ext2: Remove magic number relevant to inode's 'block' field

BUG=b:329359333
TEST=cargo test

Change-Id: I70012ed8303d01ee4cde328dcbbf40869d6587b9
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/5635974
Commit-Queue: Keiichi Watanabe <keiichiw@chromium.org>
Reviewed-by: Junichi Uekawa <uekawa@chromium.org>

ext2/src/fs.rs

@@ -31,6 +31,7 @@ use crate::blockgroup::GroupMetaData;
 use crate::blockgroup::BLOCK_SIZE;
 use crate::inode::Inode;
 use crate::inode::InodeBlock;
+use crate::inode::InodeBlocksCount;
 use crate::inode::InodeNum;
 use crate::inode::InodeType;
 use crate::superblock::Config;
@@ -277,7 +278,7 @@ impl<'a> Ext2<'a> {
             let block_id = self.allocate_block()?;
             let inode = self.get_inode_mut(parent)?;
             inode.block.set_direct_blocks(&[block_id])?;
-            inode.blocks = block_size as u32 / 512;
+            inode.blocks = InodeBlocksCount::from_bytes_len(block_size as u32);
             self.dentries.insert(
                 parent,
                 DirEntryBlock {
@@ -382,7 +383,7 @@ impl<'a> Ext2<'a> {
             &std::fs::metadata(path)?,
             block_size as u32,
             0,
-            0,
+            InodeBlocksCount::from_bytes_len(0),
             InodeBlock::default(),
         )?;
 
@@ -525,10 +526,7 @@ impl<'a> Ext2<'a> {
             unimplemented!("Triple-indirect block is not supported");
         }
 
-        // The spec says that the `blocks` field is a "32-bit value representing the total number
-        // of 512-bytes blocks". This `512` is a fixed number regardless of the actual block size,
-        // which is usuaully 4KB.
-        let blocks = used_blocks as u32 * (block_size as u32 / 512);
+        let blocks = InodeBlocksCount::from_bytes_len((used_blocks * block_size) as u32);
         let size = file_size as u32;
         let inode = Inode::from_metadata(
             arena,
@@ -575,7 +573,7 @@ impl<'a> Ext2<'a> {
             &std::fs::symlink_metadata(&link)?,
             dst.len() as u32,
             1, //links_count,
-            0, //blocks,
+            InodeBlocksCount::from_bytes_len(0),
             block,
         )?;
         self.add_inode(inode_num, inode)?;

ext2/src/inode.rs

@@ -173,7 +173,7 @@ pub(crate) struct Inode {
     _dtime: u32,
     _gid: u16,
     pub links_count: u16,
-    pub blocks: u32,
+    pub blocks: InodeBlocksCount,
     _flags: u32,
     _osd1: u32,
     pub block: InodeBlock,
@@ -189,6 +189,20 @@ pub(crate) struct Inode {
     _reserved2: u32,
 }
 
+/// Used in `Inode` to represent how many 512-byte blocks are used by a file.
+///
+/// The block size '512' byte is fixed and not related to the actual block size of the file system.
+/// For more details, see notes for `i_blocks_lo` in the specification.
+#[repr(C)]
+#[derive(Default, Debug, Copy, Clone, FromZeroes, FromBytes, AsBytes)]
+pub struct InodeBlocksCount(u32);
+
+impl InodeBlocksCount {
+    pub fn from_bytes_len(len: u32) -> Self {
+        Self(len / 512)
+    }
+}
+
 impl Inode {
     /// Size of the inode record in bytes.
     /// Its return value must be stored in `Superblock` and used to calculate the size of
@@ -266,7 +280,7 @@ impl Inode {
         m: &std::fs::Metadata,
         size: u32,
         links_count: u16,
-        blocks: u32,
+        blocks: InodeBlocksCount,
         block: InodeBlock,
     ) -> Result<&'a mut Self> {
         // (inode_num - 1) because inode is 1-indexed.