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

From: Catalin Marinas <[email-masked]>
Date: Mon, 16 Mar 2020 14:43:47 +0000

On Mon, Mar 16, 2020 at 01:35:17PM +0000, Vincenzo Frascino wrote:
> On 3/16/20 11:22 AM, Catalin Marinas wrote:
> > As I said above, I don't see how removing 'if ((u32)ts >= (1UL << 32))'
> > makes any difference. This check was likely removed by the compiler
> > already.
> > 
> > Also, userspace doesn't have a trivial way to figure out TASK_SIZE and I
> > can't see anything that tests this in the vdsotest (though I haven't
> > spent that much time looking). If it's hard-coded, note that arm32
> > TASK_SIZE is different from TASK_SIZE_32 on arm64.
> > 
> > Can you tell what actually is failing in vdsotest if you remove the
> > TASK_SIZE_32 checks in the arm64 compat vdso?
> 
> 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).

> 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)

-- 
Catalin

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