General Question: ctime, mtime, and xattrs

General Question: ctime, mtime, and xattrs

[Robert White]

So I was reading the wiki on the internal layout. The INODE description 
says "st_ctime. Also updated when xattrs change."

Why isn't changing the xattrs a modification (st_mtime) event?

It just seems odd to me...

Re: General Question: ctime, mtime, and xattrs

[Robert White]

Ah...

I've been thinking "ctime" is/was (still) "create time". It seems that 
somewhere in the last couple decades it became "change time"; Or that I 
picked up that incorrect "create time" idea back in the UNIX Sys V R 3 
days and just never had cause to think about it again...

Never mind. /sigh...

What a maroon. 8-)

On 12/05/2014 03:03 PM, cwillu wrote:
> xattrs are commonly used to implement acls, which wouldn't typically be
> considered a content modification.
> On Dec 5, 2014 4:08 PM, "Robert White" <rwhite@pobox.com> wrote:
>
>> So I was reading the wiki on the internal layout. The INODE description
>> says "st_ctime. Also updated when xattrs change."
>>
>> Why isn't changing the xattrs a modification (st_mtime) event?

Re: General Question: ctime, mtime, and xattrs

[Chris Samuel]

On Fri, 5 Dec 2014 03:28:58 PM Robert White wrote:

> I've been thinking "ctime" is/was (still) "create time". It seems that 
> somewhere in the last couple decades it became "change time"; Or that I 
> picked up that incorrect "create time" idea back in the UNIX Sys V R 3 
> days and just never had cause to think about it again...

Sadly there's never been a creation time in Linux that you can get with a 
standard system call, there was an attempt 4-5 years ago to get xstat merged 
that would include creation time from filesystems that support it (like ext4) 
but it never went anywhere (for a variety of reasons).

LWN article on the patch set:

 https://lwn.net/Articles/394298/

Linus knocking it back:

 https://lkml.org/lkml/2010/7/22/249

FreeBSD has:

 st_birthtim  Time when the inode was created.

No idea when that was added!

All the best,
Chris
-- 
 Chris Samuel  :  http://www.csamuel.org/  :  Melbourne, VIC

Re: General Question: ctime, mtime, and xattrs

[Al Viro]

On Fri, Dec 05, 2014 at 03:28:58PM -0800, Robert White wrote:
> Ah...
> 
> I've been thinking "ctime" is/was (still) "create time". It seems
> that somewhere in the last couple decades it became "change time";
> Or that I picked up that incorrect "create time" idea back in the
> UNIX Sys V R 3 days and just never had cause to think about it
> again...

v7 is the point where the third timestamp has first appeared (v6 has
only two - access and update).  And their stat(2) says this:
       st_atime is the file was last read.  For reasons of efficiency,  it  is
       not set when a directory is searched, although this would be more logi‐
       cal.  st_mtime is the time the file was last written or created.  It is
       not  set  by changes of owner, group, link count, or mode.  st_ctime is
       set both both by writing and changing the i-node.

FWIW, their /usr/sys/sys/sys4.c has
chmod()
{
        register struct inode *ip;
        register struct a {
                char    *fname;
                int     fmode;
        } *uap;

        uap = (struct a *)u.u_ap;
        if ((ip = owner()) == NULL)
                return;
        ip->i_mode &= ~07777;
        if (u.u_uid)
                uap->fmode &= ~ISVTX;
        ip->i_mode |= uap->fmode&07777;
        ip->i_flag |= ICHG;
        if (ip->i_flag&ITEXT && (ip->i_mode&ISVTX)==0)
                xrele(ip);
        iput(ip);
}
and /usr/sys/sys/iget.c has, in the end of iupdat() (called on the final
iput(), as well as on stat(2) and several other paths), this:
                if(ip->i_flag&IACC)
                        dp->di_atime = *ta;
                if(ip->i_flag&IUPD)
                        dp->di_mtime = *tm;
                if(ip->i_flag&ICHG)
                        dp->di_ctime = time;
                ip->i_flag &= ~(IUPD|IACC|ICHG);
(ta and tm are both equal to &time in that call chain).  IOW, chmod(2)
definitely sets ctime.

IOW, ctime has never been "create time"; it's "change time" and it had been
that way since its introduction.