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

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Message-ID: <8ab83e4b-9c72-4a5d-974a-7f123753e7fe@proton.me>
Date: Wed, 07 Aug 2024 07:14:13 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Danilo Krummrich <dakr@...nel.org>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com, boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, a.hindborg@...sung.com, aliceryhl@...gle.com, akpm@...ux-foundation.org, daniel.almeida@...labora.com, faith.ekstrand@...labora.com, boris.brezillon@...labora.com, lina@...hilina.net, mcanal@...lia.com, zhiw@...dia.com, acurrid@...dia.com, cjia@...dia.com, jhubbard@...dia.com, airlied@...hat.com, ajanulgu@...hat.com, lyude@...hat.com, linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4 04/28] rust: alloc: implement `Allocator` for `Kmalloc`

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