Re: [PATCH 2/2] Use fiemap internal infra-structure to handle FIBMAP

From: Matthew Wilcox <willy@infradead.org>
To: Carlos Maiolino <cmaiolino@redhat.com>
Cc: linux-fsdevel@vger.kernel.org, hch@lst.de, sandeen@redhat.com,
	david@fromorbit.com
Subject: Re: [PATCH 2/2] Use fiemap internal infra-structure to handle FIBMAP
Date: Wed, 5 Sep 2018 07:31:47 -0700	[thread overview]
Message-ID: <20180905143147.GC3729@bombadil.infradead.org> (raw)
In-Reply-To: <20180905135748.30098-3-cmaiolino@redhat.com>

On Wed, Sep 05, 2018 at 03:57:48PM +0200, Carlos Maiolino wrote:
> +	if (inode->i_op->fiemap) {
> +		fextent.fe_logical = 0;
> +		fextent.fe_physical = 0;
> +		fieinfo.fi_flags = FIEMAP_KERNEL_FIBMAP;
> +		fieinfo.fi_extents_max = 1;
> +		fieinfo.fi_extents_start = (__force struct fiemap_extent __user *) &fextent;
> +
> +		error = inode->i_op->fiemap(inode, &fieinfo, start, 1);

You'd have to play games with set_fs() and friends if you want to do this.
The fiemap implementation is going to access fi_extents_start with a call
to copy_to_user() and for machines with a 4G/4G split, you need that
address to be interpreted as kernel space, not user space.

See fiemap_fill_next_extent():

        struct fiemap_extent __user *dest = fieinfo->fi_extents_start;
...
        if (copy_to_user(dest, &extent, sizeof(extent)))
                return -EFAULT;