Interface to syscalls to partition location types (#340)

* adds syscalls to use the partition location types

* no range implementation, helper file, refactorings

* more refactoring of syscall interface

* adds and refines trait

* improved documentation and partition_length function

* simplified ModRange

* cleanup

* new aligned_iter implementation

src/embedded_flash/buffer_upgrade.rs

@@ -0,0 +1,77 @@
+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use super::helper::ModRange;
+use super::upgrade_storage::UpgradeStorage;
+use alloc::boxed::Box;
+use persistent_store::{StorageError, StorageResult};
+
+const PARTITION_LENGTH: usize = 0x40000;
+const METADATA_LENGTH: usize = 0x1000;
+
+pub struct BufferUpgradeStorage {
+    /// Content of the partition storage.
+    partition: Box<[u8]>,
+
+    /// Content of the metadata storage.
+    metadata: Box<[u8]>,
+}
+
+impl BufferUpgradeStorage {
+    pub fn new() -> StorageResult<BufferUpgradeStorage> {
+        Ok(BufferUpgradeStorage {
+            partition: vec![0xff; PARTITION_LENGTH].into_boxed_slice(),
+            metadata: vec![0xff; METADATA_LENGTH].into_boxed_slice(),
+        })
+    }
+}
+
+impl UpgradeStorage for BufferUpgradeStorage {
+    fn read_partition(&self, offset: usize, length: usize) -> StorageResult<&[u8]> {
+        let partition_range = ModRange::new(0, self.partition.len());
+        if partition_range.contains_range(&ModRange::new(offset, length)) {
+            Ok(&self.partition[offset..][..length])
+        } else {
+            Err(StorageError::OutOfBounds)
+        }
+    }
+
+    fn write_partition(&mut self, offset: usize, data: &[u8]) -> StorageResult<()> {
+        let partition_range = ModRange::new(0, self.partition.len());
+        if partition_range.contains_range(&ModRange::new(offset, data.len())) {
+            self.partition[offset..][..data.len()].copy_from_slice(data);
+            Ok(())
+        } else {
+            Err(StorageError::OutOfBounds)
+        }
+    }
+
+    fn partition_length(&self) -> usize {
+        PARTITION_LENGTH
+    }
+
+    fn read_metadata(&self) -> StorageResult<&[u8]> {
+        Ok(&self.metadata[..])
+    }
+
+    fn write_metadata(&mut self, data: &[u8]) -> StorageResult<()> {
+        if data.len() <= METADATA_LENGTH {
+            self.metadata.copy_from_slice(&[0xff; METADATA_LENGTH]);
+            self.metadata[..data.len()].copy_from_slice(data);
+            Ok(())
+        } else {
+            Err(StorageError::OutOfBounds)
+        }
+    }
+}

src/embedded_flash/helper.rs

@@ -0,0 +1,237 @@
+// Copyright 2019-2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// For compiling with std outside of tests.
+#![cfg_attr(feature = "std", allow(dead_code))]
+
+use core::iter::Iterator;
+use persistent_store::{StorageError, StorageResult};
+
+/// Reads a slice from a list of slices.
+///
+/// The returned slice contains the interval `[start, start+length)`.
+///
+/// # Errors
+///
+/// Returns [`StorageError::OutOfBounds`] if the range is not within exactly one slice.
+pub fn find_slice<'a>(
+    slices: &'a [&'a [u8]],
+    mut start: usize,
+    length: usize,
+) -> StorageResult<&'a [u8]> {
+    for slice in slices {
+        if start >= slice.len() {
+            start -= slice.len();
+            continue;
+        }
+        if start + length > slice.len() {
+            break;
+        }
+        return Ok(&slice[start..][..length]);
+    }
+    Err(StorageError::OutOfBounds)
+}
+
+/// Checks whether the address is aligned with the block size.
+///
+/// Requires `block_size` to be a power of two.
+pub fn is_aligned(block_size: usize, address: usize) -> bool {
+    debug_assert!(block_size.is_power_of_two());
+    address & (block_size - 1) == 0
+}
+
+/// A range implementation using start and length.
+///
+/// The range is treated as the interval `[start, start + length)`.
+/// All objects with length of 0, regardless of the start value, are considered empty.
+pub struct ModRange {
+    start: usize,
+    length: usize,
+}
+
+impl ModRange {
+    /// Returns a new range of given start and length.
+    pub fn new(start: usize, length: usize) -> ModRange {
+        ModRange { start, length }
+    }
+
+    /// Create a new empty range.
+    pub fn new_empty() -> ModRange {
+        ModRange::new(0, 0)
+    }
+
+    /// Returns the start of the range.
+    pub fn start(&self) -> usize {
+        self.start
+    }
+
+    /// Returns the length of the range.
+    pub fn length(&self) -> usize {
+        self.length
+    }
+
+    /// Returns whether this range contains any addresses.
+    pub fn is_empty(&self) -> bool {
+        self.length == 0
+    }
+
+    /// Returns the disjoint union with the other range, if is consecutive.
+    ///
+    /// Appending empty ranges is not possible.
+    /// Appending to the empty range returns the other range.
+    pub fn append(&self, other: ModRange) -> Option<ModRange> {
+        if self.is_empty() {
+            return Some(other);
+        }
+        if other.is_empty() {
+            return None;
+        }
+        if self.start >= other.start {
+            return None;
+        }
+        if self.length != other.start - self.start {
+            return None;
+        }
+        let new_length = self.length.checked_add(other.length);
+        new_length.map(|l| ModRange::new(self.start, l))
+    }
+
+    /// Returns whether the given range is fully contained.
+    ///
+    /// Mathematically, we calculate whether: `self ∩ range = range`.
+    pub fn contains_range(&self, range: &ModRange) -> bool {
+        range.is_empty()
+            || (self.start <= range.start
+                && range.length <= self.length
+                && range.start - self.start <= self.length - range.length)
+    }
+
+    /// Returns an iterator for all contained numbers that are divisible by the modulus.
+    pub fn aligned_iter(&self, modulus: usize) -> impl Iterator<Item = usize> {
+        (self.start..=usize::MAX)
+            .take(self.length)
+            // Skip the minimum number of elements to align.
+            .skip((modulus - self.start % modulus) % modulus)
+            // Only return aligned elements.
+            .step_by(modulus)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn find_slice_ok() {
+        assert_eq!(
+            find_slice(&[&[1, 2, 3, 4]], 0, 4).ok(),
+            Some(&[1u8, 2, 3, 4] as &[u8])
+        );
+        assert_eq!(
+            find_slice(&[&[1, 2, 3, 4], &[5, 6]], 1, 2).ok(),
+            Some(&[2u8, 3] as &[u8])
+        );
+        assert_eq!(
+            find_slice(&[&[1, 2, 3, 4], &[5, 6]], 4, 2).ok(),
+            Some(&[5u8, 6] as &[u8])
+        );
+        assert_eq!(
+            find_slice(&[&[1, 2, 3, 4], &[5, 6]], 4, 0).ok(),
+            Some(&[] as &[u8])
+        );
+        assert!(find_slice(&[], 0, 1).is_err());
+        assert!(find_slice(&[&[1, 2, 3, 4], &[5, 6]], 6, 0).is_err());
+        assert!(find_slice(&[&[1, 2, 3, 4], &[5, 6]], 3, 2).is_err());
+    }
+
+    #[test]
+    fn alignment() {
+        for exponent in 0..8 {
+            let block_size = 1 << exponent;
+            for i in 0..10 {
+                assert!(is_aligned(block_size, block_size * i));
+            }
+            for i in 1..block_size {
+                assert!(!is_aligned(block_size, block_size + i));
+            }
+        }
+    }
+
+    #[test]
+    fn mod_range_parameters() {
+        let range = ModRange::new(200, 100);
+        assert_eq!(range.start(), 200);
+        assert_eq!(range.length(), 100);
+        assert_eq!(ModRange::new_empty().length(), 0);
+    }
+
+    #[test]
+    fn mod_range_is_empty() {
+        assert!(!ModRange::new(200, 100).is_empty());
+        assert!(ModRange::new(200, 0).is_empty());
+        assert!(ModRange::new_empty().is_empty());
+        assert!(!ModRange::new(usize::MAX, 2).is_empty());
+    }
+
+    #[test]
+    fn mod_range_append() {
+        let range = ModRange::new(200, 100);
+        let new_range = range.append(ModRange::new(300, 400)).unwrap();
+        assert!(new_range.start() == 200);
+        assert!(new_range.length() == 500);
+        assert!(range.append(ModRange::new(299, 400)).is_none());
+        assert!(range.append(ModRange::new(301, 400)).is_none());
+        assert!(range.append(ModRange::new(200, 400)).is_none());
+        let empty_append = ModRange::new_empty()
+            .append(ModRange::new(200, 100))
+            .unwrap();
+        assert!(empty_append.start() == 200);
+        assert!(empty_append.length() == 100);
+    }
+
+    #[test]
+    fn mod_range_contains_range() {
+        let range = ModRange::new(200, 100);
+        assert!(!range.contains_range(&ModRange::new(199, 100)));
+        assert!(!range.contains_range(&ModRange::new(201, 100)));
+        assert!(!range.contains_range(&ModRange::new(199, 99)));
+        assert!(!range.contains_range(&ModRange::new(202, 99)));
+        assert!(!range.contains_range(&ModRange::new(200, 101)));
+        assert!(range.contains_range(&ModRange::new(200, 100)));
+        assert!(range.contains_range(&ModRange::new(200, 99)));
+        assert!(range.contains_range(&ModRange::new(201, 99)));
+        assert!(ModRange::new_empty().contains_range(&ModRange::new_empty()));
+        assert!(ModRange::new(usize::MAX, 1).contains_range(&ModRange::new(usize::MAX, 1)));
+        assert!(ModRange::new(usize::MAX, 2).contains_range(&ModRange::new(usize::MAX, 2)));
+    }
+
+    #[test]
+    fn mod_range_aligned_iter() {
+        let mut iter = ModRange::new(200, 100).aligned_iter(100);
+        assert_eq!(iter.next(), Some(200));
+        assert_eq!(iter.next(), None);
+        let mut iter = ModRange::new(200, 101).aligned_iter(100);
+        assert_eq!(iter.next(), Some(200));
+        assert_eq!(iter.next(), Some(300));
+        assert_eq!(iter.next(), None);
+        let mut iter = ModRange::new(199, 100).aligned_iter(100);
+        assert_eq!(iter.next(), Some(200));
+        assert_eq!(iter.next(), None);
+        let mut iter = ModRange::new(201, 99).aligned_iter(100);
+        assert_eq!(iter.next(), None);
+        let mut iter = ModRange::new(usize::MAX - 16, 20).aligned_iter(16);
+        assert_eq!(iter.next(), Some(0xf_fff_fff_fff_fff_ff0));
+        assert_eq!(iter.next(), None);
+    }
+}

src/embedded_flash/mod.rs

@@ -1,4 +1,4 @@
-// Copyright 2019-2020 Google LLC
+// Copyright 2019-2021 Google LLC
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -12,27 +12,36 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#[cfg(feature = "std")]
+mod buffer_upgrade;
+mod helper;
 #[cfg(not(feature = "std"))]
 mod syscall;
+mod upgrade_storage;
 
-#[cfg(not(feature = "std"))]
+pub use upgrade_storage::UpgradeStorage;
-pub use self::syscall::SyscallStorage;
 
-/// Storage definition for production.
+/// Definitions for production.
 #[cfg(not(feature = "std"))]
 mod prod {
-    pub type Storage = super::SyscallStorage;
+    use super::syscall::{SyscallStorage, SyscallUpgradeStorage};
+
+    pub type Storage = SyscallStorage;
 
     pub fn new_storage(num_pages: usize) -> Storage {
         Storage::new(num_pages).unwrap()
     }
+
+    pub type UpgradeLocations = SyscallUpgradeStorage;
 }
 #[cfg(not(feature = "std"))]
-pub use self::prod::{new_storage, Storage};
+pub use self::prod::{new_storage, Storage, UpgradeLocations};
 
-/// Storage definition for testing.
+/// Definitions for testing.
 #[cfg(feature = "std")]
 mod test {
+    use super::buffer_upgrade::BufferUpgradeStorage;
+
     pub type Storage = persistent_store::BufferStorage;
 
     pub fn new_storage(num_pages: usize) -> Storage {
@@ -47,6 +56,8 @@ mod test {
         };
         Storage::new(store, options)
     }
+
+    pub type UpgradeLocations = BufferUpgradeStorage;
 }
 #[cfg(feature = "std")]
-pub use self::test::{new_storage, Storage};
+pub use self::test::{new_storage, Storage, UpgradeLocations};

src/embedded_flash/syscall.rs

@@ -1,4 +1,4 @@
-// Copyright 2019-2020 Google LLC
+// Copyright 2019-2021 Google LLC
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use super::helper::{find_slice, is_aligned, ModRange};
+use super::upgrade_storage::UpgradeStorage;
 use alloc::vec::Vec;
 use libtock_core::syscalls;
 use persistent_store::{Storage, StorageError, StorageIndex, StorageResult};
@@ -43,6 +45,8 @@ mod memop_nr {
 
 mod storage_type {
     pub const STORE: usize = 1;
+    pub const PARTITION: usize = 2;
+    pub const METADATA: usize = 3;
 }
 
 fn get_info(nr: usize, arg: usize) -> StorageResult<usize> {
@@ -59,6 +63,37 @@ fn memop(nr: u32, arg: usize) -> StorageResult<usize> {
     }
 }
 
+fn write_slice(ptr: usize, value: &[u8]) -> StorageResult<()> {
+    let code = unsafe {
+        syscalls::raw::allow(
+            DRIVER_NUMBER,
+            allow_nr::WRITE_SLICE,
+            // We rely on the driver not writing to the slice. This should use read-only allow
+            // when available. See https://github.com/tock/tock/issues/1274.
+            value.as_ptr() as *mut u8,
+            value.len(),
+        )
+    };
+    if code < 0 {
+        return Err(StorageError::CustomError);
+    }
+
+    let code = syscalls::command(DRIVER_NUMBER, command_nr::WRITE_SLICE, ptr, value.len());
+    if code.is_err() {
+        return Err(StorageError::CustomError);
+    }
+
+    Ok(())
+}
+
+fn erase_page(ptr: usize, page_length: usize) -> StorageResult<()> {
+    let code = syscalls::command(DRIVER_NUMBER, command_nr::ERASE_PAGE, ptr, page_length);
+    if code.is_err() {
+        return Err(StorageError::CustomError);
+    }
+    Ok(())
+}
+
 pub struct SyscallStorage {
     word_size: usize,
     page_size: usize,
@@ -118,11 +153,11 @@ impl SyscallStorage {
     }
 
     fn is_word_aligned(&self, x: usize) -> bool {
-        x & (self.word_size - 1) == 0
+        is_aligned(self.word_size, x)
     }
 
     fn is_page_aligned(&self, x: usize) -> bool {
-        x & (self.page_size - 1) == 0
+        is_aligned(self.page_size, x)
     }
 }
 
@@ -157,83 +192,136 @@ impl Storage for SyscallStorage {
             return Err(StorageError::NotAligned);
         }
         let ptr = self.read_slice(index, value.len())?.as_ptr() as usize;
-
+        write_slice(ptr, value)
-        let code = unsafe {
-            syscalls::raw::allow(
-                DRIVER_NUMBER,
-                allow_nr::WRITE_SLICE,
-                // We rely on the driver not writing to the slice. This should use read-only allow
-                // when available. See https://github.com/tock/tock/issues/1274.
-                value.as_ptr() as *mut u8,
-                value.len(),
-            )
-        };
-        if code < 0 {
-            return Err(StorageError::CustomError);
-        }
-
-        let code = syscalls::command(DRIVER_NUMBER, command_nr::WRITE_SLICE, ptr, value.len());
-        if code.is_err() {
-            return Err(StorageError::CustomError);
-        }
-
-        Ok(())
     }
 
     fn erase_page(&mut self, page: usize) -> StorageResult<()> {
         let index = StorageIndex { page, byte: 0 };
         let length = self.page_size();
         let ptr = self.read_slice(index, length)?.as_ptr() as usize;
-        let code = syscalls::command(DRIVER_NUMBER, command_nr::ERASE_PAGE, ptr, length);
+        erase_page(ptr, length)
-        if code.is_err() {
+    }
+}
+
+pub struct SyscallUpgradeStorage {
+    page_size: usize,
+    partition: ModRange,
+    metadata: ModRange,
+}
+
+impl SyscallUpgradeStorage {
+    /// Provides access to the other upgrade partition and metadata if available.
+    ///
+    /// The implementation assumes that storage locations returned by the kernel through
+    /// `memop_nr::STORAGE_*` calls are in address space order.
+    ///
+    /// # Errors
+    ///
+    /// Returns `CustomError` if any of the following conditions do not hold:
+    /// - The page size is a power of two.
+    /// - The storage slices are page-aligned.
+    /// - There are not partition or metadata slices.
+    /// Returns a `NotAligned` error if partitions or metadata ranges are
+    /// - not exclusive or,
+    /// - not consecutive.
+    pub fn new() -> StorageResult<SyscallUpgradeStorage> {
+        let mut locations = SyscallUpgradeStorage {
+            page_size: get_info(command_nr::get_info_nr::PAGE_SIZE, 0)?,
+            partition: ModRange::new_empty(),
+            metadata: ModRange::new_empty(),
+        };
+        if !locations.page_size.is_power_of_two() {
             return Err(StorageError::CustomError);
         }
-        Ok(())
+        for i in 0..memop(memop_nr::STORAGE_CNT, 0)? {
+            let storage_type = memop(memop_nr::STORAGE_TYPE, i)?;
+            match storage_type {
+                storage_type::PARTITION | storage_type::METADATA => (),
+                _ => continue,
+            };
+            let storage_ptr = memop(memop_nr::STORAGE_PTR, i)?;
+            let storage_len = memop(memop_nr::STORAGE_LEN, i)?;
+            if !locations.is_page_aligned(storage_ptr) || !locations.is_page_aligned(storage_len) {
+                return Err(StorageError::CustomError);
+            }
+            let range = ModRange::new(storage_ptr, storage_len);
+            match storage_type {
+                storage_type::PARTITION => {
+                    locations.partition = locations
+                        .partition
+                        .append(range)
+                        .ok_or(StorageError::NotAligned)?
+                }
+                storage_type::METADATA => {
+                    locations.metadata = locations
+                        .metadata
+                        .append(range)
+                        .ok_or(StorageError::NotAligned)?
+                }
+                _ => (),
+            };
+        }
+        if locations.partition.is_empty() || locations.metadata.is_empty() {
+            Err(StorageError::CustomError)
+        } else {
+            Ok(locations)
+        }
+    }
+
+    fn is_page_aligned(&self, x: usize) -> bool {
+        is_aligned(self.page_size, x)
     }
 }
 
-fn find_slice<'a>(
+impl UpgradeStorage for SyscallUpgradeStorage {
-    slices: &'a [&'a [u8]],
+    fn read_partition(&self, offset: usize, length: usize) -> StorageResult<&[u8]> {
-    mut start: usize,
+        let address = self.partition.start() + offset;
-    length: usize,
+        if self
-) -> StorageResult<&'a [u8]> {
+            .partition
-    for slice in slices {
+            .contains_range(&ModRange::new(address, length))
-        if start >= slice.len() {
+        {
-            start -= slice.len();
+            Ok(unsafe { core::slice::from_raw_parts(address as *const u8, length) })
-            continue;
+        } else {
-        }
-        if start + length > slice.len() {
-            break;
-        }
-        return Ok(&slice[start..][..length]);
-    }
             Err(StorageError::OutOfBounds)
-}
+        }
+    }
 
-#[cfg(test)]
+    fn write_partition(&mut self, offset: usize, data: &[u8]) -> StorageResult<()> {
-mod tests {
+        let address = self.partition.start() + offset;
-    use super::*;
+        if self
+            .partition
+            .contains_range(&ModRange::new(address, data.len()))
+        {
+            // Erases all pages that have their first byte in the write range.
+            // Since we expect calls in order, we don't want to erase half-written pages.
+            for address in ModRange::new(address, data.len()).aligned_iter(self.page_size) {
+                erase_page(address, self.page_size)?;
+            }
+            write_slice(address, data)
+        } else {
+            Err(StorageError::OutOfBounds)
+        }
+    }
 
-    #[test]
+    fn partition_length(&self) -> usize {
-    fn find_slice_ok() {
+        self.partition.length()
-        assert_eq!(
+    }
-            find_slice(&[&[1, 2, 3, 4]], 0, 4).ok(),
+
-            Some(&[1u8, 2, 3, 4] as &[u8])
+    fn read_metadata(&self) -> StorageResult<&[u8]> {
-        );
+        Ok(unsafe {
-        assert_eq!(
+            core::slice::from_raw_parts(self.metadata.start() as *const u8, self.metadata.length())
-            find_slice(&[&[1, 2, 3, 4], &[5, 6]], 1, 2).ok(),
+        })
-            Some(&[2u8, 3] as &[u8])
+    }
-        );
+
-        assert_eq!(
+    fn write_metadata(&mut self, data: &[u8]) -> StorageResult<()> {
-            find_slice(&[&[1, 2, 3, 4], &[5, 6]], 4, 2).ok(),
+        // If less data is passed in than is reserved, assume the rest is 0xFF.
-            Some(&[5u8, 6] as &[u8])
+        if data.len() <= self.metadata.length() {
-        );
+            for address in self.metadata.aligned_iter(self.page_size) {
-        assert_eq!(
+                erase_page(address, self.page_size)?;
-            find_slice(&[&[1, 2, 3, 4], &[5, 6]], 4, 0).ok(),
+            }
-            Some(&[] as &[u8])
+            write_slice(self.metadata.start(), data)
-        );
+        } else {
-        assert!(find_slice(&[], 0, 1).is_err());
+            Err(StorageError::OutOfBounds)
-        assert!(find_slice(&[&[1, 2, 3, 4], &[5, 6]], 6, 0).is_err());
+        }
-        assert!(find_slice(&[&[1, 2, 3, 4], &[5, 6]], 3, 2).is_err());
     }
 }

src/embedded_flash/upgrade_storage.rs

@@ -0,0 +1,51 @@
+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use persistent_store::StorageResult;
+
+/// Accessors to storage locations used for upgrading from a CTAP command.
+pub trait UpgradeStorage {
+    /// Reads a slice of the partition, if within bounds.
+    ///
+    /// The offset is relative to the start of the partition.
+    ///
+    /// # Errors
+    ///
+    /// Returns [`StorageError::OutOfBounds`] if the requested slice is not inside the partition.
+    fn read_partition(&self, offset: usize, length: usize) -> StorageResult<&[u8]>;
+
+    /// Writes the given data to the given offset address, if within bounds of the partition.
+    ///
+    /// The offset is relative to the start of the partition.
+    ///
+    /// # Errors
+    ///
+    /// Returns [`StorageError::OutOfBounds`] if the data does not fit the partition.
+    fn write_partition(&mut self, offset: usize, data: &[u8]) -> StorageResult<()>;
+
+    /// Returns the length of the partition.
+    fn partition_length(&self) -> usize;
+
+    /// Reads the metadata location.
+    fn read_metadata(&self) -> StorageResult<&[u8]>;
+
+    /// Writes the given data into the metadata location.
+    ///
+    /// The passed in data is appended with 0xFF bytes if shorter than the metadata storage.
+    ///
+    /// # Errors
+    ///
+    /// Returns [`StorageError::OutOfBounds`] if the data is too long to fit the metadata storage.
+    fn write_metadata(&mut self, data: &[u8]) -> StorageResult<()>;
+}