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Re: bug: truncate to zero + setuid

To: Roger Willcocks <roger@xxxxxxxxxxxxxxxx>
Subject: Re: bug: truncate to zero + setuid
From: Timothy Shimmin <tes@xxxxxxx>
Date: Thu, 08 Nov 2007 14:13:22 +1100
Cc: xfs@xxxxxxxxxxx
In-reply-to: <000001c81f3e$eff344b0$6501a8c0@BODDINGTON>
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Hi Roger,

Roger Willcocks wrote:
Timothy Shimmin wrote:
Hi Roger,

...
I don't like all these inconsistencies.

Take a look at the attached patch relative to the current cvs (it's a bit big to put inline). The basic problem is it's currently unclear when to set the times from va_atime etc. and when to set them to the current time. So I've used the already defined XFS_AT_UPDxTIME flags to indicate that a time should be set to 'now'
and XFS_AT_xTIME to mean set it using va_xtime. This seems to fit well with
the current code and I wonder if that's how it was meant to work in the first place.

Yeah, I've looked at this a few times now ;-) and this _seems_ like
a reasonable thing to do to me.

So patch:
ATTR_ATIME_SET => XFS_AT_ATIME (& set va_atime etc) (used to set to given time)
ATTR_ATIME     => XFS_AT_UPDATIME (used to set to "now")
likewise for M variant.

Previously:
ATTR_ATIME_SET => ATTR_UTIME flag (used to set given time)
                  must expect ATTR_ATIME to be set too to get va_atime
ATTR_ATIME     => XFS_AT_ATIME (& set va_atime) (used to set to "now")
                  a bit confusing since it can store va_atime even if
                  ATTR_ATIME_SET is not on


I've also removed the now redundant ATTR_UTIME flag and pulled
the null truncate to the top, which simplifies things.


So these changes of:

        if (mask & (XFS_AT_ATIME|XFS_AT_MTIME)) {
                if (!file_owner) {
-                       if ((flags & ATTR_UTIME) &&
-                           !capable(CAP_FOWNER)) {
+                       if (!capable(CAP_FOWNER)) {

Where you take out ATTR_UTIME make sense since XFS_AT_ATIME et al,
now refer to the case where a given time is provided
instead of requiring ATTR_UTIME to be set.

One query: in both xfs_iops.c/xfs_vn_setattr and xfs_dm.c/xfs_dm_set_fileattr the
ATIME branch sets the inode's atime directly.

xfs_vn_setattr()
        if (ia_valid & ATTR_ATIME) {
                vattr.va_mask |= XFS_AT_ATIME;
                vattr.va_atime = attr->ia_atime;
                inode->i_atime = attr->ia_atime;
        }

xfs_dm_set_fileattr()
        if (mask & DM_AT_ATIME) {
                vat.va_mask |= XFS_AT_ATIME;
                vat.va_atime.tv_sec = stat.fa_atime;
                vat.va_atime.tv_nsec = 0;
                inode->i_atime.tv_sec = stat.fa_atime;
        }

Hmmm....
So this could change behavior for xfs_vn_setattr().
If previously we had ATTR_ATIME set but NOT ATTR_ATIME_SET,
then we would set inode->i_atime.
Now with the patch, in this case, we don't set inode->i_atime
at this point.
However, in this case we wouldn't want i_atime to be set to ia_atime
as we would want it to be set to "now" in xfs_ichgtime().

This is probably something to do with the comment above xfs_iops.c/xfs_ichgtime ('to make sure the access time update
will take') but it could probably be handled better.

I'll need to look.

BTW, your locking looks wrong - it appears you don't unlock when the
file is non-zero size.

Oops...

I was also thinking of a read lock here.
And initializing quot vars to zero in variable definition at top.

This stuff really needs to be QA'ed well.
It would be too easy to get a regression in expected behavior.
Need to hunt out qa tests.

Thanks for the effort,
Tim.


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