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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/97047] New: missing warning reading past the end of
 a constant string returned from a function
Date: Mon, 14 Sep 2020 16:20:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97047

            Bug ID: 97047
           Summary: missing warning reading past the end of a constant
                    string returned from a function
           Product: gcc
           Version: 11.0
            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: ---

The first call to memcpy below triggers a warning for reading past the end =
of
the string returned from f(), but the second call doesn't.

$ cat x.c && gcc -O2 -S -Wall -fdump-tree-optimized=3D/dev/stdout x.c
const char* f (void) { return "123"; }

char a[32];

void g (void)
{
  __builtin_memcpy (a, "123", sizeof a);   // warning (good)
}

void h (void)
{
  __builtin_memcpy (a, f (), sizeof a);    // missing warning (bug)
}

;; Function f (f, funcdef_no=3D0, decl_uid=3D1931, cgraph_uid=3D1, symbol_o=
rder=3D0)

f ()
{
  <bb 2> [local count: 1073741824]:
  return "123";

}


x.c: In function =E2=80=98g=E2=80=99:
x.c:7:3: warning: =E2=80=98__builtin_memcpy=E2=80=99 forming offset [4, 31]=
 is out of the
bounds [0, 4] [-Warray-bounds]
    7 |   __builtin_memcpy (a, "123", sizeof a);   // warning (good)
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

;; Function g (g, funcdef_no=3D1, decl_uid=3D1935, cgraph_uid=3D2, symbol_o=
rder=3D2)

g ()
{
  <bb 2> [local count: 1073741824]:
  __builtin_memcpy (&a, "123", 32); [tail call]
  return;

}



;; Function h (h, funcdef_no=3D2, decl_uid=3D1938, cgraph_uid=3D3, symbol_o=
rder=3D3)

h ()
{
  <bb 2> [local count: 1073741824]:
  MEM <unsigned char[32]> [(char * {ref-all})&a] =3D MEM <unsigned char[32]>
[(char * {ref-all})"123"];
  return;

}=