linux-kernel - Re: [PATCH v4 18/26] arm64: vdso32: Replace TASK_SIZE

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Date:   Fri, 20 Mar 2020 13:05:14 +0000
From:   Vincenzo Frascino <vincenzo.frascino@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        linux-mips@...r.kernel.org, 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 v4 18/26] arm64: vdso32: Replace TASK_SIZE_32 check in
 vgettimeofday

Hi Catalin,

On 3/19/20 6:10 PM, Catalin Marinas wrote:
> Hi Vincenzo,
> 
> On Thu, Mar 19, 2020 at 12:38:42PM +0000, Vincenzo Frascino wrote:
>> On 3/18/20 6:36 PM, Catalin Marinas wrote:
>>> On Wed, Mar 18, 2020 at 04:14:26PM +0000, Vincenzo Frascino wrote:
>>>> On 3/17/20 5:48 PM, Catalin Marinas wrote:
>>>>> So clock_gettime() on arm32 always falls back to the syscall?
>>>>
>>>> This seems not what you asked, and I think I answered accordingly. Anyway, in
>>>> the case of arm32 the error code path is handled via syscall fallback.
>>>>
>>>> Look at the code below as an example (I am using getres because I know this
>>>> email will be already too long, and I do not want to add pointless code, but the
>>>> concept is the same for gettime and the others):
>>>>
>>>> static __maybe_unused
>>>> int __cvdso_clock_getres(clockid_t clock, struct __kernel_timespec *res)
>>>> {
>>>> 	int ret = __cvdso_clock_getres_common(clock, res);
>>>>
>>>> 	if (unlikely(ret))
>>>> 		return clock_getres_fallback(clock, res);
>>>> 	return 0;
>>>> }
>>>>
>>>> When the return code of the "vdso" internal function returns an error the system
>>>> call is triggered.
>>>
>>> But when __cvdso_clock_getres_common() does *not* return an error, it
>>> means that it handled the clock_getres() call without a fallback to the
>>> syscall. I assume this is possible on arm32. When the clock_getres() is
>>> handled directly (not as a syscall), why doesn't arm32 need the same
>>> (res >= TASK_SIZE) check?
>>
>> Ok, I see what you mean.
> 
> I'm not sure.
> 
Thank you for the long chat this morning. As we agreed I am going to repost the
patches removing the checks discussed in this thread and we will address the
syscall ABI difference subsequently with a different series.

-- 
Regards,
Vincenzo