dynamically allocating memory in the kernel

dynamically allocating memory in the kernel

[Carter Cheng]

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Hello,

I was wondering if someone here could help me understand something about
the kernel for a project I am working on tracking dynamically allocated
memory. How many ways are there to dynamically allocate memory in the
kernel presently? I know of 4.

get_free_pages family of functions
kmalloc family (including vmalloc)
slab allocation
mempools.

Are there others? I am trying to track them all down so I have
comprehensively know when all allocations occur and all "frees".

Thanks for your help,

Carter.

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Re: dynamically allocating memory in the kernel

[Kees Cook]

On Wed, Dec 5, 2018 at 12:46 PM Carter Cheng <cartercheng@gmail.com> wrote:
>
> Hello,
>
> I was wondering if someone here could help me understand something about the kernel for a project I am working on tracking dynamically allocated memory. How many ways are there to dynamically allocate memory in the kernel presently? I know of 4.
>
> get_free_pages family of functions
> kmalloc family (including vmalloc)

vmalloc is quite different from kmalloc (see below).

> slab allocation

kmalloc is a type of slab allocation, so the base parts are the same.

> mempools.
>
> Are there others? I am trying to track them all down so I have comprehensively know when all allocations occur and all "frees".

I would recommend looking at KASAN hooks (which tracks use-after-free)
to see if there are any areas you're missing.

-Kees

-- 
Kees Cook

Re: dynamically allocating memory in the kernel

[Carter Cheng]

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Thanks, I'll check them out.

On Thu, Dec 6, 2018 at 7:01 AM Kees Cook <keescook@chromium.org> wrote:

> On Wed, Dec 5, 2018 at 12:46 PM Carter Cheng <cartercheng@gmail.com>
> wrote:
> >
> > Hello,
> >
> > I was wondering if someone here could help me understand something about
> the kernel for a project I am working on tracking dynamically allocated
> memory. How many ways are there to dynamically allocate memory in the
> kernel presently? I know of 4.
> >
> > get_free_pages family of functions
> > kmalloc family (including vmalloc)
>
> vmalloc is quite different from kmalloc (see below).
>
> > slab allocation
>
> kmalloc is a type of slab allocation, so the base parts are the same.
>
> > mempools.
> >
> > Are there others? I am trying to track them all down so I have
> comprehensively know when all allocations occur and all "frees".
>
> I would recommend looking at KASAN hooks (which tracks use-after-free)
> to see if there are any areas you're missing.
>
> -Kees
>
> --
> Kees Cook
>

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Re: dynamically allocating memory in the kernel

[Sandy Harris]

On Thu, Dec 6, 2018 at 4:46 AM Carter Cheng <cartercheng@gmail.com> wrote:

> I was wondering if someone here could help me understand something about the kernel for a project I am working on tracking dynamically allocated memory. How many ways are there to dynamically allocate memory in the kernel presently? I know of 4.
>
> get_free_pages family of functions
> kmalloc family (including vmalloc)
> slab allocation
> mempools.
>
> Are there others? I am trying to track them all down so I have comprehensively know when all allocations occur and all "frees".

You should also look at uses of alloca(3) which dynamically allocates
memory on the stack, automatically freed when the function returns.
Some programmers prefer this to other methods, mainly because it
reduces chances of unfreed memory creating memory leaks. Others
dislike it because it is not in any C standard (I think it was
originally a GNU extension but some other compilers now support it) so
it may limit portability. Some consider it dangerous because some
usages (e.g. in a recursive function) may grow a huge stack; I am not
sure how this interacts with kernel limits on stack size.