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

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Date:   Tue, 27 Mar 2018 12:54:58 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Dmitry Osipenko <digetx@...il.com>, Stefan Agner <stefan@...er.ch>,
        linux@...linux.org.uk, ard.biesheuvel@...aro.org, arnd@...db.de
Cc:     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,
        Stephen Warren <swarren@...dia.com>,
        Thierry Reding <treding@...dia.com>
Subject: Re: [PATCH v2 3/6] ARM: trusted_foundations: do not use naked
 function

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.

Robin.