Appendix 4.A Constants

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Every value below is generated from the source by pkm/tools/gen-stratafs-constants.py. Nothing here is transcribed by hand, and the generator's --check mode fails if the two drift apart.

4.A.1 Filesystem identity #

Constant	Value	Meaning
`STRATAFS_MAGIC`	`0x53545241`	Superblock magic, reported by `statfs` (§4.2.2)
`STRATAFS_NAME`	`"stratafs"`	The name the filesystem registers under
`STRATAFS_MAX_STRATA`	`16`	Longest stratum stack accepted (§4.2.1)

STRATAFS_MAGIC is an alias for STRATAFS_SUPER_MAGIC, which is declared in the header stratafs shares with KACS so that the mount policy class keyed on it cannot drift (§4.6.4).

4.A.2 Stratum flags #

Constant	Value	Meaning
`STRATAFS_F_CREATE`	`0x1`	The `create` flag (§4.2.1)
`STRATAFS_F_RO`	`0x2`	The `ro` flag
`STRATAFS_F_AM`	`0x4`	The `am` flag

4.A.3 Extended attributes #

Constant	Value	Meaning
`STRATAFS_XATTR_PREFIX`	`"system.stratafs."`	Reserved namespace; never forwarded to a provider (§4.7)
`STRATAFS_XATTR_ORIGIN`	`"system.stratafs.origin"`	Synthetic, read-only, hidden from listings (§4.7)
`STRATAFS_XATTR_STAGING`	`"security.peios.stratafs_staging"`	Copy-up staging marker; also reserved (§4.5.2)

STRATAFS_XATTR_STAGING is an alias for STRATAFS_STAGING_XATTR, declared in the shared header. Note the name it resolves to lies outside the reserved system.stratafs. namespace, yet receives the same treatment (§4.7).

The canonical security-descriptor attribute is KACS's, not stratafs's; stratafs only detects it in order to exclude it from copy-up replication (§4.6.3).

4.A.4 Copy-up and staging #

Constant	Value	Meaning
`STRATAFS_STAGE_MARKER_MAGIC`	`0x53544731`	Marker magic
`STRATAFS_STAGE_MARKER_VERSION`	`1`	Marker version
`STRATAFS_COPY_BUFFER_SIZE`	`65536`	Copy-up read/write chunk, in bytes (§4.5.2)
`STRATAFS_STAGE_PREFIX`	`".stratafs-stage-"`	Staged-name prefix
`STRATAFS_RECOVERY_BATCH`	`128`	Names scanned per staging-recovery pass

4.A.4.1 The staging marker #

struct stratafs_stage_marker is packed and 24 bytes, all fields little-endian. It is the value of the staging attribute above.

Offset	Size	Field	Type
0	4	`magic`	`__le32`
4	2	`version`	`__le16`
6	2	`size`	`__le16`
8	8	`boot_cookie`	`__le64`
16	8	`mount_cookie`	`__le64`

4.A.5 Routing #

The value route_existing returns (§4.5.1), as the Rust decision core names it and as the C glue mirrors it. The discriminants match.

C enumerator	Value	Rust
`STRATAFS_ROUTE_IN_PLACE`	`0`	`InPlace`
`STRATAFS_ROUTE_COPY_UP`	`1`	`CopyUp`
`STRATAFS_ROUTE_READ_ONLY`	`2`	`ReadOnly`

4.A.6 The decision core #

stratafs-core holds the stack-wide flag rules, provider selection, and routing. Its flag bits match the C ones above exactly.

Constant	Value
`MAX_STRATA`	`16`
`FLAG_CREATE`	`0x1`
`FLAG_READ_ONLY`	`0x2`
`FLAG_ABSENT_MAY`	`0x4`
`FLAG_MASK`	`0x7`

The crate distinguishes these configuration errors. The C boundary collapses all of them to EINVAL, so the distinction is not observable to a caller (§4.2.1).

Error	Discriminant
`Empty`	`1`
`TooMany`	`2`
`UnknownFlag`	`3`
`RepeatedCreate`	`4`
`CreateReadOnly`	`5`

4.A.7 Build configuration #

stratafs is built by CONFIG_STRATAFS_FS, a boolean option, so what it builds is linked into vmlinux rather than loaded. It depends on CONFIG_SECURITY_PKM and selects FS_STACK. Its sources are staged into the kernel tree as fs/stratafs, separate from PKM's own security/pkm. CONFIG_STRATAFS_KUNIT_TEST builds the in-kernel unit tests.

The translation units are:

super.o
lookup.o
inode.o
file.o
dir.o
xattr.o
copy_up.o

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