Re: [PATCH 10/10] powerpc: remove address space overrides using set_fs()

[Christophe Leroy]

Le 02/09/2020 à 15:51, David Laight a écrit :
> From: Christophe Leroy
> > Sent: 02 September 2020 14:25
> > Le 02/09/2020 à 15:13, David Laight a écrit :
> > > From: Christoph Hellwig
> > > > Sent: 02 September 2020 13:37
> > > >
> > > > On Wed, Sep 02, 2020 at 08:15:12AM +0200, Christophe Leroy wrote:
> > > > > > -		return 0;
> > > > > > -	return (size == 0 || size - 1 <= seg.seg - addr);
> > > > > > +	if (addr >= TASK_SIZE_MAX)
> > > > > > +		return false;
> > > > > > +	if (size == 0)
> > > > > > +		return false;
> > > > >
> > > > > __access_ok() was returning true when size == 0 up to now. Any reason to
> > > > > return false now ?
> > > >
> > > > No, this is accidental and broken.  Can you re-run your benchmark with
> > > > this fixed?
> > >
> > > Is TASK_SIZE_MASK defined such that you can do:
> > >
> > > 	return (addr | size) < TASK_SIZE_MAX) || !size;
> >
> > TASK_SIZE_MAX will usually be 0xc0000000
> >
> > With:
> > addr = 0x80000000;
> > size = 0x80000000;
> >
> > I expect it to fail ....
> >
> > With the formula you propose it will succeed, won't it ?
>
> Hmmm... Was i getting confused about some comments for 64bit
> about there being such a big hole between valid user and kernel
> addresses that it was enough to check that 'size < TASK_SIZE_MAX'.
>
> That would be true for 64bit x86 (and probably ppc (& arm??))
> if TASK_SIZE_MAX were 0x4 << 60.
> IIUC the highest user address is (much) less than 0x0 << 60
> and the lowest kernel address (much) greater than 0xf << 60
> on all these 64bit platforms.
>
> Actually if doing access_ok() inside get_user() you don't
> need to check the size at all.

You mean on 64 bit or on any platform ?

What about a word write to 0xbffffffe, won't it overwrite 0xc0000000 ?

> You don't even need to in copy_to/from_user() provided
> it always does a forwards copy.

Do you mean due to the gap ?
Is it garantied to be a gap ? Even on a 32 bits having TASK_SIZE set to 
0xc0000000 and PAGE_OFFSET set to the same ?


Christophe