Re: [PATCH] ext4: Add statx support

[Jan Kara]

On Fri 17-03-17 14:19:22, Andreas Dilger wrote:
> On Mar 16, 2017, at 11:47 PM, Eric Biggers <ebiggers3@xxxxxxxxx> wrote:
> > On Thu, Mar 16, 2017 at 11:35:33AM +0000, David Howells wrote:
> >> +
> >> +	ext4_get_inode_flags(ei);
> >> +	flags = ei->i_flags & EXT4_FL_USER_VISIBLE;
> >> +	if (flags & EXT4_APPEND_FL)
> >> +		stat->attributes |= STATX_ATTR_APPEND;
> >> +	if (flags & EXT4_COMPR_FL)
> >> +		stat->attributes |= STATX_ATTR_COMPRESSED;
> >> +	if (flags & EXT4_ENCRYPT_FL)
> >> +		stat->attributes |= STATX_ATTR_ENCRYPTED;
> >> +	if (flags & EXT4_IMMUTABLE_FL)
> >> +		stat->attributes |= STATX_ATTR_IMMUTABLE;
> >> +	if (flags & EXT4_NODUMP_FL)
> >> +		stat->attributes |= STATX_ATTR_NODUMP;
> >> 
> >> -	inode = d_inode(path->dentry);
> >> 	generic_fillattr(inode, stat);
> >> +	return 0;
> >> +}
> > 
> > I think it's the wrong approach to be calling ext4_get_inode_flags() here to
> > sync the generic inode flags (inode->i_flags) to the ext4-specific inode flags
> > (ei->i_flags).  I know the GETFLAGS and FSGETXATTR ioctls do it too, but I
> > think it's a mistake --- at least, when done so without holding the inode lock.
> > The issue is that flag syncs can occur in both directions concurrently and
> > cause an update to be lost.  For example, with thread 1 doing a stat() and
> > thread 2 doing the SETFLAGS ioctl to set the APPEND flag:
> > 
> > Thread 1, ext4_get_inode_flags()	Thread 2, ext4_ioctl_setflags()
> > -----------------------------------	---------------------------
> > Read inode->i_flags; S_APPEND clear
> > 					Set EXT4_APPEND_FL in ei->i_flags
> > Clear EXT4_APPEND_FL in ei->i_flags
> > 					Read ei->i_flags; EXT4_APPEND_FL clear
> > 					Clear S_APPEND in inode->i_flags
> > 
> > So the flag set by SETFLAGS gets lost.  This bug probably hasn't really been
> > noticable with GETFLAGS and FSGETXATTR since they're rarely used, but stat() on
> > the other hand is very common, and I'm worried that with this change people
> > would start seeing this race more often.
> > 
> > Ultimately this needs to be addressed in ext4 more fully, but how about for
> > ->getattr() just skipping the call to ext4_get_inode_flags() and instead
> > populating the generic attributes like STATX_ATTR_APPEND and
> > STATX_ATTR_IMMUTABLE from the generic inode flags, rather than from the
> > ext4-specific flags?  Actually, it could even be done in generic_fillattr(), so
> > that all filesystems benefit.
> 
> Wouldn't it make more sense to just extract the ext4 flags directly from
> ei->i_flags?  I think ext4_get_inode_flags() is only really useful when
> the VFS inode flags are changed and they need to be propagated to the ext4
> inode.
> 
> I guess the other more significant question is when/where are the VFS inode
> flags changed that they need to be propagated into the ext4 disk inode?
> The main avenue for changing these attribute flags that I know about is via
> EXT4_IOC_SETFLAGS (FS_IOC_SETFLAGS), and there is one place that I could
> find in fs/nsfs.c that sets S_IMMUTABLE but I don't think that propagates
> down to an ext4 disk inode.

Yes, you seem to be right. And actually I have checked and XFS does not
bother to copy inode->i_flags to its on-disk flags so it seems generally we
are not expected to reflect inode->i_flags in on-disk state.

> It seems like the use of ext4_get_inode_flags() is largely superfluous.
> The original commit ff9ddf7e847 indicates this was for quota inodes only.
> I think it can be removed from EXT4_IOC_FSGETXATTR, and EXT4_IOC_GETFLAGS
> and made conditional in ext4_do_update_inode():
> 
> #ifdef CONFIG_QUOTA
>         for (i = 0; i < EXT4_MAXQUOTAS; i++) {
>                 if (unlikely(sb_dqopt(sb)->files[i] == inode))
>                         ext4_get_inode_flags(EXT4_I(inode));
>         }
> #endif
> 
> Jan, what do you think?

So I think even better would be to have ext4_quota_on() and
ext4_quota_off() just update the flags as needed and avoid doing it
anywhere else... I'll have a look into it.

								Honza

-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR