Factor out a composefs-erofs crate

Cargo.toml

@@ -17,10 +17,12 @@ unsafe_code = "deny" # https://github.com/containers/composefs-rs/issues/123
 
 [workspace.dependencies]
 composefs = { version = "0.3.0", path = "crates/composefs", default-features = false }
+composefs-types = { version = "0.3.0", path = "crates/composefs-types", default-features = false }
 cfsctl = { version = "0.3.0", path = "crates/cfsctl", default-features = false }
 composefs-ioctls = { version = "0.3.0", path = "crates/composefs-ioctls", default-features = false }
 composefs-oci = { version = "0.3.0", path = "crates/composefs-oci", default-features = false }
 composefs-boot = { version = "0.3.0", path = "crates/composefs-boot", default-features = false }
+composefs-erofs = { version = "0.3.0", path = "crates/composefs-erofs", default-features = false }
 composefs-http = { version = "0.3.0", path = "crates/composefs-http", default-features = false }
 
 [profile.dev.package.sha2]

crates/composefs-erofs/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "composefs-erofs"
+description = "EROFS format support for composefs"
+publish = false
+
+edition.workspace = true
+license.workspace = true
+readme.workspace = true
+repository.workspace = true
+rust-version.workspace = true
+version.workspace = true
+
+[dependencies]
+anyhow = { version = "1.0.87", default-features = false }
+composefs-types = { workspace = true }
+hex = { version = "0.4.0", default-features = false, features = ["std"] }
+thiserror = { version = "2.0.0", default-features = false }
+zerocopy = { version = "0.8.0", default-features = false, features = ["derive", "std"] }
+
+[lints]
+workspace = true

crates/composefs-erofs/src/composefs.rs

@@ -0,0 +1,43 @@
+//! Composefs-specific EROFS structures and overlay metadata.
+//!
+//! This module defines EROFS structures specific to composefs usage,
+//! particularly overlay metadata for fs-verity integration.
+
+use zerocopy::{FromBytes, Immutable, IntoBytes, KnownLayout};
+
+use composefs_types::fsverity::FsVerityHashValue;
+
+/// Overlay metacopy xattr structure for fs-verity digest storage.
+///
+/// From linux/fs/overlayfs/overlayfs.h struct ovl_metacopy
+#[derive(Debug, FromBytes, Immutable, KnownLayout, IntoBytes)]
+#[repr(C)]
+pub struct OverlayMetacopy<H: FsVerityHashValue> {
+    version: u8,
+    len: u8,
+    flags: u8,
+    digest_algo: u8,
+    /// The fs-verity digest value.
+    pub digest: H,
+}
+
+impl<H: FsVerityHashValue> OverlayMetacopy<H> {
+    /// Creates a new overlay metacopy entry with the given digest.
+    pub fn new(digest: &H) -> Self {
+        Self {
+            version: 0,
+            len: size_of::<Self>() as u8,
+            flags: 0,
+            digest_algo: H::ALGORITHM,
+            digest: digest.clone(),
+        }
+    }
+
+    /// Checks whether this metacopy entry is valid.
+    pub fn valid(&self) -> bool {
+        self.version == 0
+            && self.len == size_of::<Self>() as u8
+            && self.flags == 0
+            && self.digest_algo == H::ALGORITHM
+    }
+}