superblock: Improve documentation and exports

Signed-off-by: Ikey Doherty <[email protected]>

crates/superblock/src/btrfs.rs

@@ -3,62 +3,110 @@
 // SPDX-License-Identifier: MPL-2.0
 
 //! BTRFS superblock handling
+//!
+//! This module provides functionality for reading and parsing BTRFS filesystem superblocks,
+//! which contain critical metadata about the filesystem including UUIDs and labels.
 
 use crate::{Error, Kind, Superblock};
 use log;
 use std::io::{self, Read};
 use uuid::Uuid;
 use zerocopy::*;
 
-/// BTRFS superblock definition (as seen in the kernel)
+/// BTRFS superblock definition that matches the on-disk format used by the Linux kernel.
+///
+/// The superblock contains critical filesystem metadata including:
+/// - Filesystem UUID and label
+/// - Size and usage information
+/// - Root tree locations
+/// - Compatibility flags
 #[derive(FromBytes, Debug)]
 #[repr(C)]
 pub struct Btrfs {
-    csum: [u8; 32],
-    fsid: [u8; 16],
-    bytenr: U64<LittleEndian>,
-    flags: U64<LittleEndian>,
-    magic: U64<LittleEndian>,
-    generation: U64<LittleEndian>,
-    root: U64<LittleEndian>,
-    chunk_root: U64<LittleEndian>,
-    log_root: U64<LittleEndian>,
-    log_root_transid: U64<LittleEndian>,
-    total_bytes: U64<LittleEndian>,
-    bytes_used: U64<LittleEndian>,
-    root_dir_objectid: U64<LittleEndian>,
-    num_devices: U64<LittleEndian>,
-    sectorsize: U32<LittleEndian>,
-    nodesize: U32<LittleEndian>,
-    leafsize: U32<LittleEndian>,
-    stripesize: U32<LittleEndian>,
-    sys_chunk_array_size: U32<LittleEndian>,
-    chunk_root_generation: U64<LittleEndian>,
-    compat_flags: U64<LittleEndian>,
-    compat_ro_flags: U64<LittleEndian>,
-    incompat_flags: U64<LittleEndian>,
-    csum_type: U16<LittleEndian>,
-    root_level: u8,
-    chunk_root_level: u8,
-    log_root_level: u8,
-    dev_item: [u8; 98],
-    label: [u8; 256],
-    cache_generation: U64<LittleEndian>,
-    uuid_tree_generation: U64<LittleEndian>,
-    metadata_uuid: [u8; 16],
-    nr_global_roots: U64<LittleEndian>,
-    reserved: [u8; 32],
-    sys_chunk_array: [u8; 2048],
-    root_backup: [u8; 256],
+    /// Checksum of the superblock data
+    pub csum: [u8; 32],
+    /// Filesystem UUID
+    pub fsid: [u8; 16],
+    /// Physical byte number where this copy of the superblock is located
+    pub bytenr: U64<LittleEndian>,
+    /// Superblock flags
+    pub flags: U64<LittleEndian>,
+    /// Magic number identifying this as a BTRFS superblock
+    pub magic: U64<LittleEndian>,
+    /// Transaction ID of the filesystem
+    pub generation: U64<LittleEndian>,
+    /// Logical address of the root tree root
+    pub root: U64<LittleEndian>,
+    /// Logical address of the chunk tree root
+    pub chunk_root: U64<LittleEndian>,
+    /// Logical address of the log tree root
+    pub log_root: U64<LittleEndian>,
+    /// Transaction ID of the log tree
+    pub log_root_transid: U64<LittleEndian>,
+    /// Total size of the filesystem in bytes
+    pub total_bytes: U64<LittleEndian>,
+    /// Number of bytes used
+    pub bytes_used: U64<LittleEndian>,
+    /// Object ID of the root directory
+    pub root_dir_objectid: U64<LittleEndian>,
+    /// Number of devices making up the filesystem
+    pub num_devices: U64<LittleEndian>,
+    /// Size of a sector in bytes
+    pub sectorsize: U32<LittleEndian>,
+    /// Size of nodes in the filesystem trees
+    pub nodesize: U32<LittleEndian>,
+    /// Size of leaf nodes in the filesystem trees
+    pub leafsize: U32<LittleEndian>,
+    /// Stripe size for the filesystem
+    pub stripesize: U32<LittleEndian>,
+    /// Size of the system chunk array
+    pub sys_chunk_array_size: U32<LittleEndian>,
+    /// Generation of the chunk tree
+    pub chunk_root_generation: U64<LittleEndian>,
+    /// Compatible feature flags
+    pub compat_flags: U64<LittleEndian>,
+    /// Compatible read-only feature flags
+    pub compat_ro_flags: U64<LittleEndian>,
+    /// Incompatible feature flags
+    pub incompat_flags: U64<LittleEndian>,
+    /// Checksum algorithm type
+    pub csum_type: U16<LittleEndian>,
+    /// Level of the root tree
+    pub root_level: u8,
+    /// Level of the chunk tree
+    pub chunk_root_level: u8,
+    /// Level of the log tree
+    pub log_root_level: u8,
+    /// Device information
+    pub dev_item: [u8; 98],
+    /// Volume label
+    pub label: [u8; 256],
+    /// Cache generation number
+    pub cache_generation: U64<LittleEndian>,
+    /// UUID tree generation
+    pub uuid_tree_generation: U64<LittleEndian>,
+    /// Metadata UUID for the filesystem
+    pub metadata_uuid: [u8; 16],
+    /// Number of global root entries
+    pub nr_global_roots: U64<LittleEndian>,
+    /// Reserved for future use
+    pub reserved: [u8; 32],
+    /// System chunk array data
+    pub sys_chunk_array: [u8; 2048],
+    /// Backup copy of root tree info
+    pub root_backup: [u8; 256],
 }
 
-// Superblock starts at 65536 for btrfs.
-const START_POSITION: u64 = 0x10000;
+/// Offset where the BTRFS superblock starts (65536 bytes)
+pub const START_POSITION: u64 = 0x10000;
 
-// "_BHRfS_M"
-const MAGIC: U64<LittleEndian> = U64::new(0x4D5F53665248425F);
+/// Magic number identifying a BTRFS superblock ("_BHRfS_M")
+pub const MAGIC: U64<LittleEndian> = U64::new(0x4D5F53665248425F);
 
-/// Attempt to decode the Superblock from the given read stream
+/// Attempt to decode the BTRFS superblock from the given read stream.
+///
+/// This will read past the initial superblock offset and validate the magic number
+/// before returning the parsed superblock structure.
 pub fn from_reader<R: Read>(reader: &mut R) -> Result<Btrfs, Error> {
     // Drop unwanted bytes (Seek not possible with zstd streamed inputs)
     io::copy(&mut reader.by_ref().take(START_POSITION), &mut io::sink())?;
@@ -78,16 +126,17 @@ pub fn from_reader<R: Read>(reader: &mut R) -> Result<Btrfs, Error> {
 }
 
 impl Superblock for Btrfs {
-    /// Return the encoded UUID for this superblock
+    /// Return the encoded UUID for this superblock as a string
     fn uuid(&self) -> Result<String, Error> {
         Ok(Uuid::from_bytes(self.fsid).hyphenated().to_string())
     }
 
+    /// Return the filesystem type
     fn kind(&self) -> Kind {
         super::Kind::Btrfs
     }
 
-    /// We don't yet support labels here.
+    /// Return the volume label as a string
     fn label(&self) -> Result<String, Error> {
         Ok(std::str::from_utf8(&self.label)?.trim_end_matches('\0').to_owned())
     }