://godbolt.org/z/x7jo7eYYh

% cat a.c
int a;
int main() {
  int *c;
  for (int d = 0; d < 1; d++)
    *c == (a = 0);
  __builtin_printf("%d\n", *c);
}
%
% gcc-tk -O0 -fsanitize=undefined -fno-sanitize-recover=all a.c &&./a.out
1
% gcc-tk -O1 -fsanitize=undefined -fno-sanitize-recover=all -w a.c &&./a.out
a.c:5:5: runtime error: load of null pointer of type 'int'
% gcc-tk -O3 -fsanitize=undefined -fno-sanitize-recover=all -w a.c &&./a.out
a.c:5:5: runtime error: load of null pointer of type 'int'
%

[Bug sanitizer/107908] A null pointer dereference bug was missed by UBsan at -O0

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107908

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
c is uninitialized so it could hold any value including but not limited to a
null pointer.
In -O0 case, it just happens to have a non null value.

[Bug sanitizer/107908] A null pointer dereference bug was missed by UBsan at -O0

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107908

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
If you used -ftrivial-auto-var-init=zero you would get the runtime error:
/app/example.cpp:6:5: runtime error: load of null pointer of type 'int'

With -ftrivial-auto-var-init=pattern you get:

/app/example.cpp:6:5: runtime error: load of misaligned address
0xfefefefefefefefe for type 'int', which requires 4 byte alignment
0xfefefefefefefefe: note: pointer points here
<memory cannot be printed>

Which is also expected.