[Bug middle-end/79221] New: missing -Wstringop-overflow= on a strcat overflow

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79221

            Bug ID: 79221
           Summary: missing -Wstringop-overflow= on a strcat overflow
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Similar to bug 79220, the -Wstringop-overflow option diagnoses the buffer
overflow in the call to strcat in f() in the program below but fails to do the
same for the strcat() overflow in g().  As in the referenced bug, GCC
transforms the second strcat() to an assignment followed by a call to memcpy,
defeating the overflow detection.  GCC should avoid this transformation when
the destination isn't big enough for the copy.

I expect this bug will be resolved by a comprehensive fix for bug 79220 but it
seems that keeping track of each of these troublesome -- even though not
invalid -- transformations separately might help assure that the fix does, in
fact, resolve all these related problems.

(Both cases of overflow are diagnosed when _FORTIFY_SOURCE is defined.)


$ cat t.c && gcc -O2 -S -Wall -Wextra -Wpedantic
-fdump-tree-optimized=/dev/stdout t.c
#include <string.h>

char d[3];

void f (int i)
{
  const char *s = i < 0 ? "01234567" : "89abcd";
  strcat (d, s);
}


void gf (int i)
{
  const char *s = i < 0 ? "12345678" : "87654321";
  strcat (d, s);
}

;; Function f (f, funcdef_no=14, decl_uid=2196, cgraph_uid=14, symbol_order=15)

Removing basic block 3
f (int i)
{
  const char * iftmp.0_1;

  <bb 2> [100.00%]:
  if (i_2(D) < 0)
    goto <bb 4>; [32.39%]
  else
    goto <bb 3>; [67.61%]

  <bb 3> [67.61%]:

  <bb 4> [100.00%]:
  # iftmp.0_1 = PHI <"01234567"(2), "89abcd"(3)>
  strcat (&d, iftmp.0_1); [tail call]
  return;

}


t.c: In function ‘f’:
t.c:8:3: warning: ‘strcat’ writing 7 bytes into a region of size 3 overflows
the destination [-Wstringop-overflow=]
   strcat (d, s);
   ^~~~~~~~~~~~~

;; Function gf (gf, funcdef_no=15, decl_uid=2200, cgraph_uid=15,
symbol_order=16)

Removing basic block 3
gf (int i)
{
  char[9] * iftmp.1_1;
  long unsigned int _5;
  char[3] * _6;

  <bb 2> [100.00%]:
  if (i_2(D) < 0)
    goto <bb 4>; [32.39%]
  else
    goto <bb 3>; [67.61%]

  <bb 3> [67.61%]:

  <bb 4> [100.00%]:
  # iftmp.1_1 = PHI <"12345678"(2), "87654321"(3)>
  _5 = __builtin_strlen (&d);
  _6 = &d + _5;
  __builtin_memcpy (_6, iftmp.1_1, 9); [tail call]
  return;

}