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

[Carlos Maiolino <cmaiolino@redhat.com>]

Hi Matthew.

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.
> 

Thanks for the review, It's the very first time I try to 'mix' __user pointers
with internal-only kernel code, I did some research on how other parts of kernel
handles it, so I'm kind of stepping on eggs here.

I do have a question about your point though:

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

Yes, I noticed it, and, that's why I added the FIEMAP_KERNEL_FIBMAP flag. This
is the changes I did to fiemap_fill_next_extent():

@@ -117,8 +117,14 @@ int fiemap_fill_next_extent(struct fiemap_extent_info *fieinfo, u64 logical,
        extent.fe_flags = flags;
 
        dest += fieinfo->fi_extents_mapped;
-       if (copy_to_user(dest, &extent, sizeof(extent)))
-               return -EFAULT;
+
+       if (fieinfo->fi_flags & FIEMAP_KERNEL_FIBMAP) {
+               memcpy((__force struct fiemap_extent *)dest, &extent,
+                      sizeof(extent));
+       } else {
+               if (copy_to_user(dest, &extent, sizeof(extent)))
+                       return -EFAULT;
+       }


So, my patch is going to use memcpy() directly, instead of copy_to_user(), if
the request comes from the bmap() function, other than that, I'm not really sure
I understand your point here.


-- 
Carlos