strfuncs: Always initialize errno to 0 after get_C_locale()
Some callers of `g_ascii_strtoull()` and similar functions assume that
they can use this pattern, similar to what they might do for
Standard C `strtoull()`:
errno = 0;
result = g_ascii_strtoull (nptr, endptr, base);
saved_errno = errno;
if (saved_errno != 0)
g_printerr ("error parsing %s\n", nptr);
This is based on the fact that it is non-trivial to tell whether
`strtoull()` and related functions succeeded (in which case the value
of `errno` is unspecified) or failed (in which case `errno` is valid).
For example, POSIX `strtoul(3)` suggests this pattern:
> Since 0, `ULONG_MAX`, and `ULLONG_MAX` are returned on error and are
> also valid returns on success, an application wishing to check for
> error situations should set `errno` to 0, then call `strtoul()` or
> `strtoull()`, then check `errno`.
However, `g_ascii_strtoull()` does not *only* call a function resembling
`strtoull()` (`strtoull_l()` or its reimplementation
`g_parse_long_long()`): it also calls `get_C_locale()`, which wraps
`newlocale()`. Even if `newlocale()` succeeds (which in practice we
expect and assume that it will), it is valid for it to clobber `errno`.
For example, it might attempt to open a file that only conditionally
exists, which would leave `errno` set to `ENOENT`.
This is difficult to reproduce in practice: I encountered what I
believe to be this bug when compiling GLib-based software for i386 in a
Debian 12 derivative via an Open Build Service instance, but I could
not reproduce the bug in a similar chroot environment locally, and I
also could not reproduce the bug when compiling for x86_64 or for a
Debian 10, 11 or 13 derivative on the same Open Build Service instance.
It also cannot be reproduced via the GTest framework, because
`g_test_init()` indirectly calls `g_ascii_strtoull()`, resulting in
the call to `newlocale()` already having happened by the time we enter
test code.
Resolves: https://gitlab.gnome.org/GNOME/glib/-/issues/3418
Signed-off-by: Simon McVittie <smcv@collabora.com>
diff --git a/glib/gstrfuncs.c b/glib/gstrfuncs.c
index 122de73..0b2a9e4 100644
--- a/glib/gstrfuncs.c
+++ b/glib/gstrfuncs.c
@@ -299,6 +299,10 @@
g_once_init_leave (&initialized, TRUE);
}
+ /* Callers of this function assume that if errno != 0, it indicates
+ * a failure by the function to which the locale_t is passed, for
+ * example strtoull_l(); so we need to make sure to clear errno. */
+ errno = 0;
return C_locale;
}
#endif
