linux-kernel - Re: [PATCH v2 3/6] ARM: trusted_foundations: do not use naked function

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Date:   Mon, 16 Apr 2018 10:08:02 -0600
From:   Stephen Warren <swarren@...dotorg.org>
To:     Stefan Agner <stefan@...er.ch>, Thierry Reding <treding@...dia.com>
Cc:     Stephen Warren <swarren@...dia.com>,
        Robin Murphy <robin.murphy@....com>, linux@...linux.org.uk,
        ard.biesheuvel@...aro.org, arnd@...db.de, nicolas.pitre@...aro.org,
        marc.zyngier@....com, behanw@...verseincode.com,
        keescook@...omium.org, Bernhard.Rosenkranzer@...aro.org,
        mka@...omium.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Dmitry Osipenko <digetx@...il.com>
Subject: Re: [PATCH v2 3/6] ARM: trusted_foundations: do not use naked
 function

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?