ext4 - getting at birth time (file create time) and getting/setting nanosecond time stamps and utime

Mingming cmm at us.ibm.com
Wed Oct 21 17:42:39 MDT 2009

On Tue, 2009-10-20 at 18:44 -0600, Andreas Dilger wrote:
> On 20-Oct-09, at 14:49, Steve French wrote:
> > On Tue, Oct 20, 2009 at 3:33 PM, Andreas Dilger <adilger at sun.com>  
> > wrote:
> >> For those users/admins that want to be able to change this (due to
> >> Samba, or whatever), great, we can give them this ability, but for
> >> users who do NOT want regular users to be able to change the file
> >> creation timestamp, that should be possible as well.
> >
> > So, are we ready for Mingming or one of the ext4 developers to propose
> > a patch for this via xattrs (I can do a similar one for cifs).
> > Sounds like various have said:
> >
> > 1) xattrs instead of ioctl
> > 2) get of create time allowed by default, but set of create time  
> > limited
> A wildly untested/uncompiled patch is attached for purposes of  
> discussion.
> One issue that I've already come across is if we are storing a raw  
> "timespec"
> we will get different xattrs on disk in the fallback case where the on- 
> disk
> inodes are not large enough to store the crtime inside the inode.
> This version of the patch will fall back to storing the timespec on disk
> in the supplied size/endianness as an xattr, but it definitely needs  
> to be
> handled more appropriately (endian/word size).  It _would_ be nice to  
> store
> a full 64-bit timespec on disk, but given that the in-inode format  
> only can
> handle 34 bits of seconds + 30 bits of nanoseconds, it isn't clear  
> whether
> the "fallback" should do better than that.
Could we transparently convert user provided raw creation time and store
the same format as whose in-inode creation timestamp in the fallback
case? Just to be consistent. User doesn't have to know what type/size of
ext4 inode it is accessing...

> The main question is whether storing a "user.crtime" as an on-disk xattr
> is a desirable fallback, or if this should just fail?  

I think it would be nice to have this fallback, just be consistent, that
for all type of ext4 inodes there is a way to set file creation time for
allowed users(whether default 256 bytes large inode which could store
the 34+30bit creation time, or the 128 bytes smaller inode, which
doesn't have space to store the creation in the inode)

> Similarly, should
> the lack of on-disk crtime return "0.0" as the creation time, or should
> it return -ENODATA or similar?
> Cheers, Andreas
> --
> Andreas Dilger
> Sr. Staff Engineer, Lustre Group
> Sun Microsystems of Canada, Inc.

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