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

From: Catalin Marinas <[email-masked]>
Date: Mon, 16 Mar 2020 11:22:06 +0000

On Mon, Mar 16, 2020 at 10:55:00AM +0000, Vincenzo Frascino wrote:
> On 3/16/20 10:34 AM, Catalin Marinas wrote:
> >> I tried to fine grain the headers as much as I could in order to avoid
> >> unneeded/unwanted inclusions:
> >>  * TASK_SIZE_32 is used to verify ABI consistency on vdso32 (please refer to
> >>    arch/arm64/kernel/vdso32/vgettimeofday.c).
> > 
> > I see. But the test is probably useless. With 4K pages, TASK_SIZE_32 is
> > 1UL << 32, so you can't have a u32 greater than this. So I'd argue that
> > the ABI compatibility here doesn't matter.
> > 
> > With 16K or 64K pages, TASK_SIZE_32 is slightly smaller but arm32 never
> > supported it.
> > 
> > What's the side-effect of dropping this check altogether?
> 
> The main side-effect is that arm32 and arm64 compat have a different behavior,
> that it is what we want to avoid.
> 
> The vdsotest [1] I am using, verifies all the side conditions with respect to
> the ABI, which we are now compatible with. Removing those checks would break
> this condition.

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?

-- 
Catalin

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