Re: [PATCH] 2.6.10 - direct-io async short read bug

[Andrew Morton <akpm@osdl.org>]

Badari Pulavarty <pbadari@us.ibm.com> wrote:
>
> On Wed, 2005-03-09 at 11:53, Andrew Morton wrote:
> > Suparna Bhattacharya <suparna@in.ibm.com> wrote:
> > >
> > >  > 	Solaris, which does forcedirectio as a mount option, actually
> > >  > will do buffered I/O on the trailing part.  Consider it like a bounce
> > >  > buffer.  That way they don't DMA the trailing data and succeed the I/O.
> > >  > The I/O returns actual bytes till EOF, just like read(2) is supposed to.
> > >  > 	Either this or a fully DMA'd number 4 is really what we should
> > >  > do.  If security can only be solved via a bounce buffer, who cares?  If
> > >  > the user created themselves a non-aligned file to open O_DIRECT, that's
> > >  > their problem if the last part-sector is negligably slower.
> > > 
> > >  If writes/truncates take care of zeroing out the rest of the sector
> > >  on disk, might we still be OK without having to do the bounce buffer
> > >  thing ?
> > 
> > We can probably rely on the rest of the sector outside i_size being zeroed
> > anyway.  Because if it contains non-zero gunk then the fs already has a
> > problem, and the user can get at that gunk with an expanding truncate and
> > mmap() anyway.
> > 
> 
> Rest of the sector or rest of the block ?

The filesystem-sized block (1<<i_blkbits) which straddles i_size should
have zeroes outside i_size.

There's one situation where it might not be zeroed out, and that's when the
final page is mapped MAP_SHARED and the application modifies that page
outside i_size while writeout is actually in flight.  We can't do much about
that.

> Are you implying that, we
> already do this, so there is no problem reading beyond EOF to user
> buffer ? Or we need to zero out the userbuffer beyond EOF ?

It should be acceptable to assume that the final (1<<i_blkbits) block of
the file contains zeroes outside i_size.

And if it doesn't contain those zeroes, well, applications are able to read
that data already.  Although I wouldn't count that as a security hole: that
data is something which an application wrote there while writing the file,
rather than being left-over uncontrolled stuff.