backend: prevent timestamp overflow

When updating timestamps, make sure that the corresponding
microsecond value fits in a single signed 64-bit integer,
so that it won't result in an integer overflow, which is UB.

Also forbid timestamps before the Unix epoch, since working with
negative time_t is problematic.

This check does not apply to current timestamp, however,
add a check on backend startup to ensure sane system time.

There's no need to validate `tv_nsec`, since the kernel already
does that for us.

src/backend_chromium.c

@@ -772,13 +772,8 @@ build_tsnode (
         ts = &now;
     }
 
-    // XXX: May overflow if system time is badly wrong,
-    // but don't bother to check.
     time_t  secs      = ts->tv_sec + EPOCH_DIFF;
     int64_t microsecs = secs * 1000000 + ts->tv_nsec / 1000;
-    if (microsecs < 0) {
-        microsecs = 0;
-    }
 
     char buf[32];
     int nbytes = snprintf(buf, sizeof(buf), "%" PRIi64, microsecs);
@@ -1713,6 +1708,13 @@ static int
 backend_init (
     uint32_t flags
 ) {
+    struct timespec now;
+    xgetrealtime(&now);
+    if (!valid_ts_sec(now.tv_sec)) {
+        log_puts("bad system time");
+        return -1;
+    }
+
     if (!(flags & BOOKMARKFS_BACKEND_LIB_READY)) {
         if (0 != bookmarkfs_lib_init()) {
             return -1;
@@ -2517,10 +2519,16 @@ bookmark_set (
         struct timespec const *times = val;
         json_t *ts_node;
         if (flags & BOOKMARK_FLAG(SET_ATIME)) {
+            if (!valid_ts_sec(times[0].tv_sec)) {
+                return -EINVAL;
+            }
             build_tsnode(&times[0], &ts_node);
             json_object_sset_new(node, "date_last_used", ts_node);
         }
         if (flags & BOOKMARK_FLAG(SET_MTIME)) {
+            if (!valid_ts_sec(times[1].tv_sec)) {
+                return -EINVAL;
+            }
             build_tsnode(&times[1], &ts_node);
             json_object_sset_new(node, "date_modified", ts_node);
         }

src/backend_firefox.c

@@ -2038,12 +2038,7 @@ static int64_t
 timespec_to_usecs (
     struct timespec const *ts
 ) {
-    int64_t microsecs = ts->tv_sec * 1000000 + ts->tv_nsec / 1000;
+    return ts->tv_sec * 1000000 + ts->tv_nsec / 1000;
-
-    if (microsecs < 0) {
-        microsecs = 0;
-    }
-    return microsecs;
 }
 
 static int
@@ -3003,6 +2998,13 @@ static int
 backend_init (
     uint32_t flags
 ) {
+    struct timespec now;
+    xgetrealtime(&now);
+    if (!valid_ts_sec(now.tv_sec)) {
+        log_puts("bad system time");
+        return -1;
+    }
+
     if (!(flags & BOOKMARKFS_BACKEND_LIB_READY)
             && !(flags & BOOKMARKFS_FRONTEND_MKFS)
     ) {
@@ -3675,10 +3677,16 @@ bookmark_set (
     if (flags & BOOKMARK_FLAG(SET_ATIME, SET_MTIME)) {
         struct timespec const *times = val;
         if (flags & BOOKMARK_FLAG(SET_ATIME)) {
+            if (!valid_ts_sec(times[0].tv_sec)) {
+                return -EINVAL;
+            }
             place_cols.last_visit_date = timespec_to_usecs(&times[0]);
             --xattr_id;
         }
         if (flags & BOOKMARK_FLAG(SET_MTIME)) {
+            if (!valid_ts_sec(times[1].tv_sec)) {
+                return -EINVAL;
+            }
             bm_cols.last_modified = timespec_to_usecs(&times[1]);
         }
     } else {

src/backend_util.h

@@ -57,6 +57,17 @@
 #define FILENAME_BADLEN   -2
 #define FILENAME_DOTDOT   -3
 
+/**
+ * The maximum valid seconds that a signed 64-bit integer can hold
+ * when converted to microseconds.
+ *
+ * Equals to `INT64_MAX / 1000000 - EPOCH_DIFF - 1`:
+ * - See the Chromium backend for `EPOCH_DIFF`.
+ * - Considering `tv_nsec`, subtract another second.
+ */
+#define TIMESPEC_SEC_MAX   INT64_C(9211727563253)
+#define valid_ts_sec(sec)  ((sec) >= 0 && (sec) <= TIMESPEC_SEC_MAX)
+
 /**
  * Opens the parent directory of a file, and stores its basename
  * to `basename_ptr`.