Re: [Xen-devel] The hypercall will fail and return EFAULT when the page becomes COW by forking process in linux

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2012-08-07 at 12:05 +0100, Wangzhenguo wrote:
> > -----Original Message-----
> > From: Ian Campbell [mailto:Ian.Campbell@xxxxxxxxxx]
> > Sent: Tuesday, August 07, 2012 6:07 PM
> 
> > The entire point of the hypercall buffer is that it needs to be safe
> > for use as a hypercall argument, therefore it does need to be protected.
> 
> We won't protect pages in hypercall buffer cache. Hypercall buffer
> cache belongs to xc_interface, parent process calls xc_interface_open
> to get xc_interface handler, after fork, child process inherits the
> handler, and also inherits hypercall buffer cache which belongs to it.
> It will cause segment fault to access pages in the hypercall buffer
> cache by being delivered   to hypercall, because they are not
> invalidate in the child process. So, We need restore the status of
> pages before putting they to hypercall buffer cache, and prevent pages
> from COW after being allocated in hyprecall buffer cache.

I don't think we should expect it to be valid to keep an xc interface
handle open after a fork. The child should open a fresh handle if it
wants to keep interacting with xc.

Ian.


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