[PATCH v2 01/15] Documentation: add newcx initramfs format description

[Taras Kondratiuk]

Quoting Arnd Bergmann (2018-01-25 01:29:12)
> On Thu, Jan 25, 2018 at 4:27 AM, Taras Kondratiuk <takondra at cisco.com> 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 usec precision and more than
> >   32-bit of seconds.
> > - removed unused checksum field.
> >
> > Signed-off-by: Taras Kondratiuk <takondra at cisco.com>
> > Signed-off-by: Mimi Zohar <zohar at linux.vnet.ibm.com>
> > Signed-off-by: Victor Kamensky <kamensky at cisco.com>
> 
> Ah nice, I like the extension of the time handling, that certainly
> addresses one of the issues with y2038 that we have previously
> hacked around in an ugly way (interpreting the 32-bit
> number as unsigned).
> 
> However, if this is to become a generally supported format
> for cpio files, could we make it use nanosecond resolution
> instead? The issue that I see with microseconds is that
> storing a file in an archive and extracting it again would
> otherwise keep the mtime stamp /almost/ identical on file
> systems that have nanosecond resolution, but most of
> the time a comparison would indicate that the files are
> not the same.
> 
> Unfortunately, the range of a 64-bit nanoseconds counter
> is still a bit limited (584 years, or half of that if we make it
> signed). While this is clearly enough for the uses in
> initramfs, it still has a similar problem: someone creating
> a fake timestamp a long time in the past or future on
> a file system would lose information after going though
> cpio.

We can match statx(2) by having 64 bits for seconds plus 32 bits for
nanoseconds. For initramfs nanoseconds field can be ignored during
unpacking.
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