Re: [PATCH v4 04/28] rust: alloc: implement `Allocator` for `Kmalloc`

[Benno Lossin]

On 06.08.24 20:55, Danilo Krummrich wrote:
> On Tue, Aug 06, 2024 at 04:51:28PM +0000, Benno Lossin wrote:
>> On 05.08.24 17:19, Danilo Krummrich wrote:
>>> +        let raw_ptr = unsafe {
>>> +            // If `size == 0` and `ptr != NULL` the memory behind the pointer is freed.
>>> +            self.0(ptr.cast(), size, flags.0).cast()
>>> +        };
>>> +
>>> +        let ptr = if size == 0 {
>>> +            NonNull::dangling()
>>
>> If we call `realloc(Some(ptr), <layout with size = 0>, ...)`, then this
>> leaks the pointer returned by the call to `self.0` above. I don't know
>> what the return value of the different functions are that can appear in
>> `self.0`, do they return NULL?
> 
> That is fine, we don't care about the return value. All `ReallocFunc` free the
> memory behind `ptr` if called with a size of zero. But to answer the question,
> they return either NULL or ZERO_SIZE_PTR.

I see, then it's fine. I think it would help if we know the exact
behavior of `kmalloc` & friends (either add a link to C docs or write it
down on `ReallocFunc`).

>> What about the following sequence:
>>
>>     let ptr = realloc(None, <layout with size = 0>, ...);
>>     let ptr = realloc(Some(ptr), <layout with size = 0>, ...);
>>
>> Then the above call to `self.0` is done with a dangling pointer, can the
>> functions that appear in `self.0` handle that?
> 
> This would be incorrect.
> 
> Calling `realloc(Some(ptr), <layout with size = 0>, ...)` frees the memory
> behind `ptr`. This is guranteed behavior for all `ReallocFunc`s, i.e.
> krealloc(), vrealloc(), kvrealloc().

Note that I don't use `ptr` afterwards, the code snippet above is
equivalent to this:

    let ptr = Kmalloc::alloc(<layout with size = 0>, ...);
    unsafe { Kmalloc::free(ptr) };

internally exactly the realloc calls that I put above should be called.

---
Cheers,
Benno