Re: [PATCH v3 01/15] Documentation: add newcx initramfs format description
From: Victor Kamensky
Date: Fri Feb 16 2018 - 16:57:18 EST
On Fri, 16 Feb 2018, Rob Landley wrote:
On 02/16/2018 02:59 PM, 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:
So rather than make minimal changes to the existing format and continue to
support the existing format (sharing as much code as possible), you recommend
gratuitous aesthetic changes?
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.
It lets encoders and decoders easily share code with the existing cpio format,
which we still intend to be able to read and write.
If you are going to use ASCII, make them
delimited so that they don't have fixed limits, or just use binary.
When it's gzipped this accomplishes what? (Other than being gratuitously
different from the previous iteration?)
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.
Again, these are minimal changes to the existing cpio format. You're complaining
about _cpio_, and that the new stuff isn't _different_ enough from it.
3. Inband encoding of EOF: if you actually have a filename "TRAILER!!!"
you have problems.
Been there, done that:
http://lkml.iu.edu/hypermail/linux/kernel/1801.3/01791.html
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)
No, he's been working with the maintainer of toybox to add support (for about a
year now), which gets him the Android command line. And the kernel has its own
built-in tool to generate cpio images anyway.
Why would anyone care what the GNU project thinks?
In our internal use of this patch series we do use gnu cpio
to create initramfs.cpio.
And reference to gnu cpio patch that supports newcx format is
posted in description for this serieis:
https://raw.githubusercontent.com/victorkamensky/initramfs-xattrs-poky/rocko/meta/recipes-extended/cpio/cpio-2.12/cpio-xattrs.patch
Whether GNU cpio maintainers will accept it is different matter.
We will try, but we need to start somewhere and agree on
new format first.
Thanks,
Victor
you should see how complex it would be to support the POSIX
tar/pax format,
That argument was had (at length) when initramfs went in over a decade ago.
There are links in Documentation/filesystems/ramfs-rootfs-initramfs.txt to the
mailing list entries about it.
which already has all the features you are seeking, and
by now is well-supported.
So... tar wasn't well-supported 15 years ago? (Hasn't the kernel source always
been distributed via tarball back since 0.0.1?)
You're suggesting having a whole second codepath that shares no code with the
existing cpio extractor. Are you suggesting abandoning support for the existing
initramfs.cpio.gz file format?
Rob