Re: [Xen-devel] Error ignored in xc_map_foreign_pages

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-02-12 at 18:53 -0800, Mukesh Rathor wrote:
> It appears that xc_map_foreign_pages() handles return incorrectly :

libxc is a complete mess in this regard (error handling) generally.

IIRC there is some subtlety on the Linux privcmd side wrt partial
failure of batches, particularly once paging/sharing gets involved and
you want to retry the missing subset, which might have something to do
with this. David and Andres might know if that is relevant here.
 
>     res = xc_map_foreign_bulk(xch, dom, prot, arr, err, num);
>     if (res) {
>         for (i = 0; i < num; i++) {
>             if (err[i]) {
>                 errno = -err[i];
>                 munmap(res, num * PAGE_SIZE);
>                 res = NULL;
>                 break;
>             }
>         }
>     }
> 
> The add to_physmap batch'd interface  actually will store errors
> in the err array, and return 0 unless EFAULT or something like that.
> See xenmem_add_to_physmap_batch(). The case I'm looking at, xentrace
> calls here to map page which fails, but the return is 0 as the error is
> succesfully copied by xen. But the error is missed above since res is 0.
> xentrace does something again, and that causes xen crash. 
> 
> It appears the fix could be just removing the check for res above...
> 
>     res = xc_map_foreign_bulk(xch, dom, prot, arr, err, num);
>     for (i = 0; i < num; i++) {
>         if (err[i]) {
>          .....
> 
> What do you guys think?
> 
> thanks,
> Mukesh



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