Messages in this thread | | | Date | Fri, 19 May 2006 13:07:12 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 5/6] nfs: check all iov segments for correct memory access rights |
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Chuck Lever <cel@citi.umich.edu> wrote: > > Andrew Morton wrote: > >> + if (unlikely(!access_ok(type, buf, len))) { > >> + retval = -EFAULT; > >> + goto out; > >> + } > > > > Now what's up here? Why does NFS, at this level, care about the page's > > virtual address? get_user_pages() will handle that? > > I guess I'm not clear on what behavior is desired for scatter/gather if > one of the segments in an iov fails. > > If one of the iov's will cause an EFAULT, how is that reported back to > the application,
If nothing has yet been transferred to/from userspace, return -EFAULT.
If something has been transferred, return the number of bytes transferred.
> and what happens to the I/O being requested in the > other segments of the vector?
The filesystem driver needs to handle it somehow.
> When do we use an "all or nothing" > semantic, and when is it OK for some segments to fail?
Actually, fs/direct-io.c cheats and doesn't implement the correct semantics. It returns either all-bytes-transferred or -EFOO. The way I justify that is to point out that returning a partial transfer count doesn't make a lot of sense when the I/Os could complete in any order - yes, we know how much data got transferred, but we don't know whereabouts in the user's memory that data ended up. So the user cannot trust _any_ of it.
NFS direct-io can do the same.
But access_ok() isn't sufficient. All it tells you is that the virtual address is a legal one for an application. But we could still get EFAULT when trying to access it.
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