fs/tar: introduce tar fs

It is a file system based on tar files and an index appended to them (to
facilitate finding fs entries without having to traverse the whole tar
file.

Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>

fs/Kconfig

@@ -331,6 +331,7 @@ source "fs/sysv/Kconfig"
 source "fs/ufs/Kconfig"
 source "fs/erofs/Kconfig"
 source "fs/vboxsf/Kconfig"
+source "fs/tarfs/Kconfig"
 
 endif # MISC_FILESYSTEMS
 

fs/Makefile

@@ -129,3 +129,4 @@ obj-$(CONFIG_EFIVAR_FS)		+= efivarfs/
 obj-$(CONFIG_EROFS_FS)		+= erofs/
 obj-$(CONFIG_VBOXSF_FS)		+= vboxsf/
 obj-$(CONFIG_ZONEFS_FS)		+= zonefs/
+obj-$(CONFIG_TARFS_FS)		+= tarfs/

fs/tarfs/Kconfig

@@ -0,0 +1,15 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+
+config TARFS_FS
+	tristate "TAR file system support"
+	depends on BLOCK
+	help
+	  This is a simple read-only file system intended for mounting
+	  tar files that have had an index appened to them.
+
+	  To compile this file system support as a module, choose M here: the
+	  module will be called tarfs.
+
+	  If you don't know whether you need it, then you don't need it:
+	  answer N.

fs/tarfs/Makefile

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: GPL-2.0
+#
+# Makefile for the linux tarfs filesystem routines.
+#
+
+obj-$(CONFIG_TARFS_FS) += tarfs.o
+
+tarfs-y := tar.o

fs/tarfs/defs.rs

@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Definitions of tarfs structures.
+
+use kernel::types::LE;
+
+/// Flags used in [`Inode::flags`].
+pub mod inode_flags {
+    /// Indicates that the inode is opaque.
+    ///
+    /// When set, inode will have the "trusted.overlay.opaque" set to "y" at runtime.
+    pub const OPAQUE: u8 = 0x1;
+}
+
+kernel::derive_readable_from_bytes! {
+    /// An inode in the tarfs inode table.
+    #[repr(C)]
+    pub struct Inode {
+        /// The mode of the inode.
+        ///
+        /// The bottom 9 bits are the rwx bits for owner, group, all.
+        ///
+        /// The bits in the [`S_IFMT`] mask represent the file mode.
+        pub mode: LE<u16>,
+
+        /// Tarfs flags for the inode.
+        ///
+        /// Values are drawn from the [`inode_flags`] module.
+        pub flags: u8,
+
+        /// The bottom 4 bits represent the top 4 bits of mtime.
+        pub hmtime: u8,
+
+        /// The owner of the inode.
+        pub owner: LE<u32>,
+
+        /// The group of the inode.
+        pub group: LE<u32>,
+
+        /// The bottom 32 bits of mtime.
+        pub lmtime: LE<u32>,
+
+        /// Size of the contents of the inode.
+        pub size: LE<u64>,
+
+        /// Either the offset to the data, or the major and minor numbers of a device.
+        ///
+        /// For the latter, the 32 LSB are the minor, and the 32 MSB are the major numbers.
+        pub offset: LE<u64>,
+    }
+
+    /// An entry in a tarfs directory entry table.
+    #[repr(C)]
+    pub struct DirEntry {
+        /// The inode number this entry refers to.
+        pub ino: LE<u64>,
+
+        /// The offset to the name of the entry.
+        pub name_offset: LE<u64>,
+
+        /// The length of the name of the entry.
+        pub name_len: LE<u64>,
+
+        /// The type of entry.
+        pub etype: u8,
+
+        /// Unused padding.
+        pub _padding: [u8; 7],
+    }
+
+    /// The super-block of a tarfs instance.
+    #[repr(C)]
+    pub struct Header {
+        /// The offset to the beginning of the inode-table.
+        pub inode_table_offset: LE<u64>,
+
+        /// The number of inodes in the file system.
+        pub inode_count: LE<u64>,
+    }
+}