krb5 commit: Add timestamp helper functions

[Greg Hudson]

https://github.com/krb5/krb5/commit/58e9155060cd93b1a7557e37fbc9b077b76465c2
commit 58e9155060cd93b1a7557e37fbc9b077b76465c2
Author: Greg Hudson <ghudson at mit.edu>
Date:   Sat Apr 22 09:49:12 2017 -0400

    Add timestamp helper functions
    
    Add k5-int.h helper functions to manipulate krb5_timestamp values,
    avoiding undefined behavior and treating negative timestamp values as
    times between 2038 and 2106.  Add a doxygen comment for krb5_timestamp
    indicating how third-party code should use it safely.
    
    ticket: 8352

 src/include/k5-int.h      |   31 +++++++++++++++++++++++++++++++
 src/include/krb5/krb5.hin |    9 +++++++++
 2 files changed, 40 insertions(+), 0 deletions(-)

diff --git a/src/include/k5-int.h b/src/include/k5-int.h
index 360e088..ff93a3b 100644
--- a/src/include/k5-int.h
+++ b/src/include/k5-int.h
@@ -2353,6 +2353,37 @@ k5memdup0(const void *in, size_t len, krb5_error_code *code)
     return ptr;
 }
 
+/* Convert a krb5_timestamp to a time_t value, treating the negative range of
+ * krb5_timestamp as times between 2038 and 2106 (if time_t is 64-bit). */
+static inline time_t
+ts2tt(krb5_timestamp timestamp)
+{
+    return (time_t)(uint32_t)timestamp;
+}
+
+/* Return the delta between two timestamps (a - b) as a signed 32-bit value,
+ * without relying on undefined behavior. */
+static inline krb5_deltat
+ts_delta(krb5_timestamp a, krb5_timestamp b)
+{
+    return (krb5_deltat)((uint32_t)a - (uint32_t)b);
+}
+
+/* Increment a timestamp by a signed 32-bit interval, without relying on
+ * undefined behavior. */
+static inline krb5_timestamp
+ts_incr(krb5_timestamp ts, krb5_deltat delta)
+{
+    return (krb5_timestamp)((uint32_t)ts + (uint32_t)delta);
+}
+
+/* Return true if a comes after b. */
+static inline krb5_boolean
+ts_after(krb5_timestamp a, krb5_timestamp b)
+{
+    return (uint32_t)a > (uint32_t)b;
+}
+
 krb5_error_code KRB5_CALLCONV
 krb5_get_credentials_for_user(krb5_context context, krb5_flags options,
                               krb5_ccache ccache,
diff --git a/src/include/krb5/krb5.hin b/src/include/krb5/krb5.hin
index 762900e..c86e782 100644
--- a/src/include/krb5/krb5.hin
+++ b/src/include/krb5/krb5.hin
@@ -181,7 +181,16 @@ typedef krb5_int32 krb5_cryptotype;
 
 typedef krb5_int32      krb5_preauthtype; /* This may change, later on */
 typedef krb5_int32      krb5_flags;
+
+/**
+ * Represents a timestamp in seconds since the POSIX epoch.  This legacy type
+ * is used frequently in the ABI, but cannot represent timestamps after 2038 as
+ * a positive number.  Code which uses this type should cast values of it to
+ * uint32_t so that negative values are treated as timestamps between 2038 and
+ * 2106 on platforms with 64-bit time_t.
+ */
 typedef krb5_int32      krb5_timestamp;
+
 typedef krb5_int32      krb5_deltat;
 
 /**