linux-kernel - Re: [PATCH -next v5 2/8] arm64: extable: make uaaccess helper use extable type EX_TYPE_UACCESS_ERR

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Message-ID: <684f0362-6e58-753d-32e1-112c6ffe6d12@huawei.com>
Date:   Mon, 20 Jun 2022 10:59:12 +0800
From:   Tong Tiangen <tongtiangen@...wei.com>
To:     Mark Rutland <mark.rutland@....com>
CC:     James Morse <james.morse@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Robin Murphy <robin.murphy@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Catalin Marinas" <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        "Alexander Viro" <viro@...iv.linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, <linuxppc-dev@...ts.ozlabs.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Xie XiuQi <xiexiuqi@...wei.com>,
        Guohanjun <guohanjun@...wei.com>
Subject: Re: [PATCH -next v5 2/8] arm64: extable: make uaaccess helper use
 extable type EX_TYPE_UACCESS_ERR_ZERO



在 2022/6/18 20:40, Mark Rutland 写道:
> On Sat, Jun 18, 2022 at 04:42:06PM +0800, Tong Tiangen wrote:
>>>>> diff --git a/arch/arm64/include/asm/asm-extable.h
>>>>> b/arch/arm64/include/asm/asm-extable.h
>>>>> index 56ebe183e78b..9c94ac1f082c 100644
>>>>> --- a/arch/arm64/include/asm/asm-extable.h
>>>>> +++ b/arch/arm64/include/asm/asm-extable.h
>>>>> @@ -28,6 +28,14 @@
>>>>>        __ASM_EXTABLE_RAW(\insn, \fixup, EX_TYPE_FIXUP, 0)
>>>>>        .endm
>>>>> +/*
>>>>> + * Create an exception table entry for uaccess `insn`, which
>>>>> will branch to `fixup`
>>>>> + * when an unhandled fault is taken.
>>>>> + * ex->data = ~0 means both reg_err and reg_zero is set to wzr(x31).
>>>>> + */
>>>>> +    .macro          _asm_extable_uaccess, insn, fixup
>>>>> +    __ASM_EXTABLE_RAW(\insn, \fixup, EX_TYPE_UACCESS_ERR_ZERO, ~0)
>>>>> +    .endm
>>>>
>>>> I'm not too keen on using `~0` here, since that also sets other bits
>>>> in the
>>>> data field, and its somewhat opaque.
>>>>
>>>> How painful is it to generate the data fields as with the C version
>>>> of this
>>>> macro, so that we can pass in wzr explciitly for the two sub-fields?
>>>>
>>>> Other than that, this looks good to me.
>>>>
>>>> Thanks,
>>>> Mark.
>>>
>>> ok, will fix next version.
>>>
>>> Thanks,
>>> Tong.
>>
>> I tried to using data filelds as with C version, but here assembly code we
>> can not using operator such as << and |, if we use lsl and orr instructions,
>> the gpr will be occupied.
>>
>> So how about using 0x3ff directly here? it means err register and zero
>> register both set to x31.
> 
> I had a go at implementing this, and it seems simple enough. Please see:
> 
>    https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/extable/asm-uaccess
> 

I made the following modifications, and the other parts are based on 
your implementation:

arch/arm64/include/asm/asm-extable.h
[...]
.macro          _asm_extable_uaccess, insn, fixup
_ASM_EXTABLE_UACCESS(\insn, \fixup)
.endm
[...]


The following errors are reported during compilation:
[...]
arch/arm64/lib/clear_user.S:45: Error: invalid operands (*ABS* and *UND* 
sections) for `<<'
[...]

"<<" is invalid operands in assembly, is there something wrong with me?

Thanks,
Tong.
> Mark.
> 
> .