From 79257fca6eb482e81325d52ada1cad099c6bb982 Mon Sep 17 00:00:00 2001
From: Nicholas Bishop <nbishop@nbishop.net>
Date: Thu, 13 Jun 2024 11:15:51 -0400
Subject: [PATCH] Add extent module

---
 src/extent.rs | 90 +++++++++++++++++++++++++++++++++++++++++++++++++++
 src/lib.rs    |  1 +
 2 files changed, 91 insertions(+)
 create mode 100644 src/extent.rs

diff --git a/src/extent.rs b/src/extent.rs
new file mode 100644
index 00000000..b3161aba
--- /dev/null
+++ b/src/extent.rs
@@ -0,0 +1,90 @@
+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use crate::error::{Corrupt, Ext4Error};
+use crate::inode::InodeIndex;
+use crate::util::read_u16le;
+
+/// Size of each entry within an extent node (including the header
+/// entry).
+const ENTRY_SIZE_IN_BYTES: usize = 12;
+
+/// Contiguous range of blocks that contain file data.
+#[derive(Debug, Eq, PartialEq)]
+pub(crate) struct Extent {
+    // Offset of the block within the file.
+    pub(crate) block_within_file: u32,
+
+    // This is the actual block within the filesystem.
+    pub(crate) start_block: u64,
+
+    // Number of blocks (both within the file, and on the filesystem).
+    pub(crate) num_blocks: u16,
+}
+
+/// Header at the start of a node in an extent tree.
+///
+/// An extent tree is made up of nodes. Each node may be internal or
+/// leaf. Leaf nodes contain `Extent`s. Internal nodes point at other
+/// nodes.
+///
+/// Each node starts with a `NodeHeader` that includes the node's depth
+/// (depth 0 is a leaf node) and the number of entries in the node.
+struct NodeHeader {
+    /// Number of entries in this node, not including the header.
+    num_entries: u16,
+
+    /// Maximum number of entries in this node, not including the header.
+    max_entries: u16,
+
+    /// Depth of this node in the tree. Zero means it's a leaf node. The
+    /// maximum depth is five.
+    depth: u16,
+}
+
+impl NodeHeader {
+    /// Size of the node, including the header.
+    fn node_size_in_bytes(&self) -> usize {
+        (usize::from(self.num_entries) + 1) * ENTRY_SIZE_IN_BYTES
+    }
+
+    /// Offset of the node's extent data.
+    fn checksum_offset(&self) -> usize {
+        (usize::from(self.max_entries) + 1) * ENTRY_SIZE_IN_BYTES
+    }
+}
+
+impl NodeHeader {
+    /// Read a `NodeHeader` from a byte slice.
+    fn from_bytes(data: &[u8], inode: InodeIndex) -> Result<Self, Ext4Error> {
+        if data.len() < ENTRY_SIZE_IN_BYTES {
+            return Err(Ext4Error::Corrupt(Corrupt::ExtentNotEnoughData(
+                inode.get(),
+            )));
+        }
+
+        let eh_magic = read_u16le(data, 0);
+        let eh_entries = read_u16le(data, 2);
+        let eh_max = read_u16le(data, 4);
+        let eh_depth = read_u16le(data, 6);
+
+        if eh_magic != 0xf30a {
+            return Err(Ext4Error::Corrupt(Corrupt::ExtentMagic(inode.get())));
+        }
+
+        if eh_depth > 5 {
+            return Err(Ext4Error::Corrupt(Corrupt::ExtentDepth(inode.get())));
+        }
+
+        Ok(Self {
+            depth: eh_depth,
+            num_entries: eh_entries,
+            max_entries: eh_max,
+        })
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index a3c5aa34..53e7f974 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -15,6 +15,7 @@ extern crate alloc;
 mod block_group;
 mod checksum;
 mod error;
+mod extent;
 mod features;
 mod file_type;
 mod inode;