From 4afd38a15026164c6da7ec667fa6ae4ecd3d7f01 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rafa=C5=82=20Harabie=C5=84?= <rafalh1992@o2.pl>
Date: Sun, 17 Jun 2018 21:58:39 +0200
Subject: [PATCH] Fix possible overflow when calculating short name checksum

Affects debug build only. Overflow was not handled when calculating
checksum used for short name generation when avoiding a conflict.
LFN checksum was working properly but got refactored in this commit to
use num::Wrapping as well.
---
 src/dir.rs | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/src/dir.rs b/src/dir.rs
index 87eef2b..0fc343b 100644
--- a/src/dir.rs
+++ b/src/dir.rs
@@ -1,6 +1,6 @@
 #[cfg(feature = "alloc")]
 use core::{slice, iter};
-use core::{str, char, cmp};
+use core::{str, char, cmp, num};
 
 use io::prelude::*;
 use io;
@@ -537,11 +537,11 @@ fn validate_long_name(name: &str) -> io::Result<()> {
 
 #[cfg(feature = "alloc")]
 fn lfn_checksum(short_name: &[u8]) -> u8 {
-    let mut chksum = 0u8;
+    let mut chksum = num::Wrapping(0u8);
     for i in 0..11 {
-        chksum = (((chksum & 1) << 7) as u16 + (chksum >> 1) as u16 + short_name[i] as u16) as u8;
+        chksum = (chksum << 7) + (chksum >> 1) + num::Wrapping(short_name[i]);
     }
-    chksum
+    chksum.0
 }
 
 #[cfg(feature = "alloc")]
@@ -796,11 +796,11 @@ impl ShortNameGenerator {
 
     fn checksum(name: &str) -> u16 {
         // BSD checksum algorithm
-        let mut chksum = 0u16;
+        let mut chksum = num::Wrapping(0u16);
         for c in name.chars() {
-            chksum = (chksum >> 1) + ((chksum & 1) << 15) + (c as u16);
+            chksum = (chksum >> 1) + (chksum << 15) + num::Wrapping(c as u16);
         }
-        chksum
+        chksum.0
     }
 
     fn generate(&self) -> io::Result<[u8; 11]> {
@@ -873,6 +873,16 @@ mod tests {
         assert_eq!(&ShortNameGenerator::new(".bashrc.swp").generate().unwrap(), "BASHRC~1SWP".as_bytes());
     }
 
+    #[test]
+    fn test_short_name_checksum_overflow() {
+        ShortNameGenerator::checksum("\u{FF5A}\u{FF5A}\u{FF5A}\u{FF5A}");
+    }
+
+    #[test]
+    fn test_lfn_checksum_overflow() {
+        lfn_checksum(&[0xFFu8, 0xFFu8, 0xFFu8, 0xFFu8, 0xFFu8, 0xFFu8, 0xFFu8, 0xFFu8, 0xFFu8, 0xFFu8, 0xFFu8]);
+    }
+
     #[test]
     fn test_generate_short_name_collisions_long() {
         let mut buf: [u8; 11];