In DCE 1.1, the time_low value is defined as an unsigned 32-bit

integer.  Presently, our implementation employs an approach that
converts the value to int64_t, then back to int, unfortunately,
this approach can be problematic when the the difference between
the two time_low is larger than 0x7fffffff, as the value is then
truncated to int.

To quote the test case from the original PR, the following is
true with the current implementation:

865e1a56-b9d9-11d9-ba27-0003476f2e88 < 062ac45c-b9d9-11d9-ba27-0003476f2e88

However, according to the DCE specification, the expected result
should be:

865e1a56-b9d9-11d9-ba27-0003476f2e88 > 062ac45c-b9d9-11d9-ba27-0003476f2e88

This commit adds a new intermediate variable which uses int64_t
to store the result of subtraction between the two time_low values,
which would not introduce different semantic of the MSB found in
time_low value.

PR:		83107
Submitted by:	Steve Sears <sjs at acm dot org>
MFC After:	1 month

1 files changed, 15 insertions, 5 deletions

diff --git a/lib/libc/uuid/uuid_compare.c b/lib/libc/uuid/uuid_compare.c
index f57a094..420f326 100644
--- a/lib/libc/uuid/uuid_compare.c
+++ b/lib/libc/uuid/uuid_compare.c
@@ -41,7 +41,8 @@
 int32_t
 uuid_compare(const uuid_t *a, const uuid_t *b, uint32_t *status)
 {
-	int res;
+	int	res;
+	int64_t res64;
 
 	if (status != NULL)
 		*status = uuid_s_ok;
@@ -54,10 +55,19 @@ uuid_compare(const uuid_t *a, const uuid_t *b, uint32_t *status)
 	if (b == NULL)
 		return ((uuid_is_nil(a, NULL)) ? 0 : 1);
 
-	/* We have to compare the hard way. */
-	res = (int)((int64_t)a->time_low - (int64_t)b->time_low);
-	if (res)
-		return ((res < 0) ? -1 : 1);
+	/*
+	 * We have to compare the hard way.
+	 *
+	 * Note that time_low is defined as unsigned 32-bit
+	 * integer, therefore, with a significantly large
+	 * a->time_low and a small b->time_low, we will end
+	 * up with a value which is larger than 0x7fffffff
+	 * which is negative if casted to signed 32-bit
+	 * integer.
+	 */
+	res64 = (int64_t)a->time_low - (int64_t)b->time_low;
+	if (res64)
+		return ((res64 < 0) ? -1 : 1);
 	res = (int)a->time_mid - (int)b->time_mid;
 	if (res)
 		return ((res < 0) ? -1 : 1);