xcursor: strncpy truncation warning with GCC 10 on s390x

[khardix]

When building 0.10.0 for Fedora (Rawhide, currently with GCC 10), there is a strncpy warning in xcursor/xcursor.c:

In file included from /usr/include/string.h:495,
                 from ../xcursor/xcursor.c:30:
In function 'strncpy',
    inlined from '_XcursorAddPathElt' at ../xcursor/xcursor.c:657:5,
    inlined from '_XcursorBuildFullname' at ../xcursor/xcursor.c:731:5,
    inlined from 'XcursorScanTheme' at ../xcursor/xcursor.c:820:13:
/usr/include/bits/string_fortified.h:106:10: error: '__builtin_strncpy' output truncated before terminating nul copying 7 bytes from a string of the same length [-Werror=stringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function 'strncpy',
    inlined from '_XcursorAddPathElt' at ../xcursor/xcursor.c:657:5,
    inlined from '_XcursorBuildFullname' at ../xcursor/xcursor.c:731:5,
    inlined from 'xcursor_load_theme' at ../xcursor/xcursor.c:959:10:
/usr/include/bits/string_fortified.h:106:10: error: '__builtin_strncpy' output truncated before terminating nul copying 7 bytes from a string of the same length [-Werror=stringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Thanks to -Werror, this aborts the build.

---

I checked the code around the specified lines, and the \0 is currently added explicitly:

wlroots/xcursor/xcursor.c

Lines 657 to 658 in f2943bd

	strncpy (path + pathlen, elt, len);
	path[pathlen + len] = '\0';

Because this only manifests on one architecture, and the warning is mitigated on the very next line, I'm inclined to blame GCC and consider this a false positive. Nevertheless, if we want to keep -Werror on, the offending code probably have to be changed.

For the time being, I will work around this by disabling the -Werror flag.