Re: [PATCH v3 18/26] arm64: Introduce asm/vdso/processor.h

Vincenzo Frascino
2020-03-16T15:33:30Z

On 3/16/20 2:43 PM, Catalin Marinas wrote[...]

>> To me does not seem optimized out. Which version of the compiler are you using?
>
> I misread the #ifdef'ery in asm/processor.h. So with 4K pages,
> TASK_SIZE_32 is (1UL<<32)-PAGE_SIZE. However, with 64K pages _and_
> CONFIG_KUSER_HELPERS, TASK_SIZE_32 is 1UL<<32 and the check is removed
> by the compiler.
>
> With the 4K build, __vdso_clock_gettime starts as:
>
> 00000194 <__vdso_clock_gettime>:
> 194: f511 5f80 cmn.w r1, #4096 ; 0x1000
> 198: d214 bcs.n 1c4 <__vdso_clock_gettime+0x30>
> 19a: b5b0 push {r4, r5, r7, lr}
> ...
> 1c4: f06f 000d mvn.w r0, #13
> 1c8: 4770 bx lr
>
> With 64K pages:
>
> 00000194 <__vdso_clock_gettime>:
> 194: b5b0 push {r4, r5, r7, lr}
> ...
> 1be: bdb0 pop {r4, r5, r7, pc}
>
> I haven't tried but it's likely that the vdsotest fails with 64K pages
> and compat enabled (requires EXPERT).
>

This makes more sense. Thanks for the clarification.

I agree on the behavior of 64K pages and I think as well that the
"compatibility" issue is still there. However as you correctly stated in your
first email arm32 never supported 16K or 64K pages, hence I think we should not
be concerned about compatibility in this cases.

To make it more explicit we could make COMPAT_VDSO on arm64 depend on
ARM64_4K_PAGES. What do you think?

>> Please find below the list of errors for clock_gettime (similar for the other):
>>
>> passing UINTPTR_MAX to clock_gettime (VDSO): terminated by unexpected signal 7
>> clock-gettime-monotonic/abi: 1 failures/inconsistencies encountered
>
> Ah, so it uses UINTPTR_MAX in the test. Fair enough but I don't think
> the arm64 check is entirely useful. On arm32, the check was meant to
> return -EFAULT for addresses beyond TASK_SIZE that may enter into the
> kernel or module space. On arm64 compat, the kernel space is well above
> the reach of the 32-bit code.
>
> If you want to preserve some compatibility for this specific test, what
> about checking for wrapping around 0, I think it would make more sense.
> Something like:
>
> if ((u32)ts > UINTPTR_MAX - sizeof(*ts) + 1)
>

Ok, sounds good to me. But it is something that this patch series inherited,
hence I would prefer to send a separate patch that introduces what you are
proposing and removes TASK_SIZE_32 from the headers. How does it sound?

--
Regards,
Vincenzo