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

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Date:   Sun, 26 Jul 2020 18:34:22 +0200
From:   Christoph Hellwig <hch@....de>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>, x86@...nel.org,
        Jan Kara <jack@...e.com>, linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH 4/4] quota: simplify the quotactl compat handling

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..