linux-kernel - Re: [PATCH v1 01/15] powerpc/uaccess: Remove __get_user_allowed() and unsafe_op

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Message-ID: <e57ae14b-806e-854d-d43b-e6278b89ae04@csgroup.eu>
Date:   Wed, 10 Mar 2021 09:14:23 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Daniel Axtens <dja@...ens.net>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 01/15] powerpc/uaccess: Remove __get_user_allowed() and
 unsafe_op_wrap()



Le 01/03/2021 à 23:02, Daniel Axtens a écrit :
> 
> 
> Christophe Leroy <christophe.leroy@...roup.eu> writes:
> 
>> Those two macros have only one user which is unsafe_get_user().
>>
>> Put everything in one place and remove them.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>> ---
>>   arch/powerpc/include/asm/uaccess.h | 10 +++++-----
>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
>> index 78e2a3990eab..8cbf3e3874f1 100644
>> --- a/arch/powerpc/include/asm/uaccess.h
>> +++ b/arch/powerpc/include/asm/uaccess.h
>> @@ -53,9 +53,6 @@ static inline bool __access_ok(unsigned long addr, unsigned long size)
>>   #define __put_user(x, ptr) \
>>   	__put_user_nocheck((__typeof__(*(ptr)))(x), (ptr), sizeof(*(ptr)))
>>   
>> -#define __get_user_allowed(x, ptr) \
>> -	__get_user_nocheck((x), (ptr), sizeof(*(ptr)), false)
>> -
>>   #define __get_user_inatomic(x, ptr) \
>>   	__get_user_nosleep((x), (ptr), sizeof(*(ptr)))
>>   #define __put_user_inatomic(x, ptr) \
>> @@ -482,8 +479,11 @@ user_write_access_begin(const void __user *ptr, size_t len)
>>   #define user_write_access_begin	user_write_access_begin
>>   #define user_write_access_end		prevent_current_write_to_user
>>   
>> -#define unsafe_op_wrap(op, err) do { if (unlikely(op)) goto err; } while (0)
>> -#define unsafe_get_user(x, p, e) unsafe_op_wrap(__get_user_allowed(x, p), e)
>> +#define unsafe_get_user(x, p, e) do {					\
>> +	if (unlikely(__get_user_nocheck((x), (p), sizeof(*(p)), false)))\
>> +		goto e;							\
>> +} while (0)
>> +
> 
> This seems correct to me.
> 
> Checkpatch does have one check that is relevant:
> 
> CHECK: Macro argument reuse 'p' - possible side-effects?
> #36: FILE: arch/powerpc/include/asm/uaccess.h:482:
> +#define unsafe_get_user(x, p, e) do {					\
> +	if (unlikely(__get_user_nocheck((x), (p), sizeof(*(p)), false)))\
> +		goto e;							\
> +} while (0)
> 
> Given that we are already creating a new block, should we do something
> like this (completely untested):
> 
> #define unsafe_get_user(x, p, e) do {					\
>          __typeof__(p) __p = (p);
> 	if (unlikely(__get_user_nocheck((x), (__p), sizeof(*(__p)), false)))\
> 		goto e;							\
> } while (0)
> 

As mentioned by Segher, this is not needed, sizeof(p) doesn't evaluate (p) so (p) is only evaluated 
once in the macro, so no risk of side-effects with that.

Christophe