Re: [PATCH 1/2] mm: vmalloc: implement vrealloc()

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Jul 18, 2024 at 12:24:01AM +0200, Danilo Krummrich wrote:
> Implement vrealloc() analogous to krealloc().
> 
> Currently, krealloc() requires the caller to pass the size of the
> previous memory allocation, which, instead, should be self-contained.
> 
> We attempt to fix this in a subsequent patch which, in order to do so,
> requires vrealloc().
> 
> Besides that, we need realloc() functions for kernel allocators in Rust
> too. With `Vec` or `KVec` respectively, potentially growing (and
> shrinking) data structures are rather common.
> 
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  include/linux/vmalloc.h |  4 +++
>  mm/vmalloc.c            | 58 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 62 insertions(+)
> 
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index e4a631ec430b..9ff0a8e5c323 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -189,6 +189,10 @@ extern void *__vcalloc_noprof(size_t n, size_t size, gfp_t flags) __alloc_size(1
>  extern void *vcalloc_noprof(size_t n, size_t size) __alloc_size(1, 2);
>  #define vcalloc(...)		alloc_hooks(vcalloc_noprof(__VA_ARGS__))
>  
> +extern void * __must_check vrealloc_noprof(const void *p, size_t size,
> +					   gfp_t flags) __realloc_size(2);
> +#define vrealloc(...)		alloc_hooks(vrealloc_noprof(__VA_ARGS__))
> +
>  extern void vfree(const void *addr);
>  extern void vfree_atomic(const void *addr);
>  
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index e34ea860153f..4ec949ac9d9d 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -4036,6 +4036,64 @@ void *vzalloc_node_noprof(unsigned long size, int node)
>  }
>  EXPORT_SYMBOL(vzalloc_node_noprof);
>  
> +/**
> + * vrealloc - reallocate virtually contiguous memory; contents remain unchanged
> + * @p: object to reallocate memory for
> + * @size: the size to reallocate
> + * @flags: the flags for the page level allocator
> + *
> + * The contents of the object pointed to are preserved up to the lesser of the
> + * new and old size (__GFP_ZERO flag is effectively ignored).
> + *
> + * If @p is %NULL, vrealloc() behaves exactly like vmalloc(). If @size is 0 and
> + * @p is not a %NULL pointer, the object pointed to is freed.
> + *
> + * Return: pointer to the allocated memory; %NULL if @size is zero or in case of
> + *         failure
> + */
> +void *vrealloc_noprof(const void *p, size_t size, gfp_t flags)
> +{
> +	size_t old_size = 0;
> +	void *n;
> +
> +	if (!size) {
> +		vfree(p);
> +		return NULL;
> +	}
> +
> +	if (p) {
> +		struct vm_struct *vm;
> +
> +		vm = find_vm_area(p);
>
Concurrent vfree() will lead to use-after-free. Either add a comment
that a user is responsible for not using vrealloc()/vfree() on the same
pointer concurrently or use find_unlink_vmap_area() which might be more
complex when it comes to design of the vrealloc().

--
Uladzislau Rezki