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

[Christoph Hellwig <hch@lst.de>]

On Wed, Sep 05, 2018 at 07:31:47AM -0700, Matthew Wilcox wrote:
> 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():

Yeah.  I think we need to pass fiemap_fill_next_extent() as a function
pointer to fiemap in a prep patch, and then pass a different pointer
for the in-kernel usage.  Which is good API design anyway, so we should
have done this from the beginning.