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

[hpa@zytor.com]

On February 17, 2018 4:15:12 PM PST, Mimi Zohar <zohar@linux.vnet.ibm.com> wrote:
>On Fri, 2018-02-16 at 12:59 -0800, H. Peter Anvin wrote:
>> On 02/16/18 12:33, Taras Kondratiuk wrote:
>> > 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

I'm not trying to derail anything, but I do want to see it done well if it is going to be done.  I have several ideas already, but I may not have a chance to write them down properly until after the weekend due to family obligations.

The assessment of pax/tar is useful; it should be added to the patch documentation in a future set.
-- 
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