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
