Re: process 'stuck' at exit.

[Thomas Gleixner]

On Tue, 10 Dec 2013, Linus Torvalds wrote:

> On Tue, Dec 10, 2013 at 2:19 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Shouldn't we do something like the attached?
> 
> So I think that kernel/futex.c part of the patch might be a good idea,
> but on x86-64 (which is what Dave is running), the
> 
>         if (end >> __VIRTUAL_MASK_SHIFT)
> 
> test in get_user_pages_fast() should have been equivalent. And even on
> 32-bit, we do check the _PAGE_USER bits in the page tables, so I guess
> it's all good on a get_user_pages_fast() side.
> 
> So never mind. It's not the address checking.
> 
> And I think I see what's up.
> 
> I think what happens is:
>  - get_user_pages_fast(address, 1, 1, &page) fails (because it's read-only)
>  - get_user_pages_fast(address, 1, 0, &page) succeeds and gets a large-page
>  - __get_user_pages_fast(address, 1, 1, &page) fails (because it's read-only).
> 
> so what triggers this is likely that Dave now does large-pages, and
> one of them is a read-only mapping.
> 
> So I would suggest replacing the second "1" in the
> __get_user_pages_fast() call with a "!ro" instead. So how about this
> second patch instead (the access_ok() move remains).

Yes, that's what I decoded as well.

But how does the access_ok() move do anything helpful here?

We really need it for the fastpath !fshared case, but for the fshared
case you actively break working code, because you force a VERIFY_WRITE
check into it. The VERIFY_WRITE is necessary for !fshared, because
there is no way that one thread can map the futex RO and the other RW,
right?

But for fshared it's legitimate to have a RO mapping if you just wait
for the futex. Note, that futexes are (ab)used as user space
waitqueues, so RO makes sense. And your move would break them.

If [__]get_user_pages_fast() does not do the right checks, then we
need to fix that and not add random access_ok() checks into the call
sites.

Thanks,

	tglx





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