Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions

[David Hildenbrand <david@redhat.com>]

On 08.05.24 04:34, Steven Rostedt wrote:
> On Tue, 30 Apr 2024 12:13:51 +0100
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
>> +#ifdef CONFIG_MMU
>> +static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
>> +			struct vm_area_struct *vma)
>> +{
>> +	unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff;
>> +	unsigned int subbuf_pages, subbuf_order;
>> +	struct page **pages;
>> +	int p = 0, s = 0;
>> +	int err;
>> +
>> +	/* Refuse MP_PRIVATE or writable mappings */
>> +	if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC ||
>> +	    !(vma->vm_flags & VM_MAYSHARE))
>> +		return -EPERM;
>> +
>> +	/*
>> +	 * Make sure the mapping cannot become writable later. Also tell the VM
>> +	 * to not touch these pages (VM_DONTCOPY | VM_DONTEXPAND). Finally,
>> +	 * prevent migration, GUP and dump (VM_IO).
>> +	 */
>> +	vm_flags_mod(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_IO, VM_MAYWRITE);
> 
> Do we really need the VM_IO?
> 
> When testing this in gdb, I would get:
> 
> (gdb) p tmap->map->subbuf_size
> Cannot access memory at address 0x7ffff7fc2008
> 
> It appears that you can't ptrace IO memory. When I removed that flag,
> gdb has no problem reading that memory.
> 
> I think we should drop that flag.
> 
> Can you send a v23 with that removed, Shuah's update, and also the
> change below:
> 
>> +
>> +	lockdep_assert_held(&cpu_buffer->mapping_lock);
>> +
>> +	subbuf_order = cpu_buffer->buffer->subbuf_order;
>> +	subbuf_pages = 1 << subbuf_order;
>> +
>> +	nr_subbufs = cpu_buffer->nr_pages + 1; /* + reader-subbuf */
>> +	nr_pages = ((nr_subbufs) << subbuf_order) - pgoff + 1; /* + meta-page */
>> +
>> +	vma_pages = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
>> +	if (!vma_pages || vma_pages > nr_pages)
>> +		return -EINVAL;
>> +
>> +	nr_pages = vma_pages;
>> +
>> +	pages = kcalloc(nr_pages, sizeof(*pages), GFP_KERNEL);
>> +	if (!pages)
>> +		return -ENOMEM;
>> +
>> +	if (!pgoff) {
>> +		pages[p++] = virt_to_page(cpu_buffer->meta_page);
>> +
>> +		/*
>> +		 * TODO: Align sub-buffers on their size, once
>> +		 * vm_insert_pages() supports the zero-page.
>> +		 */
>> +	} else {
>> +		/* Skip the meta-page */
>> +		pgoff--;
>> +
>> +		if (pgoff % subbuf_pages) {
>> +			err = -EINVAL;
>> +			goto out;
>> +		}
>> +
>> +		s += pgoff / subbuf_pages;
>> +	}
>> +
>> +	while (s < nr_subbufs && p < nr_pages) {
>> +		struct page *page = virt_to_page(cpu_buffer->subbuf_ids[s]);
>> +		int off = 0;
>> +
>> +		for (; off < (1 << (subbuf_order)); off++, page++) {
>> +			if (p >= nr_pages)
>> +				break;
>> +
>> +			pages[p++] = page;
>> +		}
>> +		s++;
>> +	}
> 
> The above can be made to:
> 
> 	while (p < nr_pages) {
> 		struct page *page;
> 		int off = 0;
> 
> 		if (WARN_ON_ONCE(s >= nr_subbufs))
> 			break;

I'm not particularly happy about us calling vm_insert_pages with NULL 
pointers stored in pages.

Should we instead do

if (WARN_ON_ONCE(s >= nr_subbufs)) {
	err = -EINVAL;
	goto out;
}

?

-- 
Cheers,

David / dhildenb