linux-kernel - Re: [PATCH v4 5/5] powerpc/vdso: Wire up getrandom() vDSO implementation on PPC64

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Message-ID: <c411b0c6-1806-4e4d-8bcf-51f0747fcd19@csgroup.eu>
Date: Mon, 2 Sep 2024 16:16:48 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>,
 Naveen N Rao <naveen@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling
 <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-kselftest@...r.kernel.org,
 llvm@...ts.linux.dev, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
 linux-trace-kernel@...r.kernel.org,
 Adhemerval Zanella <adhemerval.zanella@...aro.org>,
 Xi Ruoyao <xry111@...111.site>
Subject: Re: [PATCH v4 5/5] powerpc/vdso: Wire up getrandom() vDSO
 implementation on PPC64

Le 02/09/2024 à 16:00, Jason A. Donenfeld a écrit :
> On Mon, Sep 02, 2024 at 03:12:47PM +0200, Christophe Leroy wrote:
>>
>>
>> Le 02/09/2024 à 14:41, Jason A. Donenfeld a écrit :
>>> On Mon, Sep 02, 2024 at 02:04:42PM +0200, Christophe Leroy wrote:
>>>>    SYM_FUNC_START(__arch_chacha20_blocks_nostack)
>>>>    #ifdef __powerpc64__
>>>> -	blr
>>>> +	std	r5, -216(r1)
>>>> +
>>>> +	std	r14, -144(r1)
>>>> +	std	r15, -136(r1)
>>>> +	std	r16, -128(r1)
>>>> +	std	r17, -120(r1)
>>>> +	std	r18, -112(r1)
>>>> +	std	r19, -104(r1)
>>>> +	std	r20, -96(r1)
>>>> +	std	r21, -88(r1)
>>>> +	std	r22, -80(r1)
>>>> +	std	r23, -72(r1)
>>>> +	std	r24, -64(r1)
>>>> +	std	r25, -56(r1)
>>>> +	std	r26, -48(r1)
>>>> +	std	r27, -40(r1)
>>>> +	std	r28, -32(r1)
>>>> +	std	r29, -24(r1)
>>>> +	std	r30, -16(r1)
>>>> +	std	r31, -8(r1)
>>>>    #else
>>>>    	stwu	r1, -96(r1)
>>>>    	stw	r5, 20(r1)
>>>> +#ifdef __BIG_ENDIAN__
>>>>    	stmw	r14, 24(r1)
>>>> +#else
>>>> +	stw	r14, 24(r1)
>>>> +	stw	r15, 28(r1)
>>>> +	stw	r16, 32(r1)
>>>> +	stw	r17, 36(r1)
>>>> +	stw	r18, 40(r1)
>>>> +	stw	r19, 44(r1)
>>>> +	stw	r20, 48(r1)
>>>> +	stw	r21, 52(r1)
>>>> +	stw	r22, 56(r1)
>>>> +	stw	r23, 60(r1)
>>>> +	stw	r24, 64(r1)
>>>> +	stw	r25, 68(r1)
>>>> +	stw	r26, 72(r1)
>>>> +	stw	r27, 76(r1)
>>>> +	stw	r28, 80(r1)
>>>> +	stw	r29, 84(r1)
>>>> +	stw	r30, 88(r1)
>>>> +	stw	r31, 92(r1)
>>>> +#endif
>>>> +#endif
>>>
>>> This confuses me. Why are you adding code to the !__powerpc64__ branch
>>> in this commit? (Also, why does stmw not work on LE?)
>>
>> That's for the VDSO32 ie running 32 bits binaries on a 64 bits kernel.
>>
>> "Programming Environments Manual for 32-Bit Implementations of the
>> PowerPC™ Architecture" say: In some implementations operating with
>> little-endian byte order, execution of an lmw or stmw instruction
>> causes the system alignment error handler to be invoked
>>
>> And GCC doesn't like it either:
>>
>> tools/arch/powerpc/vdso/vgetrandom-chacha.S:84: Error: `stmw' invalid
>> when little-endian
> 
> Does it make sense to do all the 32-bit stuff in the PPC32 commit (and
> then you can introduce the selftests there without the error you
> mentioned), and then add the 64-bit stuff in this commit?

Can do that, but there will still be a problem with chacha selftests if 
I don't opt-out the entire function content when it is ppc64. It will 
build properly but if someone runs it on a ppc64 it will likely crash 
because only the low 32 bits of registers will be saved.

That's the reason why I really prefered the approach where I set 
something in vdso_config.h so that the assembly is used only for 
powerpc32 and when building powerpc64 the assembly part is kept out and 
vdso_test_chacha simply tells it is not supported.

Christophe