Re: [PATCH bpf-next v3 1/6] vmalloc: introduce execmem_alloc, execmem_free, and execmem_fill

[Luis Chamberlain <mcgrof@kernel.org>]

On Wed, Nov 16, 2022 at 05:06:16PM -0800, Song Liu wrote:
> +static void move_vmap_to_free_text_tree(void *addr)
> +{
> +	struct vmap_area *va;
> +
> +	/* remove from vmap_area_root */
> +	spin_lock(&vmap_area_lock);
> +	va = __find_vmap_area((unsigned long)addr, &vmap_area_root);
> +	if (WARN_ON_ONCE(!va)) {
> +		spin_unlock(&vmap_area_lock);
> +		return;
> +	}
> +	unlink_va(va, &vmap_area_root);
> +	spin_unlock(&vmap_area_lock);
> +
> +	/* make the memory RO+X */
> +	memset(addr, 0, va->va_end - va->va_start);
> +	set_memory_ro(va->va_start, (va->va_end - va->va_start) >> PAGE_SHIFT);
> +	set_memory_x(va->va_start, (va->va_end - va->va_start) >> PAGE_SHIFT);
> +
> +	/* add to all_text_vm */
> +	va->vm->next = all_text_vm;
> +	all_text_vm = va->vm;
> +
> +	/* add to free_text_area_root */
> +	spin_lock(&free_text_area_lock);
> +	merge_or_add_vmap_area_augment(va, &free_text_area_root, &free_text_area_list);
> +	spin_unlock(&free_text_area_lock);
> +}

<-- snip -->

> +void *execmem_alloc(unsigned long size, unsigned long align)
> +{
> +	struct vmap_area *va, *tmp;
> +	unsigned long addr;
> +	enum fit_type type;
> +	int ret;
> +
> +	va = kmem_cache_alloc_node(vmap_area_cachep, GFP_KERNEL, NUMA_NO_NODE);
> +	if (unlikely(!va))
> +		return NULL;
> +
> +again:
> +	preload_this_cpu_lock(&free_text_area_lock, GFP_KERNEL, NUMA_NO_NODE);
> +	tmp = find_vmap_lowest_match(&free_text_area_root, size, align, 1, false);
> +
> +	if (!tmp) {
> +		unsigned long alloc_size;
> +		void *ptr;
> +
> +		spin_unlock(&free_text_area_lock);
> +
> +		/*
> +		 * Not enough continuous space in free_text_area_root, try
> +		 * allocate more memory. The memory is first added to
> +		 * vmap_area_root, and then moved to free_text_area_root.
> +		 */
> +		alloc_size = roundup(size, PMD_SIZE * num_online_nodes());
> +		ptr = __vmalloc_node_range(alloc_size, PMD_SIZE, EXEC_MEM_START,
> +					   EXEC_MEM_END, GFP_KERNEL, PAGE_KERNEL,
> +					   VM_ALLOW_HUGE_VMAP | VM_NO_GUARD,
> +					   NUMA_NO_NODE, __builtin_return_address(0));
> +		if (unlikely(!ptr))
> +			goto err_out;
> +
> +		move_vmap_to_free_text_tree(ptr);

It's not perfectly clear to me how we know for sure nothing can take
this underneath our noses.

  Luis