Fix underflow when input is shorter than one sector

This also changes the error to InvalidSignature when the input is too short (it used to be ReadError in release mode and panic in debug mode).

Author: frankdavid

src/lib.rs

@@ -729,6 +729,10 @@ impl GPT {
         reader.seek(SeekFrom::Start(sector_size))?;
         let header = GPTHeader::read_from(&mut reader).or_else(|primary_err| {
             let len = reader.seek(SeekFrom::End(0))?;
+            if len < sector_size {
+                return Err(InvalidSignature);
+            }
+
             reader.seek(SeekFrom::Start((len / sector_size - 1) * sector_size))?;
 
             GPTHeader::read_from(&mut reader).map_err(|backup_err| {
@@ -1427,6 +1431,15 @@ mod test {
         assert!(GPT::find_from(&mut fs::File::open(DISK2).unwrap()).is_ok());
     }
 
+    #[test]
+    fn input_too_short() {
+        let mut empty = io::Cursor::new(vec![1; 5]);
+        assert!(matches!(
+            GPT::read_from(&mut empty, 512).expect_err("Should fail on short input"),
+            Error::InvalidSignature
+        ));
+    }
+
     #[test]
     fn find_backup() {
         fn test(path: &str, ss: u64) {