Messages in this thread | | | Date | Wed, 16 May 2007 23:04:11 +1000 | From | Nick Piggin <> | Subject | Re: [PATCH 1 of 2] block_page_mkwrite() Implementation V2 |
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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.
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