Re:[PATCH] x86/mm/mem_encrypt_identity : fix error useage to sizeof

Re:[PATCH] x86/mm/mem_encrypt_identity : fix error useage to sizeof

[Thomas Gleixner]

On Mon, 7 Jan 2019, peng.hao2@zte.com.cn wrote:

> >> Fix error usage to sizeof. It should not use sizeof to pointer.
> >
> >.... because?
> >
> >The commit message needs to explain what the potential issue could be
> >and why it doesn't matter in this case.
> I see the definition of pte_t may be more than sizeof(unsigned long).
> So I think sizeof(pte_t) is safer.

What exactly is the difference between:

	pte_t	*p;

	sizeof(*p)

and

	sizeof(pte_t)

and what is safer about the latter?

Answer: No difference and nothing is safer because it's exactly the same.

In general we use sizeof(*p) simply because when the data type of p changes
you don't have to update the code, it just works and stays correct.

Thanks,

	tglx

Re: [PATCH] x86/mm/mem_encrypt_identity : fix error useage to sizeof

[Juergen Gross]

On 15/01/2019 11:13, Thomas Gleixner wrote:
> On Mon, 7 Jan 2019, peng.hao2@zte.com.cn wrote:
> 
>>>> Fix error usage to sizeof. It should not use sizeof to pointer.
>>>
>>> .... because?
>>>
>>> The commit message needs to explain what the potential issue could be
>>> and why it doesn't matter in this case.
>> I see the definition of pte_t may be more than sizeof(unsigned long).
>> So I think sizeof(pte_t) is safer.
> 
> What exactly is the difference between:
> 
> 	pte_t	*p;
> 
> 	sizeof(*p)
> 
> and
> 
> 	sizeof(pte_t)
> 
> and what is safer about the latter?

Please note that the current code is using sizeof(p) instead of sizeof(*p).

And this is really different for X86_32 PAE.


Juergen

> 
> Answer: No difference and nothing is safer because it's exactly the same.
> 
> In general we use sizeof(*p) simply because when the data type of p changes
> you don't have to update the code, it just works and stays correct.
> 
> Thanks,
> 
> 	tglx
> 

Re: [PATCH] x86/mm/mem_encrypt_identity : fix error useage to sizeof

[Thomas Gleixner]

On Tue, 15 Jan 2019, Juergen Gross wrote:
> On 15/01/2019 11:13, Thomas Gleixner wrote:
> > On Mon, 7 Jan 2019, peng.hao2@zte.com.cn wrote:
> > 
> >>>> Fix error usage to sizeof. It should not use sizeof to pointer.
> >>>
> >>> .... because?
> >>>
> >>> The commit message needs to explain what the potential issue could be
> >>> and why it doesn't matter in this case.
> >> I see the definition of pte_t may be more than sizeof(unsigned long).
> >> So I think sizeof(pte_t) is safer.
> > 
> > What exactly is the difference between:
> > 
> > 	pte_t	*p;
> > 
> > 	sizeof(*p)
> > 
> > and
> > 
> > 	sizeof(pte_t)
> > 
> > and what is safer about the latter?
> 
> Please note that the current code is using sizeof(p) instead of sizeof(*p).

Ooops. That's wrong indeed, but we should not change it to sizeof(pte_t)
and change it to sizeof(*p) instead.

Thanks,

	tglx

Re: [PATCH] x86/mm/mem_encrypt_identity : fix error useage to sizeof

[Juergen Gross]

On 15/01/2019 11:25, Thomas Gleixner wrote:
> On Tue, 15 Jan 2019, Juergen Gross wrote:
>> On 15/01/2019 11:13, Thomas Gleixner wrote:
>>> On Mon, 7 Jan 2019, peng.hao2@zte.com.cn wrote:
>>>
>>>>>> Fix error usage to sizeof. It should not use sizeof to pointer.
>>>>>
>>>>> .... because?
>>>>>
>>>>> The commit message needs to explain what the potential issue could be
>>>>> and why it doesn't matter in this case.
>>>> I see the definition of pte_t may be more than sizeof(unsigned long).
>>>> So I think sizeof(pte_t) is safer.
>>>
>>> What exactly is the difference between:
>>>
>>> 	pte_t	*p;
>>>
>>> 	sizeof(*p)
>>>
>>> and
>>>
>>> 	sizeof(pte_t)
>>>
>>> and what is safer about the latter?
>>
>> Please note that the current code is using sizeof(p) instead of sizeof(*p).
> 
> Ooops. That's wrong indeed, but we should not change it to sizeof(pte_t)
> and change it to sizeof(*p) instead.

And that's what the patch does.


Juergen

Re: [PATCH] x86/mm/mem_encrypt_identity : fix error useage to sizeof

[Thomas Gleixner]

On Tue, 15 Jan 2019, Thomas Gleixner wrote:

> On Tue, 15 Jan 2019, Juergen Gross wrote:
> > On 15/01/2019 11:13, Thomas Gleixner wrote:
> > > On Mon, 7 Jan 2019, peng.hao2@zte.com.cn wrote:
> > > 
> > >>>> Fix error usage to sizeof. It should not use sizeof to pointer.
> > >>>
> > >>> .... because?
> > >>>
> > >>> The commit message needs to explain what the potential issue could be
> > >>> and why it doesn't matter in this case.
> > >> I see the definition of pte_t may be more than sizeof(unsigned long).
> > >> So I think sizeof(pte_t) is safer.
> > > 
> > > What exactly is the difference between:
> > > 
> > > 	pte_t	*p;
> > > 
> > > 	sizeof(*p)
> > > 
> > > and
> > > 
> > > 	sizeof(pte_t)
> > > 
> > > and what is safer about the latter?
> > 
> > Please note that the current code is using sizeof(p) instead of sizeof(*p).
> 
> Ooops. That's wrong indeed, but we should not change it to sizeof(pte_t)
> and change it to sizeof(*p) instead.

Which is what the patch actually does. Just the above reply:

> > >> So I think sizeof(pte_t) is safer.

confused the hell out of me. -ENOTENOUGHCOFFEE