This patch introduces on-disk and runtime data structure of Extended
Attributes implementation in erofs_sys crate. This will be later used to
implement the op handler.

Signed-off-by: Yiyang Wu <toolmanp@tlmp.cc>
---
 fs/erofs/rust/erofs_sys.rs        |  12 +++
 fs/erofs/rust/erofs_sys/xattrs.rs | 124 ++++++++++++++++++++++++++++++
 2 files changed, 136 insertions(+)
 create mode 100644 fs/erofs/rust/erofs_sys/xattrs.rs

diff --git a/fs/erofs/rust/erofs_sys.rs b/fs/erofs/rust/erofs_sys.rs
index 2bd1381da5ab..6f3c12665ed6 100644
--- a/fs/erofs/rust/erofs_sys.rs
+++ b/fs/erofs/rust/erofs_sys.rs
@@ -25,4 +25,16 @@
 
 pub(crate) mod errnos;
 pub(crate) mod superblock;
+pub(crate) mod xattrs;
 pub(crate) use errnos::Errno;
+
+/// Helper macro to round up or down a number.
+#[macro_export]
+macro_rules! round {
+    (UP, $x: expr, $y: expr) => {
+        ($x + $y - 1) / $y * $y
+    };
+    (DOWN, $x: expr, $y: expr) => {
+        ($x / $y) * $y
+    };
+}
diff --git a/fs/erofs/rust/erofs_sys/xattrs.rs b/fs/erofs/rust/erofs_sys/xattrs.rs
new file mode 100644
index 000000000000..d1a110ef10dd
--- /dev/null
+++ b/fs/erofs/rust/erofs_sys/xattrs.rs
@@ -0,0 +1,124 @@
+// Copyright 2024 Yiyang Wu
+// SPDX-License-Identifier: MIT or GPL-2.0-or-later
+
+use alloc::vec::Vec;
+
+/// The header of the xattr entry index.
+/// This is used to describe the superblock's xattrs collection.
+#[derive(Clone, Copy)]
+#[repr(C)]
+pub(crate) struct XAttrSharedEntrySummary {
+    pub(crate) name_filter: u32,
+    pub(crate) shared_count: u8,
+    pub(crate) reserved: [u8; 7],
+}
+
+impl From<[u8; 12]> for XAttrSharedEntrySummary {
+    fn from(value: [u8; 12]) -> Self {
+        Self {
+            name_filter: u32::from_le_bytes([value[0], value[1], value[2], value[3]]),
+            shared_count: value[4],
+            reserved: value[5..12].try_into().unwrap(),
+        }
+    }
+}
+
+pub(crate) const XATTR_ENTRY_SUMMARY_BUF: [u8; 12] = [0u8; 12];
+
+/// Represented as a inmemory memory entry index header used by SuperBlockInfo.
+pub(crate) struct XAttrSharedEntries {
+    pub(crate) name_filter: u32,
+    pub(crate) shared_indexes: Vec<u32>,
+}
+
+/// Represents the name index for infixes or prefixes.
+#[repr(C)]
+#[derive(Clone, Copy)]
+pub(crate) struct XattrNameIndex(u8);
+
+impl core::cmp::PartialEq<u8> for XattrNameIndex {
+    fn eq(&self, other: &u8) -> bool {
+        if self.0 & EROFS_XATTR_LONG_PREFIX != 0 {
+            self.0 & EROFS_XATTR_LONG_MASK == *other
+        } else {
+            self.0 == *other
+        }
+    }
+}
+
+impl XattrNameIndex {
+    pub(crate) fn is_long(&self) -> bool {
+        self.0 & EROFS_XATTR_LONG_PREFIX != 0
+    }
+}
+
+impl From<u8> for XattrNameIndex {
+    fn from(value: u8) -> Self {
+        Self(value)
+    }
+}
+
+#[allow(clippy::from_over_into)]
+impl Into<usize> for XattrNameIndex {
+    fn into(self) -> usize {
+        if self.0 & EROFS_XATTR_LONG_PREFIX != 0 {
+            (self.0 & EROFS_XATTR_LONG_MASK) as usize
+        } else {
+            self.0 as usize
+        }
+    }
+}
+
+/// This is on-disk representation of xattrs entry header.
+/// This is used to describe one extended attribute.
+#[repr(C)]
+#[derive(Clone, Copy)]
+pub(crate) struct XAttrEntryHeader {
+    pub(crate) suffix_len: u8,
+    pub(crate) name_index: XattrNameIndex,
+    pub(crate) value_len: u16,
+}
+
+impl From<[u8; 4]> for XAttrEntryHeader {
+    fn from(value: [u8; 4]) -> Self {
+        Self {
+            suffix_len: value[0],
+            name_index: value[1].into(),
+            value_len: u16::from_le_bytes(value[2..4].try_into().unwrap()),
+        }
+    }
+}
+
+/// Xattr Common Infix holds the prefix index in the first byte and all the common infix data in
+/// the rest of the bytes.
+pub(crate) struct XAttrInfix(pub(crate) Vec<u8>);
+
+impl XAttrInfix {
+    fn prefix_index(&self) -> u8 {
+        self.0[0]
+    }
+    fn name(&self) -> &[u8] {
+        &self.0[1..]
+    }
+}
+
+pub(crate) const EROFS_XATTR_LONG_PREFIX: u8 = 0x80;
+pub(crate) const EROFS_XATTR_LONG_MASK: u8 = EROFS_XATTR_LONG_PREFIX - 1;
+
+/// Supported xattr prefixes
+pub(crate) const EROFS_XATTRS_PREFIXS: [&[u8]; 7] = [
+    b"",
+    b"user.",
+    b"system.posix_acl_access",
+    b"system.posix_acl_default",
+    b"trusted.",
+    b"",
+    b"security.",
+];
+
+/// Represents the value of an xattr entry or the size of it if the buffer is present in the query.
+#[derive(Debug)]
+pub(crate) enum XAttrValue {
+    Buffer(usize),
+    Vec(Vec<u8>),
+}
-- 
2.46.0