Re: [PATCH 4/4] quota: simplify the quotactl compat handling

[Christoph Hellwig]

On Sun, Jul 26, 2020 at 05:32:14PM +0100, Al Viro wrote:
> > +static int compat_copy_fs_qfilestat(struct compat_fs_qfilestat __user *to,
> > +		struct fs_qfilestat *from)
> > +{
> > +	if (copy_to_user(to, from, sizeof(*to)) ||
> > +	    put_user(from->qfs_nextents, &to->qfs_nextents))
> > +		return -EFAULT;
> > +	return 0;
> > +}
> 
> do we have any need of that put_user()?  Note that you don't even call
> that thing unless compat_need_64bit_alignment_fixup() is true.  And AFAICS
> all such cases are little-endian...

The main reason it is there is to preserve the previous semantics.
And no, I don't think we actually need it on x86.  But what if some
poor souls adds a BE version that needs this?  E.g. arm oabi has similar
weird alignment, and now imagine someone adding arm64 compat code for
that..