linux-kernel - Re: [PATCH 14/14] powerpc: remove address space overrides using set

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Date:   Thu, 3 Sep 2020 18:03:56 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Christoph Hellwig <hch@....de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>, x86@...nel.org,
        linux-arch@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, Luis Chamberlain <mcgrof@...nel.org>,
        linux-fsdevel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [PATCH 14/14] powerpc: remove address space overrides using
 set_fs()



Le 03/09/2020 à 17:56, Christoph Hellwig a écrit :
> On Thu, Sep 03, 2020 at 05:49:09PM +0200, Christoph Hellwig wrote:
>> On Thu, Sep 03, 2020 at 05:43:25PM +0200, Christophe Leroy wrote:
>>>
>>>
>>> Le 03/09/2020 à 16:22, Christoph Hellwig a écrit :
>>>> Stop providing the possibility to override the address space using
>>>> set_fs() now that there is no need for that any more.
>>>>
>>>> Signed-off-by: Christoph Hellwig <hch@....de>
>>>> ---
>>>
>>>
>>>>    -static inline int __access_ok(unsigned long addr, unsigned long size,
>>>> -			mm_segment_t seg)
>>>> +static inline bool __access_ok(unsigned long addr, unsigned long size)
>>>>    {
>>>> -	if (addr > seg.seg)
>>>> -		return 0;
>>>> -	return (size == 0 || size - 1 <= seg.seg - addr);
>>>> +	if (addr >= TASK_SIZE_MAX)
>>>> +		return false;
>>>> +	return size == 0 || size <= TASK_SIZE_MAX - addr;
>>>>    }
>>>
>>> You don't need to test size == 0 anymore. It used to be necessary because
>>> of the 'size - 1', as size is unsigned.
>>>
>>> Now you can directly do
>>>
>>> 	return size <= TASK_SIZE_MAX - addr;
>>>
>>> If size is 0, this will always be true (because you already know that addr
>>> is not >= TASK_SIZE_MAX
>>
>> True.  What do you think of Linus' comment about always using the
>> ppc32 version on ppc64 as well with this?

I have nothing against it. That's only adding a substract, all args are 
already in registers so that will be in the noise for a modern CPU.

> 
> i.e. something like this folded in:
> 
> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> index 5363f7fc6dd06c..be070254e50943 100644
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -11,26 +11,14 @@
>   #ifdef __powerpc64__
>   /* We use TASK_SIZE_USER64 as TASK_SIZE is not constant */
>   #define TASK_SIZE_MAX		TASK_SIZE_USER64
> -
> -/*
> - * This check is sufficient because there is a large enough gap between user
> - * addresses and the kernel addresses.
> - */
> -static inline bool __access_ok(unsigned long addr, unsigned long size)
> -{
> -	return addr < TASK_SIZE_MAX && size < TASK_SIZE_MAX;
> -}
> -
>   #else
>   #define TASK_SIZE_MAX		TASK_SIZE
> +#endif
>   
>   static inline bool __access_ok(unsigned long addr, unsigned long size)
>   {
> -	if (addr >= TASK_SIZE_MAX)
> -		return false;
> -	return size == 0 || size <= TASK_SIZE_MAX - addr;
> +	return addr < TASK_SIZE_MAX && size <= TASK_SIZE_MAX - addr;
>   }
> -#endif /* __powerpc64__ */
>   
>   #define access_ok(addr, size)		\
>   	(__chk_user_ptr(addr),		\
> 


Christophe