Date:   Fri, 13 Jul 2018 10:07:41 +0200
From:   Stefan Agner <stefan@...er.ch>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Kees Cook <keescook@...omium.org>,
        Thierry Reding <treding@...dia.com>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Stephen Warren <swarren@...dia.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Stephen Warren <swarren@...dotorg.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Dmitry Osipenko <digetx@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Bero Rosenkränzer 
        <Bernhard.Rosenkranzer@...aro.org>
Subject: Re: [PATCH v2 3/6] ARM: trusted_foundations: do not use naked
 function

On 13.07.2018 01:01, Russell King - ARM Linux wrote:
> On Thu, Jul 12, 2018 at 03:43:10PM -0700, Kees Cook wrote:
>> On Tue, Apr 17, 2018 at 1:11 AM, Thierry Reding <treding@...dia.com> wrote:
>> > On Mon, Apr 16, 2018 at 08:21:09PM +0200, Stefan Agner wrote:
>> >> On 16.04.2018 18:08, Stephen Warren wrote:
>> >> > On 04/16/2018 09:56 AM, Stefan Agner wrote:
>> >> >> On 27.03.2018 14:16, Dmitry Osipenko wrote:
>> >> >>> On 27.03.2018 14:54, Robin Murphy wrote:
>> >> >>>> On 26/03/18 22:20, Dmitry Osipenko wrote:
>> >> >>>>> On 25.03.2018 21:09, Stefan Agner wrote:
>> >> >>>>>> As documented in GCC naked functions should only use Basic asm
>> >> >>>>>> syntax. The Extended asm or mixture of Basic asm and "C" code is
>> >> >>>>>> not guaranteed. Currently this works because it was hard coded
>> >> >>>>>> to follow and check GCC behavior for arguments and register
>> >> >>>>>> placement.
>> >> >>>>>>
>> >> >>>>>> Furthermore with clang using parameters in Extended asm in a
>> >> >>>>>> naked function is not supported:
>> >> >>>>>>     arch/arm/firmware/trusted_foundations.c:47:10: error: parameter
>> >> >>>>>>             references not allowed in naked functions
>> >> >>>>>>                   : "r" (type), "r" (arg1), "r" (arg2)
>> >> >>>>>>                          ^
>> >> >>>>>>
>> >> >>>>>> Use a regular function to be more portable. This aligns also with
>> >> >>>>>> the other smc call implementations e.g. in qcom_scm-32.c and
>> >> >>>>>> bcm_kona_smc.c.
>> >> >>>>>>
>> >> >>>>>> Cc: Dmitry Osipenko <digetx@...il.com>
>> >> >>>>>> Cc: Stephen Warren <swarren@...dia.com>
>> >> >>>>>> Cc: Thierry Reding <treding@...dia.com>
>> >> >>>>>> Signed-off-by: Stefan Agner <stefan@...er.ch>
>> >> >>>>>> ---
>> >> >>>>>> Changes in v2:
>> >> >>>>>> - Keep stmfd/ldmfd to avoid potential ABI issues
>> >> >>>>>>
>> >> >>>>>>    arch/arm/firmware/trusted_foundations.c | 14 +++++++++-----
>> >> >>>>>>    1 file changed, 9 insertions(+), 5 deletions(-)
>> >> >>>>>>
>> >> >>>>>> diff --git a/arch/arm/firmware/trusted_foundations.c
>> >> >>>>>> b/arch/arm/firmware/trusted_foundations.c
>> >> >>>>>> index 3fb1b5a1dce9..689e6565abfc 100644
>> >> >>>>>> --- a/arch/arm/firmware/trusted_foundations.c
>> >> >>>>>> +++ b/arch/arm/firmware/trusted_foundations.c
>> >> >>>>>> @@ -31,21 +31,25 @@
>> >> >>>>>>      static unsigned long cpu_boot_addr;
>> >> >>>>>>    -static void __naked tf_generic_smc(u32 type, u32 arg1, u32 arg2)
>> >> >>>>>> +static void tf_generic_smc(u32 type, u32 arg1, u32 arg2)
>> >> >>>>>>    {
>> >> >>>>>> +    register u32 r0 asm("r0") = type;
>> >> >>>>>> +    register u32 r1 asm("r1") = arg1;
>> >> >>>>>> +    register u32 r2 asm("r2") = arg2;
>> >> >>>>>> +
>> >> >>>>>>        asm volatile(
>> >> >>>>>>            ".arch_extension    sec\n\t"
>> >> >>>>>> -        "stmfd    sp!, {r4 - r11, lr}\n\t"
>> >> >>>>>> +        "stmfd    sp!, {r4 - r11}\n\t"
>> >> >>>>>>            __asmeq("%0", "r0")
>> >> >>>>>>            __asmeq("%1", "r1")
>> >> >>>>>>            __asmeq("%2", "r2")
>> >> >>>>>>            "mov    r3, #0\n\t"
>> >> >>>>>>            "mov    r4, #0\n\t"
>> >> >>>>>>            "smc    #0\n\t"
>> >> >>>>>> -        "ldmfd    sp!, {r4 - r11, pc}"
>> >> >>>>>> +        "ldmfd    sp!, {r4 - r11}\n\t"
>> >> >>>>>>            :
>> >> >>>>>> -        : "r" (type), "r" (arg1), "r" (arg2)
>> >> >>>>>> -        : "memory");
>> >> >>>>>> +        : "r" (r0), "r" (r1), "r" (r2)
>> >> >>>>>> +        : "memory", "r3", "r12", "lr");
>> >> >>>>>
>> >> >>>>> Although seems "lr" won't be affected by SMC invocation because it should be
>> >> >>>>> banked and hence could be omitted entirely from the code. Maybe somebody could
>> >> >>>>> confirm this.
>> >> >>>> Strictly per the letter of the architecture, the SMC could be trapped to Hyp
>> >> >>>> mode, and a hypervisor might clobber LR_usr in the process of forwarding the
>> >> >>>> call to the firmware secure monitor (since Hyp doesn't have a banked LR of its
>> >> >>>> own). Admittedly there are probably no real systems with the appropriate
>> >> >>>> hardware/software combination to hit that, but on the other hand if this gets
>> >> >>>> inlined where the compiler has already created a stack frame then an LR clobber
>> >> >>>> is essentially free, so I reckon we're better off keeping it for reassurance.
>> >> >>>> This isn't exactly a critical fast path anyway.
>> >> >>>
>> >> >>> Okay, thank you for the clarification.
>> >> >>
>> >> >> So it seems this change is fine?
>> >> >>
>> >> >> Stephen, you picked up changes for this driver before, is this patch
>> >> >> going through your tree?
>> >> >
>> >> > You had best ask Thierry; he's taken over Tegra maintenance upstream.
>> >> > But that said, don't files in arch/arm go through Russell?
>> >>
>> >> I think the last patches applied to that file went through your tree.
>> >>
>> >> Thierry, Russel, any preferences?
>> >
>> > I don't mind picking this up into the Tegra tree. Might be a good idea
>> > to move this into drivers/firmware, though, since that's where all the
>> > other firmware-related drivers reside.
>> >
>> > Firmware code, such as the BPMP driver, usually goes through ARM-SoC
>> > these days. I think this is in the same category.
>> >
>> > Russell, any objections to me picking this patch up and moving it into
>> > drivers/firmware?
>>
>> Please take this -- without it I'm seeing build failures on the arm
>> allmodconfig under gcc 7.3.0:
> 
> Sorry, I'd completely missed this... now replied on the original patch.

Thierry merged this patch just two days ago, so it is already in -next.

--
Stefan

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