Re: [PATCH 1 of 2] block_page_mkwrite() Implementation V2

Chris Mason
2007-05-16T13:13:59Z

On Wed, May 16, 2007 at 11:04:11PM +1000, Nick Piggin wrote:
> Chris Mason wrote:
> >On Wed, May 16, 2007 at 08:09:19PM +0800, David Woodhouse wrote:
> >
> >>On Wed, 2007-05-16 at 11:19 +0100, David Howells wrote:
> >>
> >>>The start and end points passed to block_prepare_write() delimit the
> >>>region of
> >>>the page that is going to be modified. This means that prepare_write()
> >>>doesn't need to fill it in if the page is not up to date.
> >>
> >>Really? Is it _really_ going to be modified? Even if the pointer
> >>userspace gave to write() is bogus, and is going to fault half-way
> >>through the copy_from_user()?
> >
> >
> >This is why there are so many variations on copy_from_user that zero on
> >faults. One way or another, the prepare_write/commit_write pair are
> >responsible for filling it in.
>
> I'll add to David's question about David's comment on David's patch, yes
> it will be modified but in that case it would be zero-filled as Chris
> says. However I believe this is incorrect behaviour.
>
> It is possible to easily fix that so it would only happen via a tiny race
> window (where the source memory gets unmapped at just the right time)
> however nobody seemed to interested (just by checking the return value of
> fault_in_pages_readable).
>
> The buffered write patches I'm working on fix that (among other things) of
> course. But they do away with prepare_write and introduce new aops, and
> they indeed must not expect the full range to have been written to.

I was also wrong to say prepare_write and commit_write are
responsible, they work together with their callers to make the right
things happen. Oh well, so much for trying to give a short answer for a
chunk of code full of corner cases ;)

-chris