UBSAN does not catch overflow of short, chart

milianw commented 6 years ago

Overflows for short, char and signed char are not caught by UBSAN. It does work for long long, long and int though:

┌milian:/tmp
└$ cat test.c
#include <limits.h>

int main()
{
  long long ll = LLONG_MAX;
  ++ll;

  long l = LONG_MAX;
  ++l;

  int i = INT_MAX;
  ++i;

  short s = SHRT_MAX;
  ++s;

  char c = CHAR_MAX;
  ++c;

  signed char sc = SCHAR_MAX;
  ++sc;

  return 0;
}
┌milian:/tmp
└$ clang -fsanitize=undefined -g test.c && ./a.out 
test.c:6:3: runtime error: signed integer overflow: 9223372036854775807 + 1 cannot be represented in type 'long long'
    #0 0x55ca3af09f67 in main /tmp/test.c:6:3
    #1 0x7f6ad1feaf49 in __libc_start_main (/usr/lib/libc.so.6+0x20f49)
    #2 0x55ca3aee4249 in _start (/tmp/a.out+0x3249)

test.c:9:3: runtime error: signed integer overflow: 9223372036854775807 + 1 cannot be represented in type 'long'
    #0 0x55ca3af09fbb in main /tmp/test.c:9:3
    #1 0x7f6ad1feaf49 in __libc_start_main (/usr/lib/libc.so.6+0x20f49)
    #2 0x55ca3aee4249 in _start (/tmp/a.out+0x3249)

test.c:12:3: runtime error: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'
    #0 0x55ca3af0a004 in main /tmp/test.c:12:3
    #1 0x7f6ad1feaf49 in __libc_start_main (/usr/lib/libc.so.6+0x20f49)
    #2 0x55ca3aee4249 in _start (/tmp/a.out+0x3249)

dvyukov commented 6 years ago

Isn't it defined in C/C++ due to promotion rules? AFAIK if you add two shorts and assign the result to short, arguments are first promoted to int's, int's are added and then the result is truncated back to short.

milianw commented 6 years ago

ugh TIL... Considering that this is often not what the developer intended, can we maybe get a flag for this similar to -fsanitize=unsigned-integer-overflow to catch these?

yugr commented 6 years ago

I once filed LLVM bug #21530 about this and feedback was positive but it never got beyond prototype. Maybe try asking in Bugzilla?