On Tue, May 25, 2010 at 12:20:47AM +0100, Jamie Lokier wrote:
> Philippe De Muyter wrote:
> > On Mon, May 24, 2010 at 06:26:08PM +0100, Jamie Lokier wrote:
> > > Philippe De Muyter wrote:
> > > > On Mon, May 24, 2010 at 04:59:18PM +0100, David Howells wrote:
> > > > > Philippe De Muyter <p...@macqel.be> wrote:
> > > > >
> > > > > > > +#else
> > > > > > > +#define TASK_SIZE (0xFFFFFFFFUL)
> > > > > > > +#endif
> > > > > >
> > > > > > Because of do_getname() :
> > > > > >
> > > > > > len = TASK_SIZE - (unsigned long) filename;
> > > > > >
> > > > > > we should rather have
> > > > > >
> > > > > > #define TASK_SIZE (0x100000000ull)
> > > > >
> > > > > Do you guarantee that will work everywhere on a 32-bit system, though?
> > > > >
> > > > > Note that it also makes things slower as gcc has to start using 64-bit
> > > > > arithmetic where it could otherwise use 32-bit arithmetic.
> > > >
> > > > Except if gcc notices that this simplifies to
> > > >
> > > > len = (unsigned long)(-filename);
> > > >
> > > > I don't know if it does.
> > >
> > > It did simplify on the x86 (gcc 4.4) and ARM (gcc 3.4.3) tests I just did.
> > > Also the comparison "addr < TASK_SIZE" simplified to "1".
> > >
> > > However I can imagine some logic like this going awry with that
> > > definition:
> > >
> > > if (addr >= TASK_SIZE || len > TASK_SIZE - addr)
> > > return -EINVAL;
> > >
> > > Think about the case addr == 0.
> >
> > It would give ( 0 || len > (unsigned long)(-addr))
Sorry. I should not have answered that late.
Of course, for (addr == 0), it would give
( 0 || len > TASK_SIZE)
and hence
( 0 || 0)
> > I don't see a problem there.
>
> The problem is it would return -EINVAL for any non-zero value of len,
> which isn't what the code looks like it's meant to do.
No, see above.
Of course, if one computes the maximum len (TASK_SIZE - addr) separately
and stores it in a unsigned 32bit, then there is a design error in the
program.
Philippe
--
Philippe De Muyter phdm at macqel dot be Tel +32 27029044
Macq Electronique SA rue de l'Aeronef 2 B-1140 Bruxelles Fax +32 27029077
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