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Arbitrary Attribute Interchange Protocol
Draft version 0.2
Jan 30 2008
Interchange of Persistent File Attributes
by Thomas Schmitt - mailto:scdbackup@gmx.net
Libburnia project - mailto:libburn-hackers@pykix.org
AAIP is intended as companion of the Rock Ridge Interchange Protocol RRIP
which under the general design of System Use Sharing Protocol SUSP extends
ISO 9660 aka ECMA-119 filesystem semantics to match POSIX needs.
Goal is to have for each file an arbitrary number of attributes which consist
of two components (Name and Value) of arbitrary length and to have a compact
representation of ACLs.
This document describes a SUSP entry with Signature Word "AA" which collides
neither with SUSP 1.12 nor with RRIP 1.12. The AA entry has been designed
to be as similar to the RRIP entry SL as possible.
The presence of AAIP shall be announced by a particular ER entry.
Since the size of a SUSP entry is limited to 255, multiple entries may be
needed to describe one component. The CE mechanism of SUSP shall be used to
address enough storage if needed.
AA entries and the ER entry of AAIP shall only be present if the ER entry
of RRIP is present.
-------------------------------------------------------------------------------
System Entries Provided by this Specification
* AA
Description of the "AA" System Use Entry
The entry has exactly the same layout as RRIP entry SL. One has to expect
more data bytes than with SL, though, and any of the 256 possible byte values.
The reader shall be prepared to detect and handle oversized data.
One or more AA entries form the Attribute List of a file object with
an even number of components. Each two consequtive components form a pair of
Name and Value. The empty name is reserved for a compact representation of
ALCs. The meaning of any other name is not specified by this document.
All AA entries except the last one shall have the CONTINUE flag set. An AA
entry with CONTINUE set to 0 indicates the end of the Attribute List.
The format of the AA System Use Field is as follows:
[1] "BP 1 to BP 2 - Signature Word" shall be (41)(41) ("AA").
[2] "BP 3 - Length" shall specify as an 8-bit number the length in bytes of
the AA entry recorded according to ISO 9660:7.1.1.
[3] "BP 4 - System Use Entry Version" shall be 1 as in ISO 9660:7.1.1.
[4] "BP 5 - Flags" shall contain bit field flags numbered 0 to 7 starting
with the least significant bit as follows:
0 CONTINUE This AA entry continues in the next AA entry.
All other bits shall be set to 0.
[5] "BP 6 to Length - Component Area" shall contain Component Records
as described below.
| 'A' | 'A' | LENGTH | 1 | FLAGS | COMPONENT AREA |
Within AA entries each component (Name or Value) shall be recorded as one
or more component records. If a component does not fit into the remaining
space of an AA entry then it shall be continued in following AA entries.
All Component Records of a component except the last one shall have the
CONTINUE flag set. A Component Record with CONTINUE set to 0 indicates the end
of the component. An eventually following Component Record starts the next
component.
-------------------------------------------------------------------------------
The Component Record format is identical to the one of the SL entry.
The complete form of the following summary can be found in RRIP 1.12 "4.1.3.1".
In case of discrepancies, RRIP 1.12 is the decisive specification. Please
inform the author of this document if you find such a discrepancy.
Component Records shall be recorded contiguously within each Component Area,
starting in the first byte of the Component Area. The last Component Record
in the Component Area of an AA System Use Entry may be continued in the
Component Area of the next recorded AA System Use Entry in the same
System Use Area.
Each Component Record shall have the following format:
[A] "BP 1 - Component Flags" shall contain bit field flags numbered 0 to 7,
starting with the least significant bit, as follows:
0 CONTINUE This Component Record continues in the next
AA Component Record.
all others are RESERVED and shall be 0.
[B] "BP 2 - Component Length (LEN_CP)" shall specify as an 8-bit number the
number of component bytes in the Component Record. This length shall not
include the first two bytes of the Component Record.
If any of the bit positions 1-3 is set, the value of this field shall be
set to ZERO and no Component Content shall be recorded.
This field shall be recorded according to ISO 9660 Format section 7.1.1.
[C] "BP 3 to 2 + LEN_CP - Component Content" shall contain the component
bytes in the Component Record.
| COMPONENT FLAGS | LEN_CP | COMPONENT BYTES |
Example: Two pairs of "name"="long...content" and "one"="more" encoded as
two AA entries
Field 1 contains the Component Record of Name and one Component Record of
Value :
{ 'A', 'A', 255, 1, 1,
0, 4, 'n', 'a', 'm', 'e',
1, 255, 'l', 'o', 'n', 'g', ... 238 more bytes, 13 go to next AA ... }
Field 2 contains the rest of "long...content" and the complete second pair.
It marks the end of the Attribute List :
{ 'A', 'A', 38, 1, 0,
... 13 remaining bytes of the Component Record in first entry ...
0, 7, 'c', 'o', 'n', 't', 'e', 'n', 't',
0, 3, 'o', 'n', 'e',
0, 4, 'm', 'o', 'r', 'e' }
-------------------------------------------------------------------------------
Specification of binary ACL representation as special Arbitrary Attribute
The Name component of a binary ACL shall be of length 0.
The Value shall be an arbitrary number of ACL Entries:
[a] "BP 1 - Entry Flags" shall contain bit field flags numbered 0 to 7,
starting with the least significant bit, as follows:
0 EXEC indicates that this entry grants execute permission
1 WRITE write permission
2 READ read permission
3 QUALIFIER indicates that one or more Qualifier Records follow
4 - 7 TYPE
shall contain the tag type of the ACL entry as four bit code:
0 TRANSLATE entry for a global map of name to numeric id
1 ACL_USER_OBJ permissions of owning user (as of PX entry)
2 ACL_USER of arbitrary user, with name as qualifier
3 ACL_GROUP_OBJ permissions of owning group (as of PX entry)
4 ACL_GROUP of arbitrary group, with name as qualifier
5 ACL_MASK restricts 2, 3, and 4 via logical AND
6 ACL_OTHER permissions of non-listed, non-owning users
8 SWITCH_MARK switch from "access" ACL to "default" ACL
10 ACL_USER_N like 2, with numeric user id as qualifier
12 ACL_GROUP_N like 4, with numeric group id as qualifier
15 FUTURE_VERSION will indicate that this document
does not apply to the entry.
The other values are reserved. Readers shall ignore them if
they are not aware of updates of this document which would
assign a meaning to them.
The entries ACL_USER_OBJ, ACL_GROUP_OBJ, ACL_OTHER must match the permission
bits of the PX entry. This shall obey the peculiar rule that
ACL_USER_OBJ must match S_IRWXU, ACL_OTHER must match S_IRWXO,
ACL_MASK - if present - must match S_IRWXG, else ACL_GROUP must match S_IRWXG.
If there is ACL_USER_OBJ or ACL_GROUP_OBJ there must also be ACL_MASK.
A numeric qualifier is a binary number of variable length. The Most Significant
Byte comes first. The number shall be the "POSIX File User ID" resp.
"POSIX File Group ID" as also used in RRIP PX entries. The ids of owning user
and owning group shall be taken from the PX entry of the file object.
Optional TRANSLATE entries may associate user or group names with numeric
ids to allow the reading system to remap the numeric ids. See below.
The writer is not obliged to write them and the reader is not obliged to
interpret them.
The ACL entries belong to the "access" ACL of a file object. An optional
SWITCH_MARK entry may direct further entries to the "default" ACL which
is defined for directory objects. The switching is controlled by the EXEC bit.
0 "access" ACL
1 "default" ACL
The bits for WRITE, READ, QUALIFIER shall be 0 with SWITCH_MARK.
The eventually needed qualifier is stored in one or more Qualifier Records.
[b] "BP 2 - Qualifier Record Head" shall be present only if QUALIFIER is set
to 1. It shall give the number of Qualifier Bytes and eventually
indicate that the qualifier continues in a Qualifier Record which comes
imediately after this record.
0 to 127 Q_LENGTH, the qualifier is complete by this record
128 to 255 Q_LENGTH+128, the qualifier is continued by next record
So a Qualifier Record can contain at most 127 Qualifier Bytes.
This field shall be recorded according to ISO 9660 Format section 7.1.1.
[c] "BP 3 to BP 2 + Q_LENGTH - Qualifier Bytes" shall be present only if
QUALIFIER is set to 1 and hold the announced number of bytes of the
user or group name.
| ENTRY FLAGS [ | QUALIFIER HEAD | QUALIFIER BYTES | ]
Example: From man 5 acl: u::rw-,u:lisa:rw-,g::r--,g:toolies:rw-,m::r--,o::r--
{ 'A', 'A', 28, 1, 0,
0, 0,
0, 19, 0x16,
0x2E, 4, 'l', 'i', 's', 'a',
0x34,
0x4E, 7, 't', 'o', 'o', 'l', 'i', 'e', 's',
0x54,
0x64 }
Example: An entry with very long qualifier u:His_Excellency_..._the_Boss:r--
0x2C, 255, 'H', 'i', 's', '_', 'E', 'x', 'c', 'e', 'l', 'e',
... 117 more bytes ...,
8, 't', 'h', 'e', '_', 'B', 'o', 's', 's',
Example: User number 71 in numerical form gets rwx
0xAF, 1, 71,
Group number 65534 gets r-x
0xCD, 2, 255, 254,
-------------------------------------------------------------------------------
About Names and Numeric Identifiers
It makes an important difference whether qualifiers are represented as names
or as id numbers. By storing names (and usually wasting space) it is possible
to control permissions in a way that is portable between uncoordinated
computers as long as the human readable user/group names are present on both
sides. POSIX File ID numbers make most sense in backups which shall be read
by the same system which wrote it.
Rock Ridge can only record POSIX File Ids but not user or group names.
The entry flag value 0x08 TRANSLATE is not a ACL entry of the hosting object
but rather a global hint about the relation of roles, names and numeric ids.
If it is recorded at all, then it shall be recorded with the first Directory
Entry of the volume's root directory. According to the description of SUSP
entry ER, this has to be "dot" or (00). Other than with ER, a TRANSLATE entry
may not appear in the root of directory sub trees.
An interested reader shall examine the Arbitrary Attributes of this Directory
Entry in order to collect a translation table.
The advised translation is: PX or AA Id number -> name -> local id number.
The Qualifier Bytes of a TRANSLATE entry shall have the following format:
[i] "BP 0 - Role" shall tell whether it is about a user name (role 0) or
a group name (role 1). Other values are not allowed.
[ii] "BP 1 to BP 8 - Numeric Id" shall hold the 32 bit POSIX Id number of the
entry. This field shall be recorded according to ISO 9660:7.3.3.
[iii] "BP 9 to End Of Qualifier - Name" shall hold the name bytes of this
entry.
| ROLE | NUMERIC ID | NAME |
Example: User id number 1001 gets associated with user name "lisa"
0x08, 13, 0, 233,3,0,0, 0,0,3,233, 'l', 'i', 's', 'a',
-------------------------------------------------------------------------------
Specification of the ER System Use Entry Values for AAIP:
This ER system entry shall only be present if the ER entry of RRIP is present.
To be compliant with SUSP-1.12, this ER entry must be present if AA entries
are present, and ES entries have to mark RRIP and AAIP entries.
If for some reason compliance with SUSP-1.10 is intended, then this ER entry
and the ES entries must not be present, although SUSP-1.10 would allow ER.
(See below: Compatibility considerations.)
The Extension Version number for this version of AAIP shall be 1.
The Extension Identifier field shall be "AAIP_0002" with Identifier Length 9.
The mandatory content form of the Extension Descriptor is
"AA PROVIDES VIA AAIP 0.2 SUPPORT FOR ARBITRARY FILE ATTRIBUTES IN ISO 9660 IMAGES"
The Description Length is 81.
The recommended content of the Extension Source is
"PLEASE CONTACT THE LIBBURNIA PROJECT VIA LIBBURNIA-PROJECT.ORG".
The corresponding Source Length is 62.
-------------------------------------------------------------------------------
Compatibility Considerations
This extension is supposed not to disturb any reader system which complies
to SUSP-1.10:
"6.2 Requirements for a Receiving System
[...]
Any System Use Field which the receiving system does not recognize
is to be ignored and skipped."
SUSP-1.12 extends this prescription by:
"Any System Use Entry, with the exception of the set of System Use Entries
defined in this document, following an "ES" System Use Entry that indicates
an extension specification which the receiving system does not recognize
shall be ignored and skipped."
According to SUSP-1.12 the ER entry is mandatory for a conformant extension.
It also prescribes that in the case that ER entries of RRIP and AAIP are
present, then ES entries shall be used to separate RRIP entries from AAIP
entries.
SUSP-1.12 frowns on extensions which are not announced by ER. Nevertheless
is does not totally outrule them.
SUSP-1.10 does not specify ES entries at all and allows to have extension
entries without announcing them by an ER entry. So if a second ER entry is
not bearable, then the SUSP-1.10 downgrade of AAIP allows to omit the
AAIP ER and the ES entries. But if there is the AAIP ER then there must be ES
at the appropriate places. Else the format would explicitely violate SUSP-1.12.
-------------------------------------------------------------------------------
Model Relations:
Attribute List ------------- [1:0..1] ------------- ACL
[1:0..n] [1:0..n]
Arbitrary Attribute ( [1:0..1] ACL ) Entry
[1:2..2n] [1:0..1]
Component ( [1..m:1..n] AA Field ) Qualifier
[1:1..n] << one of >>
Component Record / \ \
[1..m:1..n] Translation Entry , Name , Numeric Id
AA Field | | |
[1:1..n] [1:1..n] [1:1]
\ | /
Qualifier Record
-------------------------------------------------------------------------------
Revoked drafts:
There was a draft AAIP 0.0 with ER signature "AAIP_2008A". It did not resemble
the existing entry SL and therefore shall not be used by writers of ISO images.
-------------------------------------------------------------------------------
References:
ECMA-119 http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-119.pdf
SUSP 1.12 ftp://ftp.ymi.com/pub/rockridge/susp112.ps
RRIP 1.12 ftp://ftp.ymi.com/pub/rockridge/rrip112.ps
(especially entry SL)
-------------------------------------------------------------------------------
Pending considerations:
- shall the tag types ACL_USER and ACL_GROUP with non-numeric qualifier be
revoked ?
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