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

From: Christophe Leroy

Sent: 02 September 2020 14:25
Le 02/09/2020 à 15:13, David Laight a écrit :

quoted

From: Christoph Hellwig

quoted

Sent: 02 September 2020 13:37

On Wed, Sep 02, 2020 at 08:15:12AM +0200, Christophe Leroy wrote:

quoted

quoted

-		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 don't even need to in copy_to/from_user() provided
it always does a forwards copy.
(Rather that copying the last word first for misaligned lengths.)

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)