Re: [PATCH V11 04/14] xfs: Check for extent overflow when adding/removing xattrs

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Dec 04, 2020 at 02:34:17PM +0530, Chandan Babu R wrote:
> On Thu, 03 Dec 2020 10:45:59 -0800, Darrick J. Wong wrote:
> > On Tue, Nov 17, 2020 at 07:14:06PM +0530, Chandan Babu R wrote:
> > > Adding/removing an xattr can cause XFS_DA_NODE_MAXDEPTH extents to be
> > > added. One extra extent for dabtree in case a local attr is large enough
> > > to cause a double split.  It can also cause extent count to increase
> > > proportional to the size of a remote xattr's value.
> > > 
> > > To be able to always remove an existing xattr, when adding an xattr we
> > > make sure to reserve inode fork extent count required for removing max
> > > sized xattr in addition to that required by the xattr add operation.
> > > 
> > > Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
> > > ---
> > >  fs/xfs/libxfs/xfs_attr.c       | 20 ++++++++++++++++++++
> > >  fs/xfs/libxfs/xfs_inode_fork.h | 10 ++++++++++
> > >  2 files changed, 30 insertions(+)
> > > 
> > > diff --git a/fs/xfs/libxfs/xfs_attr.c b/fs/xfs/libxfs/xfs_attr.c
> > > index fd8e6418a0d3..d53b3867b308 100644
> > > --- a/fs/xfs/libxfs/xfs_attr.c
> > > +++ b/fs/xfs/libxfs/xfs_attr.c
> > > @@ -396,6 +396,8 @@ xfs_attr_set(
> > >  	struct xfs_trans_res	tres;
> > >  	bool			rsvd = (args->attr_filter & XFS_ATTR_ROOT);
> > >  	int			error, local;
> > > +	int			iext_cnt;
> > > +	int			rmt_blks;
> > >  	unsigned int		total;
> > >  
> > >  	if (XFS_FORCED_SHUTDOWN(dp->i_mount))
> > > @@ -416,6 +418,9 @@ xfs_attr_set(
> > >  	 */
> > >  	args->op_flags = XFS_DA_OP_OKNOENT;
> > >  
> > > +	rmt_blks = xfs_attr3_rmt_blocks(mp, XFS_XATTR_SIZE_MAX);
> > > +	iext_cnt = XFS_IEXT_ATTR_MANIP_CNT(rmt_blks);
> > 
> > These values are only relevant for the xattr removal case, right?
> > AFAICT the args->value != NULL case immediately after will set new
> > values, so why not just move this into...
> 
> The above statements compute the extent count required to remove a maximum
> sized remote xattr.
> 
> To guarantee that a user can always remove an xattr, the "args->value != NULL"
> case adds to the value of iext_cnt that has been computed above. I had
> extracted and placed the above set of statements since they were now common to
> both "insert" and "remove" xattr operations.

D'oh, you're right.

> > 
> > > +
> > >  	if (args->value) {
> > >  		XFS_STATS_INC(mp, xs_attr_set);
> > >  
> > > @@ -442,6 +447,13 @@ xfs_attr_set(
> > >  		tres.tr_logcount = XFS_ATTRSET_LOG_COUNT;
> > >  		tres.tr_logflags = XFS_TRANS_PERM_LOG_RES;
> > >  		total = args->total;
> > > +
> > > +		if (local)
> > > +			rmt_blks = 0;
> > > +		else
> > > +			rmt_blks = xfs_attr3_rmt_blocks(mp, args->valuelen);
> > > +
> > > +		iext_cnt += XFS_IEXT_ATTR_MANIP_CNT(rmt_blks);
> > >  	} else {
> > >  		XFS_STATS_INC(mp, xs_attr_remove);
> > 
> > ...the bottom of this clause here.
> > 
> > >  
> > > @@ -460,6 +472,14 @@ xfs_attr_set(
> > >  
> > >  	xfs_ilock(dp, XFS_ILOCK_EXCL);
> > >  	xfs_trans_ijoin(args->trans, dp, 0);
> > > +
> > > +	if (args->value || xfs_inode_hasattr(dp)) {
> > 
> > Can this simply be "if (iext_cnt != 0)" ?
> 
> That would lead to a bug since iext_cnt is computed unconditionally at the
> beginning of the function. An extent count reservation will be attempted when
> xattr delete operation is executed against an inode which does not have an
> associated attr fork. This will cause xfs_iext_count_may_overflow() to
> dereference the NULL pointer at xfs_inode->i_afp->if_nextents.

Ah, right, got it.  This looks fine to me then...

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> > 
> > --D
> > 
> > > +		error = xfs_iext_count_may_overflow(dp, XFS_ATTR_FORK,
> > > +				iext_cnt);
> > > +		if (error)
> > > +			goto out_trans_cancel;
> > > +	}
> > > +
> > >  	if (args->value) {
> > >  		unsigned int	quota_flags = XFS_QMOPT_RES_REGBLKS;
> > >  
> > > diff --git a/fs/xfs/libxfs/xfs_inode_fork.h b/fs/xfs/libxfs/xfs_inode_fork.h
> > > index bcac769a7df6..5de2f07d0dd5 100644
> > > --- a/fs/xfs/libxfs/xfs_inode_fork.h
> > > +++ b/fs/xfs/libxfs/xfs_inode_fork.h
> > > @@ -47,6 +47,16 @@ struct xfs_ifork {
> > >   */
> > >  #define XFS_IEXT_PUNCH_HOLE_CNT		(1)
> > >  
> > > +/*
> > > + * Adding/removing an xattr can cause XFS_DA_NODE_MAXDEPTH extents to
> > > + * be added. One extra extent for dabtree in case a local attr is
> > > + * large enough to cause a double split.  It can also cause extent
> > > + * count to increase proportional to the size of a remote xattr's
> > > + * value.
> > > + */
> > > +#define XFS_IEXT_ATTR_MANIP_CNT(rmt_blks) \
> > > +	(XFS_DA_NODE_MAXDEPTH + max(1, rmt_blks))
> > > +
> > >  /*
> > >   * Fork handling.
> > >   */
> > 
> 
> 
> -- 
> chandan
> 
> 
>