[PATCH 1/3] arm/syscalls: Move address limit check in loop

[Thomas Garnier]

On Wed, Jul 26, 2017 at 5:02 AM, Will Deacon <will.deacon at arm.com> wrote:
> On Tue, Jul 25, 2017 at 01:01:17PM -0700, Thomas Garnier wrote:
>> On Tue, Jul 25, 2017 at 3:38 AM, Russell King - ARM Linux
>> <linux at armlinux.org.uk> wrote:
>> > On Tue, Jul 25, 2017 at 01:28:01PM +0300, Leonard Crestez wrote:
>> >> On Mon, 2017-07-24 at 10:07 -0700, Thomas Garnier wrote:
>> >> > On Wed, Jul 19, 2017 at 10:58 AM, Thomas Garnier <thgarnie at google.com
>> >> > > wrote:
>> >> > >
>> >> > > The work pending loop can call set_fs after addr_limit_user_check
>> >> > > removed the _TIF_FSCHECK flag. To prevent the infinite loop, move
>> >> > > the addr_limit_user_check call at the beginning of the loop.
>> >> > >
>> >> > > Fixes: 73ac5d6a2b6a ("arm/syscalls: Check address limit on user-
>> >> > > mode return")
>> >> > > Reported-by: Leonard Crestez <leonard.crestez at nxp.com>
>> >> > > Signed-off-by: Thomas Garnier <thgarnie at google.com>
>> >>
>> >> > Any comments on this patch set?
>> >>
>> >> Tested-by: Leonard Crestez <leonard.crestez at nxp.com>
>> >>
>> >> This appears to fix the original issue of failing to boot from NFS when
>> >> there are lots of alignment faults. But this is a very basic test
>> >> relative to the reach of this change.
>> >>
>> >> However the original patch has been in linux-next for a while and
>> >> apparently nobody else noticed system calls randomly hanging on arm.
>> >>
>> >> I assume maintainers need to give their opinion.
>> >
>> > I've already stated my opinion, which is different from what Linus has
>> > requested of Thomas.  IMHO, the current approach is going to keep on
>> > causing problems along the lines that I've already pointed out.
>>
>> I understand. Do you think this problem apply to arm64 as well?
>
> It's probably less of an issue for arm64 because we don't take alignment
> faults from the kernel and I think the perf case would resolve itself by
> throttling the event. However, I also don't see the advantage of doing
> this in the work loop as opposed to leaving it until we're actually doing
> the return to userspace.

I think the idea is doing the check as early as possible to catch any
error before any additional logic.

>
> I looked to see what you've done for x86, but it looks like you check/clear
> the flag before the work pending loop (exit_to_usermode_loop), which
> subsequently re-enables interrupts and exits when
> EXIT_TO_USERMODE_LOOP_FLAGS are all clear. Since TIF_FSCHECK isn't included
> in those flags, what stops it being set again by an irq and remaining set
> for the return to userspace?

Nothing, I plan to improve the x86 logic later. I focused on ARM/ARM64
right now based on Leonard report.

For the next iteration, I plan on having an updated version of the
previous implementation for ARM. I will send it soon.

>
> Will



-- 
Thomas