[Bug middle-end/91205] -fstack-protector-strong -D_FORTIFY_SOURCE=2 breaks tftpd

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91205

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
_FORTIFY_SOURCE=1 checks for all covered functions just whether they don't
overflow whole objects, while _FORTIFY_SOURCE=2 goes for a subset of functions
(e.g. those str*/stp* but not mem*) further and requires that they don't cross
structure field boundaries, say for
  struct S { char s[4]; char t[4]; } s;
  memcpy (s.s, "abcdefg", 8); // ok
  strcpy (s.s, "abcdefg"); // invalid
The only valid field that can go beyond end of the structure is flexible array
member, which is b in struct T { int a; char b[]; };, but GCC goes beyond that,
and allows [0], or even [23] as the last array in the structure, and handles I
believe even the union case you have in #c2, but it certainly isn't allowing
e.g. to cross from element to element of a multi-dimensional array or your #c0
case.