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From: "shaohua.li at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug sanitizer/107431] New: UBSan has inconsistent behaviors in
 certain code snippet
Date: Thu, 27 Oct 2022 08:39:02 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: sanitizer
X-Bugzilla-Version: 13.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107431

            Bug ID: 107431
           Summary: UBSan has inconsistent behaviors in certain code
                    snippet
           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, marxi=
n at gcc dot gnu.org
  Target Milestone: ---

Hi, for the following code, there is a misaligned load at line 5 `long d =
=3D *c`.
UBSan detected it successfully. However, if you change the definition of `a`
from `long a` to `int a`, UBSan would report nothing.

% cat example1.c
int main() {
    long a=3D2;
    int b=3D1;
    long *c =3D &b;
    long d =3D *c;
    &a;
}
% gcc -O0 -fsanitize=3Dundefined example1.c && ./a.out
example1.c:5:10: runtime error: load of misaligned address 0x7fffd97a993c f=
or
type 'long int', which requires 8 byte alignment
0x7fffd97a993c: note: pointer points here
  f0 71 77 cc 01 00 00 00  02 00 00 00 00 00 00 00  3c 99 7a d9 ff 7f 00 00=
  50
9a 7a d9 ff 7f 00 00
%
% cat example2.c
int main() {
    int a=3D2;
    int b=3D1;
    long *c =3D &b;
    long d =3D *c;
    &a;
}
% gcc -O0 -fsanitize=3Dundefined example1.c && ./a.out
%

Compiler explorer: https://godbolt.org/z/4oKaMjbee=