On 3/20/26 23:39, Lorenzo Stoakes (Oracle) wrote: > While the conversion of mmap hooks to mmap_prepare is underway, we will > encounter situations where mmap hooks need to invoke nested mmap_prepare > hooks. > > The nesting of mmap hooks is termed 'stacking'. In order to flexibly > facilitate the conversion of custom mmap hooks in drivers which stack, we > must split up the existing __compat_vma_mmap() function into two separate > functions: > > * compat_set_desc_from_vma() - This allows the setting of a vm_area_desc > object's fields to the relevant fields of a VMA. > > * __compat_vma_mmap() - Once an mmap_prepare hook has been executed upon a > vm_area_desc object, this function performs any mmap actions specified by > the mmap_prepare hook and then invokes its vm_ops->mapped() hook if any > were specified. > > In ordinary cases, where a file's f_op->mmap_prepare() hook simply needs > to be invoked in a stacked mmap() hook, compat_vma_mmap() can be used. > > However some drivers define their own nested hooks, which are invoked in > turn by another hook. > > A concrete example is vmbus_channel->mmap_ring_buffer(), which is invoked > in turn by bin_attribute->mmap(): > > vmbus_channel->mmap_ring_buffer() has a signature of: > > int (*mmap_ring_buffer)(struct vmbus_channel *channel, > struct vm_area_struct *vma); > > And bin_attribute->mmap() has a signature of: > > int (*mmap)(struct file *, struct kobject *, > const struct bin_attribute *attr, > struct vm_area_struct *vma); > > And so compat_vma_mmap() cannot be used here for incremental conversion of > hooks from mmap() to mmap_prepare(). > > There are many such instances like this, where conversion to mmap_prepare > would otherwise cascade to a huge change set due to nesting of this kind. > > The changes in this patch mean we could now instead convert > vmbus_channel->mmap_ring_buffer() to > vmbus_channel->mmap_prepare_ring_buffer(), and implement something like: > > struct vm_area_desc desc; > int err; > > compat_set_desc_from_vma(&desc, file, vma); > err = channel->mmap_prepare_ring_buffer(channel, &desc); > if (err) > return err; > > return __compat_vma_mmap(&desc, vma); > > Allowing us to incrementally update this logic, and other logic like it. > > Unfortunately, as part of this change, we need to be able to flexibly > assign to the VMA descriptor, so have to remove some of the const > declarations within the structure. > > Also update the VMA tests to reflect the changes. > > Signed-off-by: Lorenzo Stoakes (Oracle) <[email protected]>
Acked-by: Vlastimil Babka (SUSE) <[email protected]>
