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

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Date:   Mon, 16 Mar 2020 11:22:06 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org,
        clang-built-linux@...glegroups.com, x86@...nel.org,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        Paul Burton <paul.burton@...s.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Collingbourne <pcc@...gle.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrei Vagin <avagin@...nvz.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <Mark.Rutland@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v3 18/26] arm64: Introduce asm/vdso/processor.h

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