Implement checked_add_duration for SystemTime

Since SystemTime is opaque there is no way to check if the result
of an addition will be in bounds. That makes the Add<Duration>
trait completely unusable with untrusted data. This is a big problem
because adding a Duration to UNIX_EPOCH is the standard way of
constructing a SystemTime from a unix timestamp.

This commit implements checked_add_duration(&self, &Duration) -> Option<SystemTime>
for std::time::SystemTime and as a prerequisite also for all platform
specific time structs. This also led to the refactoring of many
add_duration(&self, &Duration) -> SystemTime functions to avoid
redundancy (they now unwrap the result of checked_add_duration).

Some basic unit tests for the newly introduced function were added
too.

5 files changed, 71 insertions, 22 deletions

diff --git a/src/libstd/sys/cloudabi/time.rs b/src/libstd/sys/cloudabi/time.rs
index ee12731619a..191324e26a4 100644
--- a/src/libstd/sys/cloudabi/time.rs
+++ b/src/libstd/sys/cloudabi/time.rs
@@ -19,10 +19,14 @@ pub struct Instant {
     t: abi::timestamp,
 }
 
-pub fn dur2intervals(dur: &Duration) -> abi::timestamp {
+fn checked_dur2intervals(dur: &Duration) -> Option<abi::timestamp> {
     dur.as_secs()
         .checked_mul(NSEC_PER_SEC)
         .and_then(|nanos| nanos.checked_add(dur.subsec_nanos() as abi::timestamp))
+}
+
+pub fn dur2intervals(dur: &Duration) -> abi::timestamp {
+    checked_dur2intervals(dur)
         .expect("overflow converting duration to nanoseconds")
 }
 
@@ -92,11 +96,14 @@ impl SystemTime {
     }
 
     pub fn add_duration(&self, other: &Duration) -> SystemTime {
-        SystemTime {
-            t: self.t
-                .checked_add(dur2intervals(other))
-                .expect("overflow when adding duration to instant"),
-        }
+        self.checked_add_duration(other)
+            .expect("overflow when adding duration to instant")
+    }
+
+    pub fn checked_add_duration(&self, other: &Duration) -> Option<SystemTime> {
+        checked_dur2intervals(other)
+            .and_then(|d| self.t.checked_add(d))
+            .map(|t| SystemTime {t})
     }
 
     pub fn sub_duration(&self, other: &Duration) -> SystemTime {
diff --git a/src/libstd/sys/redox/time.rs b/src/libstd/sys/redox/time.rs
index 5c491115c55..5f8799489aa 100644
--- a/src/libstd/sys/redox/time.rs
+++ b/src/libstd/sys/redox/time.rs
@@ -42,27 +42,36 @@ impl Timespec {
     }
 
     fn add_duration(&self, other: &Duration) -> Timespec {
-        let mut secs = other
+        self.checked_add_duration(other).expect("overflow when adding duration to time")
+    }
+
+    fn checked_add_duration(&self, other: &Duration) -> Option<Timespec> {
+        let mut secs = match other
             .as_secs()
             .try_into() // <- target type would be `i64`
             .ok()
             .and_then(|secs| self.t.tv_sec.checked_add(secs))
-            .expect("overflow when adding duration to time");
+        {
+            Some(ts) => ts,
+            None => return None,
+        };
 
         // Nano calculations can't overflow because nanos are <1B which fit
         // in a u32.
         let mut nsec = other.subsec_nanos() + self.t.tv_nsec as u32;
         if nsec >= NSEC_PER_SEC as u32 {
             nsec -= NSEC_PER_SEC as u32;
-            secs = secs.checked_add(1).expect("overflow when adding \
-                                               duration to time");
+            secs = match secs.checked_add(1) {
+                Some(ts) => ts,
+                None => return None,
+            }
         }
-        Timespec {
+        Some(Timespec {
             t: syscall::TimeSpec {
                 tv_sec: secs,
                 tv_nsec: nsec as i32,
             },
-        }
+        })
     }
 
     fn sub_duration(&self, other: &Duration) -> Timespec {
@@ -180,6 +189,10 @@ impl SystemTime {
         SystemTime { t: self.t.add_duration(other) }
     }
 
+    pub fn checked_add_duration(&self, other: &Duration) -> Option<SystemTime> {
+        self.t.checked_add_duration(other).map(|t| SystemTime { t })
+    }
+
     pub fn sub_duration(&self, other: &Duration) -> SystemTime {
         SystemTime { t: self.t.sub_duration(other) }
     }
diff --git a/src/libstd/sys/unix/time.rs b/src/libstd/sys/unix/time.rs
index 0b1fb726357..50c3c00382e 100644
--- a/src/libstd/sys/unix/time.rs
+++ b/src/libstd/sys/unix/time.rs
@@ -43,27 +43,36 @@ impl Timespec {
     }
 
     fn add_duration(&self, other: &Duration) -> Timespec {
-        let mut secs = other
+        self.checked_add_duration(other).expect("overflow when adding duration to time")
+    }
+
+    fn checked_add_duration(&self, other: &Duration) -> Option<Timespec> {
+        let mut secs = match other
             .as_secs()
             .try_into() // <- target type would be `libc::time_t`
             .ok()
             .and_then(|secs| self.t.tv_sec.checked_add(secs))
-            .expect("overflow when adding duration to time");
+        {
+            Some(ts) => ts,
+            None => return None,
+        };
 
         // Nano calculations can't overflow because nanos are <1B which fit
         // in a u32.
         let mut nsec = other.subsec_nanos() + self.t.tv_nsec as u32;
         if nsec >= NSEC_PER_SEC as u32 {
             nsec -= NSEC_PER_SEC as u32;
-            secs = secs.checked_add(1).expect("overflow when adding \
-                                               duration to time");
+            secs = match secs.checked_add(1) {
+                Some(ts) => ts,
+                None => return None,
+            }
         }
-        Timespec {
+        Some(Timespec {
             t: libc::timespec {
                 tv_sec: secs,
                 tv_nsec: nsec as _,
             },
-        }
+        })
     }
 
     fn sub_duration(&self, other: &Duration) -> Timespec {
@@ -201,6 +210,10 @@ mod inner {
             SystemTime { t: self.t.add_duration(other) }
         }
 
+        pub fn checked_add_duration(&self, other: &Duration) -> Option<SystemTime> {
+            self.t.checked_add_duration(other).map(|t| SystemTime { t })
+        }
+
         pub fn sub_duration(&self, other: &Duration) -> SystemTime {
             SystemTime { t: self.t.sub_duration(other) }
         }
@@ -325,6 +338,10 @@ mod inner {
             SystemTime { t: self.t.add_duration(other) }
         }
 
+        pub fn checked_add_duration(&self, other: &Duration) -> Option<SystemTime> {
+            self.t.checked_add_duration(other).map(|t| SystemTime { t })
+        }
+
         pub fn sub_duration(&self, other: &Duration) -> SystemTime {
             SystemTime { t: self.t.sub_duration(other) }
         }
diff --git a/src/libstd/sys/wasm/time.rs b/src/libstd/sys/wasm/time.rs
index e52435e6339..991e8176edf 100644
--- a/src/libstd/sys/wasm/time.rs
+++ b/src/libstd/sys/wasm/time.rs
@@ -51,6 +51,10 @@ impl SystemTime {
         SystemTime(self.0 + *other)
     }
 
+    pub fn checked_add_duration(&self, other: &Duration) -> Option<SystemTime> {
+        self.0.checked_add(*other).map(|d| SystemTime(d))
+    }
+
     pub fn sub_duration(&self, other: &Duration) -> SystemTime {
         SystemTime(self.0 - *other)
     }
diff --git a/src/libstd/sys/windows/time.rs b/src/libstd/sys/windows/time.rs
index 07e64d386a1..8eebe4a85fe 100644
--- a/src/libstd/sys/windows/time.rs
+++ b/src/libstd/sys/windows/time.rs
@@ -128,9 +128,13 @@ impl SystemTime {
     }
 
     pub fn add_duration(&self, other: &Duration) -> SystemTime {
-        let intervals = self.intervals().checked_add(dur2intervals(other))
-                            .expect("overflow when adding duration to time");
-        SystemTime::from_intervals(intervals)
+        self.checked_add_duration(other).expect("overflow when adding duration to time")
+    }
+
+    pub fn checked_add_duration(&self, other: &Duration) -> Option<SystemTime> {
+        checked_dur2intervals(other)
+            .and_then(|d| self.intervals().checked_add(d))
+            .map(|i| SystemTime::from_intervals(i))
     }
 
     pub fn sub_duration(&self, other: &Duration) -> SystemTime {
@@ -180,11 +184,15 @@ impl Hash for SystemTime {
     }
 }
 
-fn dur2intervals(d: &Duration) -> i64 {
+fn checked_dur2intervals(d: &Duration) -> Option<i64> {
     d.as_secs()
         .checked_mul(INTERVALS_PER_SEC)
         .and_then(|i| i.checked_add(d.subsec_nanos() as u64 / 100))
         .and_then(|i| i.try_into().ok())
+}
+
+fn dur2intervals(d: &Duration) -> i64 {
+    checked_dur2intervals(d)
         .expect("overflow when converting duration to intervals")
 }