[PATCH v3 01/15] Documentation: add newcx initramfs format description

On Fri, 2018-02-16 at 12:59 -0800, H. Peter Anvin wrote:

On 02/16/18 12:33, Taras Kondratiuk wrote:

quoted

Many of the Linux security/integrity features are dependent on file
metadata, stored as extended attributes (xattrs), for making decisions.
These features need to be initialized during initcall and enabled as
early as possible for complete security coverage.

Initramfs (tmpfs) supports xattrs, but newc CPIO archive format does not
support including them into the archive.

This patch describes "extended" newc format (newcx) that is based on
newc and has following changes:
- extended attributes support
- increased size of filesize to support files >4GB
- increased mtime field size to have 64 bits of seconds and added a
  field for nanoseconds
- removed unused checksum field

If you are going to implement a new, non-backwards-compatible format,
you shouldn't replicate the mistakes of the current format.  Specifically:

1. The use of ASCII-encoded fixed-length numbers is an idiotic legacy
from an era before there were any portable way of dealing with numbers
with prespecified endianness.  If you are going to use ASCII, make them
delimited so that they don't have fixed limits, or just use binary.

The cpio header isn't fixed size, so that argument goes away, in fact
the only way to determine the end of the header is to scan forward.

2. Alignment sensitivity!  Because there is no header length
information, the above scan tells you where the header ends, but there
is padding before the data, and the size of that padding is only defined
by alignment.

3. Inband encoding of EOF: if you actually have a filename "TRAILER!!!"
you have problems.

But first, before you define a whole new format for which no tools exist
(you will have to work with the maintainers of the GNU tools to add
support) you should see how complex it would be to support the POSIX
tar/pax format, which already has all the features you are seeking, and
by now is well-supported.

The discussion about including xattrs in the initramfs didn't start
yesterday. ?It's been on the list of measurement/appraisal gaps that
need to be closed for years. ?Initially I planned on using tar, but at
the 2014 Kernel Summit I spoke with Al at length. ?At the time, he was
very clear that tar is unnecessarily overly complicated and
recommended extending CPIO.

I took his advice. ?Unfortunately, as soon as I posted an initial
patch set to include xattrs in CPIO, all of the problems with CPIO had
to be addressed before defining a new CPIO number. ?Unfortunately,
this wasn't the only measurement/appraisal gap that needed to be
addressed. ?I've been working on closing other gaps.

I'm really happy that someone has taken the time to work on this.
?Instead of derailing their attempt of extending CPIO to include
xattrs, I'd appreciate your making constructive suggestions.

Mimi

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