Fix out of range panic for 248+ long file names when alloc is disabled

Author: rafalh

CHANGELOG.md

@@ -28,6 +28,7 @@ New features:
 * Create directory entry with `VOLUME_ID` attribute when formatting if volume label was set in `FormatVolumeOptions`.
 * Fix creating directory entries when `lfn` feature is enabled and `alloc` feature is disabled
 * Fix `format_volume` function panicking in debug build for FAT12 volumes with size below 1 MB
+* Fix index out of range panic when reading 248+ characters long file names with `alloc` feature disabled
 * Remove `byteorder` dependency.
 * Bump up minimal Rust compiler version to 1.46.0.
 * Build the crate using the 2018 edition.

src/dir.rs

@@ -732,13 +732,17 @@ pub(crate) struct LfnBuffer {
     ucs2_units: Vec<u16>,
 }
 
+const MAX_LONG_NAME_LEN: usize = 255;
+
+const MAX_LONG_DIR_ENTRIES: usize = (MAX_LONG_NAME_LEN + LFN_PART_LEN - 1) / LFN_PART_LEN;
+
 #[cfg(not(feature = "alloc"))]
-const MAX_LFN_LEN: usize = 256;
+const LONG_NAME_BUFFER_LEN: usize = MAX_LONG_DIR_ENTRIES * LFN_PART_LEN;
 
 #[cfg(all(feature = "lfn", not(feature = "alloc")))]
 #[derive(Clone)]
 pub(crate) struct LfnBuffer {
-    ucs2_units: [u16; MAX_LFN_LEN],
+    ucs2_units: [u16; LONG_NAME_BUFFER_LEN],
     len: usize,
 }
 
@@ -777,14 +781,14 @@ impl LfnBuffer {
 impl LfnBuffer {
     fn new() -> Self {
         Self {
-            ucs2_units: [0_u16; MAX_LFN_LEN],
+            ucs2_units: [0_u16; LONG_NAME_BUFFER_LEN],
             len: 0,
         }
     }
 
     fn from_ucs2_units<I: Iterator<Item = u16>>(usc2_units: I) -> Self {
         let mut lfn = Self {
-            ucs2_units: [0_u16; MAX_LFN_LEN],
+            ucs2_units: [0_u16; LONG_NAME_BUFFER_LEN],
             len: 0,
         };
         for (i, usc2_unit) in usc2_units.enumerate() {
@@ -795,7 +799,7 @@ impl LfnBuffer {
     }
 
     fn clear(&mut self) {
-        self.ucs2_units = [0_u16; MAX_LFN_LEN];
+        self.ucs2_units = [0_u16; LONG_NAME_BUFFER_LEN];
         self.len = 0;
     }
 
@@ -875,7 +879,7 @@ impl LongNameBuilder {
     fn process(&mut self, data: &DirLfnEntryData) {
         let is_last = (data.order() & LFN_ENTRY_LAST_FLAG) != 0;
         let index = data.order() & 0x1F;
-        if index == 0 {
+        if index == 0 || usize::from(index) > MAX_LONG_DIR_ENTRIES {
             // Corrupted entry
             warn!("currupted lfn entry! {:x}", data.order());
             self.clear();

src/dir_entry.rs

@@ -1,12 +1,12 @@
 #[cfg(all(not(feature = "std"), feature = "alloc"))]
 use alloc::string::String;
+use bitflags::bitflags;
 use core::char;
 use core::convert::TryInto;
 use core::fmt;
 #[cfg(not(feature = "unicode"))]
 use core::iter;
 use core::str;
-use bitflags::bitflags;
 
 #[cfg(feature = "lfn")]
 use crate::dir::LfnBuffer;