Fix lookahead buffer size reported to littlefs2-sys

Previously, we reported the lookahead buffer size in bytes but
littlefs2-sys expects the lookahead buffer size as a multiple of 8
bytes.  This could lead to a buffer overflow causing filesystem
corruption.  This patch fixes the reported lookahead buffer size.

Note that Storage::LOOKAHEAD_WORDS_SIZE allows users to set invalid
values (as it is measured in 4 bytes, not in 8 bytes).  Invalid values
that were previously accepted because of the wrong buffer size
calculation can now be rejected by littlefs2-sys.

This is a combination of two previous patches:
	trussed-dev#19
	#1

Fixes: trussed-dev#16

CHANGELOG.md

@@ -14,6 +14,15 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Changed
 - Made `Path::from_bytes_with_nul_unchecked` `const`.
 
+### Fixed
+- Fixed the lookahead size reported to `littlefs2-sys`.  Previously, the
+  reported size was too large by the factor of 8, potentially leading to a
+  buffer overflow causing filesystem corruption.  Fixing this means that
+  `Storage::LOOKAHEADWORD_SIZE` values that are not a multiple of 2 can now
+  lead to an error.  Fixes [#16].
+
+[#16]: https://github.com/trussed-dev/littlefs2/issues/16
+
 ## [v0.2.2] - 2021-03-20
 
 ### Changed

src/driver.rs

@@ -52,10 +52,9 @@ pub trait Storage {
     /// Must be a factor of `BLOCK_SIZE`.
     type CACHE_SIZE: ArrayLength<u8>;
 
-    /// littlefs itself has a `LOOKAHEAD_SIZE`, which must be a multiple of 8,
-    /// as it stores data in a bitmap. It also asks for 4-byte aligned buffers.
-    /// Hence, we further restrict `LOOKAHEAD_SIZE` to be a multiple of 32.
-    /// Our LOOKAHEADWORDS_SIZE is this multiple.
+    /// littlefs itself has a `LOOKAHEAD_SIZE`, which must be a multiple of 8 bytes.  For
+    /// historical reasons, `LOOKAHEADWORDS_SIZE` is measured in 4 bytes.  This means that users
+    /// must always provide a multiple of 2 here.
     type LOOKAHEADWORDS_SIZE: ArrayLength<u32>;
     // type LOOKAHEAD_SIZE: ArrayLength<u8>;
 

src/fs.rs

@@ -20,6 +20,7 @@ struct Cache<Storage: driver::Storage> {
     read: Bytes<Storage::CACHE_SIZE>,
     write: Bytes<Storage::CACHE_SIZE>,
     // lookahead: aligned::Aligned<aligned::A4, Bytes<Storage::LOOKAHEAD_SIZE>>,
+    // lookahead buffer must be aligned to 32 bytes
     lookahead: generic_array::GenericArray<u32, Storage::LOOKAHEADWORDS_SIZE>,
 }
 
@@ -28,7 +29,6 @@ impl<S: driver::Storage> Cache<S> {
         Self {
             read: Default::default(),
             write: Default::default(),
-            // lookahead: aligned::Aligned(Default::default()),
             lookahead: Default::default(),
         }
     }
@@ -60,8 +60,7 @@ impl<Storage: driver::Storage> Allocation<Storage> {
         let write_size: u32 = Storage::WRITE_SIZE as _;
         let block_size: u32 = Storage::BLOCK_SIZE as _;
         let cache_size: u32 = <Storage as driver::Storage>::CACHE_SIZE::U32;
-        let lookahead_size: u32 =
-            32 * <Storage as driver::Storage>::LOOKAHEADWORDS_SIZE::U32;
+        let lookahead_size: u32 = 4 * <Storage as driver::Storage>::LOOKAHEADWORDS_SIZE::U32;
         let block_cycles: i32 = Storage::BLOCK_CYCLES as _;
         let block_count: u32 = Storage::BLOCK_COUNT as _;
 
@@ -89,6 +88,10 @@ impl<Storage: driver::Storage> Allocation<Storage> {
         debug_assert!(cache_size <= block_size);
         debug_assert!(block_size % cache_size == 0);
 
+        // lookahead words size (measured in 4 bytes) must be a multiple of 2 so that the actual
+        // lookahead size is a multiple of 8 bytes
+        debug_assert!(lookahead_size % 2 == 0);
+
         let cache = Cache::new();
 
         let filename_max_plus_one: u32 = crate::consts::FILENAME_MAX_PLUS_ONE;

src/macros.rs

@@ -93,7 +93,7 @@ macro_rules! ram_storage { (
             cache_size_ty=$crate::consts::U32,
             block_size=128,
             block_count=$bytes/128,
-            lookaheadwords_size_ty=$crate::consts::U1,
+            lookaheadwords_size_ty=$crate::consts::U2,
             filename_max_plus_one_ty=$crate::consts::U256,
             path_max_plus_one_ty=$crate::consts::U256,
             result=LfsResult,
@@ -110,7 +110,7 @@ macro_rules! ram_storage { (
             cache_size_ty=$crate::consts::U32,
             block_size=128,
             block_count=8,
-            lookaheadwords_size_ty=$crate::consts::U1,
+            lookaheadwords_size_ty=$crate::consts::U2,
             filename_max_plus_one_ty=$crate::consts::U256,
             path_max_plus_one_ty=$crate::consts::U256,
             result=Result,
@@ -221,7 +221,7 @@ macro_rules! const_ram_storage { (
             cache_size_ty=$crate::consts::U512,
             block_size=512,
             block_count=$bytes/512,
-            lookaheadwords_size_ty=$crate::consts::U1,
+            lookaheadwords_size_ty=$crate::consts::U2,
             filename_max_plus_one_ty=$crate::consts::U256,
             path_max_plus_one_ty=$crate::consts::U256,
             result=LfsResult,

src/tests.rs

@@ -26,7 +26,7 @@ ram_storage!(
     cache_size_ty=consts::U32,
     block_size=256,
     block_count=512,
-    lookaheadwords_size_ty=consts::U1,
+    lookaheadwords_size_ty=consts::U2,
     filename_max_plus_one_ty=consts::U256,
     path_max_plus_one_ty=consts::U256,
     result=Result,

tests/ui/constructors-fail.rs

@@ -15,7 +15,7 @@ ram_storage!(
     cache_size_ty=consts::U32,
     block_size=256,
     block_count=512,
-    lookaheadwords_size_ty=consts::U1,
+    lookaheadwords_size_ty=consts::U2,
     filename_max_plus_one_ty=consts::U256,
     path_max_plus_one_ty=consts::U256,
     result=Result,
@@ -31,7 +31,7 @@ ram_storage!(
     cache_size_ty=consts::U700,
     block_size=20*35,
     block_count=32,
-    lookaheadwords_size_ty=consts::U1,
+    lookaheadwords_size_ty=consts::U2,
     filename_max_plus_one_ty=consts::U256,
     path_max_plus_one_ty=consts::U256,
     result=Result,

tests/ui/sync-fail.rs

@@ -15,7 +15,7 @@ ram_storage!(
     cache_size_ty=consts::U32,
     block_size=256,
     block_count=512,
-    lookaheadwords_size_ty=consts::U1,
+    lookaheadwords_size_ty=consts::U2,
     filename_max_plus_one_ty=consts::U256,
     path_max_plus_one_ty=consts::U256,
     result=Result,
@@ -31,7 +31,7 @@ ram_storage!(
     cache_size_ty=consts::U700,
     block_size=20*35,
     block_count=32,
-    lookaheadwords_size_ty=consts::U1,
+    lookaheadwords_size_ty=consts::U2,
     filename_max_plus_one_ty=consts::U256,
     path_max_plus_one_ty=consts::U256,
     result=Result,