fortify: Capture __bos() results in const temp vars

In two recent run-time memcpy() bound checking bug reports (NFS[1] and
JFS[2]), the _detection_ was working correctly (in the sense that the
requested copy size was larger than the destination field size), but
the _warning text_ was showing the destination field size as SIZE_MAX
("unknown size"). This should be impossible, since the detection function
will explicitly give up if the destination field size is unknown. For
example, the JFS warning was:

  memcpy: detected field-spanning write (size 132) of single field "ip->i_link" at fs/jfs/namei.c:950 (size 18446744073709551615)

Other cases of this warning (e.g.[3]) have reported correctly,
and the reproducer only happens under GCC (at least 10.2 and 12.1),
so this currently appears to be a GCC bug. Explicitly capturing the
__builtin_object_size() results in const temporary variables fixes the
report. For example, the JFS reproducer now correctly reports the field
size (128):

  memcpy: detected field-spanning write (size 132) of single field "ip->i_link" at fs/jfs/namei.c:950 (size 128)

Examination of the .text delta (which is otherwise identical), shows
the literal value used in the report changing:

-     mov    $0xffffffffffffffff,%rcx
+     mov    $0x80,%ecx

[1] https://lore.kernel.org/lkml/Y0zEzZwhOxTDcBTB@codemonkey.org.uk/
[2] https://syzkaller.appspot.com/bug?id=23d613df5259b977dac1696bec77f61a85890e3d
[3] https://lore.kernel.org/all/202210110948.26b43120-yujie.liu@intel.com/

Cc: "Dr. David Alan Gilbert" <linux@treblig.org>
Cc: llvm@lists.linux.dev
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/fortify-string.h

@@ -441,13 +441,18 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size,
 
 #define __fortify_memcpy_chk(p, q, size, p_size, q_size,		\
 			     p_size_field, q_size_field, op) ({		\
-	size_t __fortify_size = (size_t)(size);				\
-	WARN_ONCE(fortify_memcpy_chk(__fortify_size, p_size, q_size,	\
-				     p_size_field, q_size_field, #op),	\
+	const size_t __fortify_size = (size_t)(size);			\
+	const size_t __p_size = (p_size);				\
+	const size_t __q_size = (q_size);				\
+	const size_t __p_size_field = (p_size_field);			\
+	const size_t __q_size_field = (q_size_field);			\
+	WARN_ONCE(fortify_memcpy_chk(__fortify_size, __p_size,		\
+				     __q_size, __p_size_field,		\
+				     __q_size_field, #op),		\
 		  #op ": detected field-spanning write (size %zu) of single %s (size %zu)\n", \
 		  __fortify_size,					\
 		  "field \"" #p "\" at " __FILE__ ":" __stringify(__LINE__), \
-		  p_size_field);					\
+		  __p_size_field);					\
 	__underlying_##op(p, q, __fortify_size);			\
 })