Re: [PATCH 2/5] Protectable Memory Allocator

[Igor Stoppa]

On 06/06/17 15:08, Tetsuo Handa wrote:
> Igor Stoppa wrote:
>>>> +struct pmalloc_node {
>>>> +	struct hlist_node nodes_list;
>>>> +	atomic_t used_words;
>>>> +	unsigned int total_words;
>>>> +	__PMALLOC_ALIGNED align_t data[];
>>>> +};
>>>
>>> Is this __PMALLOC_ALIGNED needed? Why not use "long" and "BITS_PER_LONG" ?
>>
>> In an earlier version I actually asked the same question.
>> It is currently there because I just don't know enough about various
>> architectures. The idea of having "align_t" was that it could be tied
>> into what is the most desirable alignment for each architecture.
>> But I'm actually looking for advise on this.
> 
> I think that let the compiler use natural alignment is OK.

On a 64 bit machine the preferred alignment might be either 32 or 64,
depending on the application. How can the compiler choose?


>>> You need to check for node != NULL before dereference it.
>>
>> So, if I understood correctly, there shouldn't be a case where node is
>> NULL, right?
>> Unless it has been tampered/damaged. Is that what you mean?
> 
> I meant to say
> 
> +	node = __pmalloc_create_node(req_words);
> // this location.
> +	starting_word = atomic_fetch_add(req_words, &node->used_words);

argh, yes


>>>> +const char *__pmalloc_check_object(const void *ptr, unsigned long n)
>>>> +{
>>>> +	unsigned long p;
>>>> +
>>>> +	p = (unsigned long)ptr;
>>>> +	n += (unsigned long)ptr;
>>>> +	for (; (PAGE_MASK & p) <= (PAGE_MASK & n); p += PAGE_SIZE) {
>>>> +		if (is_vmalloc_addr((void *)p)) {
>>>> +			struct page *page;
>>>> +
>>>> +			page = vmalloc_to_page((void *)p);
>>>> +			if (!(page && PagePmalloc(page)))
>>>> +				return msg;
>>>> +		}
>>>> +	}
>>>> +	return NULL;
>>>> +}
>>>
>>> I feel that n is off-by-one if (ptr + n) % PAGE_SIZE == 0
>>> according to check_page_span().
>>
>> It seems to work. If I am missing your point, could you please
>> use the same format of the example I made, to explain me?
> 
> If ptr == NULL and n == PAGE_SIZE so that (ptr + n) % PAGE_SIZE == 0,
> this loop will access two pages (one page containing p == 0 and another
> page containing p == PAGE_SIZE) when this loop should access only one
> page containing p == 0. When checking n bytes, it's range is 0 to n - 1.

oh, so:

p = (unsigned long) ptr;
n = p + n - 1;


--
igor