Re: [PATCH] mem-pool: fix big allocations

[phillip.wood123@gmail.com]

On 28/12/2023 18:56, René Scharfe wrote:
> Am 28.12.23 um 17:48 schrieb phillip.wood123@gmail.com:
>> On 28/12/2023 16:05, René Scharfe wrote:
>>> Am 28.12.23 um 16:10 schrieb Phillip Wood:
>>>> The diff at the end of
>>>> this email shows a possible implementation of a check_ptr() macro for
>>>> the unit test library. I'm wary of adding it though because I'm not sure
>>>> printing the pointer values is actually very useful most of the
>>>> time. I'm also concerned that the rules around pointer arithmetic and
>>>> comparisons mean that many pointer tests such as
>>>>
>>>>       check_ptr(pool->mp_block->next_free, <=, pool->mp_block->end);
>>>>
>>>> will be undefined if they fail.
>>>
>>> True, the compiler could legally emit mush when it finds out that the
>>> pointers are for different objects.  And the error being fixed produces
>>> such unrelated pointer pairs -- oops.
>>>
>>> This check is not important here, we can just drop it.
>>>
>>> mem_pool_contains() has the same problem, by the way.
>>>
>>> Restricting ourselves to only equality comparisons for pointers prevents
>>> some interesting sanity checks, though.  Casting to intptr_t or
>>> uintptr_t would allow arbitrary comparisons without risk of undefined
>>> behavior, though.  Perhaps that would make a check_ptr() macro viable
>>> and useful.
>>
>> That certainly helps and the check_ptr() macro in my previous email
>> casts the pointers to uintptr_t before comparing them. Maybe I'm
>> worrying too much, but my concern is that in a failing comparison it
>> is likely one of the pointers is invalid (for example it is the
>> result of some undefined pointer arithmetic) and the program is
>> undefined from the point the invalid pointer is created.
> 
> There are no restrictions on integer comparisons.  So comparing after
> casting to uintptr_t should not invoke undefined behavior.  If undefined
> behavior was involved in calculating the pointers in the first place
> then the compiler might still legally go crazy, but not due to the
> comparison.  Right?

Exactly, my worry is that if the comparison fails it is likely that 
there will have been undefined behavior involved in calculating the 
pointer before we get to the comparison in which case so casting to 
uintptr_t in the comparison does not help.

> Whether the result of a uintptr_t-cast comparison of pointers to
> different objects is meaningful is a different question.  Hopefully
> range checks are possible.
> 
>> The
>> documentation for check_ptr() in my previous mail contains the
>> following example
>>
>>      For example if `start` and `end` are pointers to the beginning and
>>      end of an allocation and `offset` is an integer then
>>
>>          check_ptr(start + offset, <=, end)
>>
>>      is undefined when `offset` is larger than `end - start`. Rewriting
>>      the comparison as
>>
>>          check_uint(offset, <=, end - start)
>>
>>      avoids undefined behavior when offset is too large, but is still
>>      undefined if there is a bug that means `start` and `end` do not
>>      point to the same allocation.
> 
> True, but in such a unit test we'd need additional checks verifying
> that start and end belong to the same object.  Or perhaps use a
> numerical size instead of an end pointer.

Agreed, but I think the implication is that there will be cases we 
should be using check_uint() as in the second comparison above rather 
than check_ptr() as in the first comparison above. I'm not opposed to 
adding check_ptr() if we think it will be useful but I am worried it is 
easy to misuse it. If we do add check_ptr() we should have some 
guidelines about when it makes sense to use it.

Best Wishes

Phillip