misc: Fix rare fortify crash on wchar funcs. [BZ 29030]

If `__glibc_objsize (__o) == (size_t) -1` (i.e. `__o` is unknown size), fortify
checks should pass, and `__whatever_alias` should be called.

Previously, `__glibc_objsize (__o) == (size_t) -1` was explicitly checked, but
on commit a643f60c53876b, this was moved into `__glibc_safe_or_unknown_len`.

A comment says the -1 case should work as: "The -1 check is redundant because
since it implies that __glibc_safe_len_cond is true.". But this fails when:
* `__s > 1`
* `__osz == -1` (i.e. unknown size at compile time)
* `__l` is big enough
* `__l * __s <= __osz` can be folded to a constant
(I only found this to be true for `mbsrtowcs` and other functions in wchar2.h)

In this case `__l * __s <= __osz` is false, and `__whatever_chk_warn` will be
called by `__glibc_fortify` or `__glibc_fortify_n` and crash the program.

This commit adds the explicit `__osz == -1` check again.
moc crashes on startup due to this, see: https://bugs.archlinux.org/task/74041

Minimal test case (test.c):
    #include <wchar.h>

    int main (void)
    {
        const char *hw = "HelloWorld";
        mbsrtowcs (NULL, &hw, (size_t)-1, NULL);
        return 0;
    }

Build with:
    gcc -O2 -Wp,-D_FORTIFY_SOURCE=2 test.c -o test && ./test

Output:
    *** buffer overflow detected ***: terminated

Fixes: BZ #29030
Signed-off-by: Joan Bruguera <joanbrugueram@gmail.com>
Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

2 files changed, 11 insertions, 6 deletions

diff --git a/debug/tst-fortify.c b/debug/tst-fortify.c
index d65a2fe6e1..03c9867714 100644
--- a/debug/tst-fortify.c
+++ b/debug/tst-fortify.c
@@ -1504,6 +1504,11 @@ do_test (void)
       CHK_FAIL_END
 #endif
 
+      /* Bug 29030 regresion check */
+      cp = "HelloWorld";
+      if (mbsrtowcs (NULL, &cp, (size_t)-1, &s) != 10)
+        FAIL ();
+
       cp = "A";
       if (mbstowcs (wenough, cp, 10) != 1
 	  || wcscmp (wenough, L"A") != 0)
diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
index 44d3826bca..f1faf8292c 100644
--- a/misc/sys/cdefs.h
+++ b/misc/sys/cdefs.h
@@ -162,13 +162,13 @@
    || (__builtin_constant_p (__l) && (__l) > 0))
 
 /* Length is known to be safe at compile time if the __L * __S <= __OBJSZ
-   condition can be folded to a constant and if it is true.  The -1 check is
-   redundant because since it implies that __glibc_safe_len_cond is true.  */
+   condition can be folded to a constant and if it is true, or unknown (-1) */
 #define __glibc_safe_or_unknown_len(__l, __s, __osz) \
-  (__glibc_unsigned_or_positive (__l)					      \
-   && __builtin_constant_p (__glibc_safe_len_cond ((__SIZE_TYPE__) (__l),     \
-						   __s, __osz))		      \
-   && __glibc_safe_len_cond ((__SIZE_TYPE__) (__l), __s, __osz))
+  ((__osz) == (__SIZE_TYPE__) -1					      \
+   || (__glibc_unsigned_or_positive (__l)				      \
+       && __builtin_constant_p (__glibc_safe_len_cond ((__SIZE_TYPE__) (__l), \
+						       (__s), (__osz)))	      \
+       && __glibc_safe_len_cond ((__SIZE_TYPE__) (__l), (__s), (__osz))))
 
 /* Conversely, we know at compile time that the length is unsafe if the
    __L * __S <= __OBJSZ condition can be folded to a constant and if it is