Re: [PATCH 1/2] mm/madvise: allow process_madvise operations on entire memory range

[Christoph Hellwig <hch@infradead.org>]

On Fri, Dec 11, 2020 at 09:27:46PM +0100, Jann Horn wrote:
> > Can we just use one element in iovec to indicate entire address rather
> > than using up the reserved flags?
> >
> >         struct iovec {
> >                 .iov_base = NULL,
> >                 .iov_len = (~(size_t)0),
> >         };
> 
> In addition to Suren's objections, I think it's also worth considering
> how this looks in terms of compat API. If a compat process does
> process_madvise() on another compat process, it would be specifying
> the maximum 32-bit number, rather than the maximum 64-bit number, so
> you'd need special code to catch that case, which would be ugly.
> 
> And when a compat process uses this API on a non-compat process, it
> semantically gets really weird: The actual address range covered would
> be larger than the address range specified.
> 
> And if we want different access checks for the two flavors in the
> future, gating that different behavior on special values in the iovec
> would feel too magical to me.
> 
> And the length value SIZE_MAX doesn't really make sense anyway because
> the length of the whole address space would be SIZE_MAX+1, which you
> can't express.
> 
> So I'm in favor of a new flag, and strongly against using SIZE_MAX as
> a magic number here.

Yes, using SIZE_MAX is a horrible interface in this case.  I'm not
a huge fan of a flag either.  What is the use case for the madvise
to all of a processes address space anyway?