ext: Filesystem init implementation.

PiperOrigin-RevId: 258645957

3 files changed, 19 insertions, 3 deletions

diff --git a/pkg/sentry/fs/ext/disklayout/block_group.go b/pkg/sentry/fs/ext/disklayout/block_group.go
index 32ea3d97d..ad6f4fef8 100644
--- a/pkg/sentry/fs/ext/disklayout/block_group.go
+++ b/pkg/sentry/fs/ext/disklayout/block_group.go
@@ -18,6 +18,16 @@ package disklayout
 // is split into a series of block groups. This provides an access layer to
 // information needed to access and use a block group.
 //
+// Location:
+//   - The block group descriptor table is always placed in the blocks
+//     immediately after the block containing the superblock.
+//   - The 1st block group descriptor in the original table is in the
+//      (sb.FirstDataBlock() + 1)th block.
+//   - See SuperBlock docs to see where the block group descriptor table is
+//     replicated.
+//   - sb.BgDescSize() must be used as the block group descriptor entry size
+//     while reading the table from disk.
+//
 // See https://www.kernel.org/doc/html/latest/filesystems/ext4/globals.html#block-group-descriptors.
 type BlockGroup interface {
 	// InodeTable returns the absolute block number of the block containing the
diff --git a/pkg/sentry/fs/ext/disklayout/superblock.go b/pkg/sentry/fs/ext/disklayout/superblock.go
index e4b8f46fb..7a337a5e0 100644
--- a/pkg/sentry/fs/ext/disklayout/superblock.go
+++ b/pkg/sentry/fs/ext/disklayout/superblock.go
@@ -14,6 +14,11 @@
 
 package disklayout
 
+const (
+	// SbOffset is the absolute offset at which the superblock is placed.
+	SbOffset = 1024
+)
+
 // SuperBlock should be implemented by structs representing the ext superblock.
 // The superblock holds a lot of information about the enclosing filesystem.
 // This interface aims to provide access methods to important information held
@@ -57,8 +62,6 @@ type SuperBlock interface {
 	//
 	// If the filesystem has 1kb data blocks then this should return 1. For all
 	// other configurations, this typically returns 0.
-	//
-	// The first block group descriptor is in (FirstDataBlock() + 1)th block.
 	FirstDataBlock() uint32
 
 	// BlockSize returns the size of one data block in this filesystem.
@@ -128,7 +131,7 @@ type SuperBlock interface {
 }
 
 // SbRevision is the type for superblock revisions.
-type SbRevision int
+type SbRevision uint32
 
 // Super block revisions.
 const (
diff --git a/pkg/sentry/fs/ext/disklayout/superblock_old.go b/pkg/sentry/fs/ext/disklayout/superblock_old.go
index c74953610..aada8b550 100644
--- a/pkg/sentry/fs/ext/disklayout/superblock_old.go
+++ b/pkg/sentry/fs/ext/disklayout/superblock_old.go
@@ -44,6 +44,9 @@ type SuperBlockOld struct {
 	DefResGID           uint16
 }
 
+// Compiles only if SuperBlockOld implements SuperBlock.
+var _ SuperBlock = (*SuperBlockOld)(nil)
+
 // InodesCount implements SuperBlock.InodesCount.
 func (sb *SuperBlockOld) InodesCount() uint32 { return sb.InodesCountRaw }