Hi Yang.

On Wed, Nov 25, 2015 at 02:45:43PM -0800, Yang Shi wrote:

Check if user address is accessible in atomic version __get_user_pages_fast() before walking the page table. And, check if end > start in get_user_pages_fast(), otherwise fallback to slow path.

Two different but related things in one patch is often a bad thing. It would have been better to split it up.

Signed-off-by: Yang Shi yang.shi@linaro.org

Just found slow_irqon label is not defined, added it to avoid compile error.

arch/sparc/mm/gup.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/sparc/mm/gup.c b/arch/sparc/mm/gup.c index 2e5c4fc..cf4fb47 100644 --- a/arch/sparc/mm/gup.c +++ b/arch/sparc/mm/gup.c @@ -173,6 +173,9 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write, addr = start; len = (unsigned long) nr_pages << PAGE_SHIFT; end = start + len;

• if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,

• 			(void __user *)start, len)))
  

• return 0;
  

This change is not justified. Why would we take the time to first do the access_ok() stuff. If this had been an expensive operation then we had made this function slower in the normal case ( assuming there were no access violations in the normal case). When I look at the implementation of access_ok() I get the impression that this is not really a check we need.

access_ok() always returns 1.

local_irq_save(flags); pgdp = pgd_offset(mm, addr); @@ -203,6 +206,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write, addr = start; len = (unsigned long) nr_pages << PAGE_SHIFT; end = start + len;

• if (end < start)

• goto slow_irqon;
  

end can only be smaller than start if there is some overflow. See how end is calculated just the line above.

This looks like a highly suspicious change.

Sam

...

access_ok() always returns 1.

Thanks for pointing it out. And, I didn't notice that gup is just built for SPARC64. I though it is built by both 64 bit and 32 bit.

A follow-up question, is there any reason to just have sparc specific fast gup for 64 bit not for 32 bit?

I do not know - sorry.

...

...

@@ -203,6 +206,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write, addr = start; len = (unsigned long) nr_pages << PAGE_SHIFT; end = start + len;

• if (end < start)

• goto slow_irqon;
  

end can only be smaller than start if there is some overflow. See how end is calculated just the line above.

This looks like a highly suspicious change.

I'm supposed this is used to protect the overflow. I copied the code from other arch. Actually, every arch has this except sparc.

The the other archs are likely confused as well - there is most likely some history behind that can be found if digging a little.

The code is not present in the generic version.

Sam