[Bug c/93653] New: diagnose calls to strncmp with bound less than constant string length

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93653

            Bug ID: 93653
           Summary: diagnose calls to strncmp with bound less than
                    constant string length
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

-Wstring-compare diagnoses equality expressions involving calls to strcmp and
strncmp that evaluate to constant s based on the size of one argument and the
length of another.  Such calls are likely mistakes (it makes little sense to
compare a longer string for equality to a smaller array).

Another class of mistakes -Wstring-compare could help detect is those pointed
out in pr93640 and pr93641: calls with constant bounds that are less than the
length of the constant string argument.  These should probably be detected and
diagnosed early, before non-constant expressions have been folded into
constants.  Another question is whether the string arguments should be limited
to literals or whether all constant strings should be considered.

$ cat x.c && gcc -O2 -S -Wall -Wextra -Wpedantic x.c
char a[2];

int f (void)
{
  return __builtin_strncmp (a, "123", 3) == 0;   // warning
}

int g (const char *s)
{
  return __builtin_strncmp (s, "123", 2) == 0;   // should warn
}
x.c: In function ‘f’:
x.c:5:10: warning: ‘__builtin_strncmp’ of a string of length 3, an array of
size 2 and bound of 3 evaluates to nonzero [-Wstring-compare]
    5 |   return __builtin_strncmp (a, "123", 3) == 0;   // warning
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~