[Bug general/29141] _FORTIFY_SOURCE=3 fail for gcc 12/glibc 2.35

["mark at klomp dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29141

Mark Wielaard <mark at klomp dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mark at klomp dot org,
                   |                            |siddhesh at sourceware dot org

--- Comment #1 from Mark Wielaard <mark at klomp dot org> ---
This is interesting. I recently fixed a similar warning in gdb (also for
-Werror=stringop-overflow, but not _FORTIFY_SOURCE related):
https://sourceware.org/pipermail/gdb-patches/2022-May/188694.html

I have added Siddhesh to the CC who might have seen this earlier.
Background is that in elfutils we are trying to use _FORTIFY_SOURCE=3 when it
is available using this configure snippet:

# See if we can add -D_FORTIFY_SOURCE=2 or =3. Don't do it if it is already
# (differently) defined or if it generates warnings/errors because we
# don't use the right optimisation level (string.h will warn about that).
AC_MSG_CHECKING([whether to add -D_FORTIFY_SOURCE=2 or =3 to CFLAGS])
case "$CFLAGS" in
  *-D_FORTIFY_SOURCE=*)
    AC_MSG_RESULT([no, already there])
    ;;
  *)
    save_CFLAGS="$CFLAGS"
    # Try 3 first.
    CFLAGS="-D_FORTIFY_SOURCE=3 $save_CFLAGS -Werror"
    fortified_cflags=""
    AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
      #include <string.h>
      int main() { return 0; }
    ]])], [ AC_MSG_RESULT([yes -D_FORTIFY_SOURCE=3])
            fortified_cflags="-D_FORTIFY_SOURCE=3" ], [])

    # If that didn't work, try 2.
    if test -z "$fortified_cflags"; then
      CFLAGS="-D_FORTIFY_SOURCE=2 $save_CFLAGS -Werror"
      AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
        #include <string.h>
        int main() { return 0; }
      ]])], [ AC_MSG_RESULT([yes -D_FORTIFY_SOURCE=2])
              fortified_cflags="-D_FORTIFY_SOURCE=2" ],
              [ AC_MSG_RESULT([no, cannot be used])])
    fi
    CFLAGS="$fortified_cflags $save_CFLAGS"
    CXXFLAGS="$fortified_cflags $CXXFLAGS"
    ;;
esac


But it seems to produce gcc warnings like the above, so either our configure
foo is bad or we simply have to always use _FORTIFY_SOURCE=2.

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