[PATCH 10/13] PCI: revoke mappings like devmem

[Dan Williams]

On Wed, Oct 7, 2020 at 11:11 AM Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>
> Since 3234ac664a87 ("/dev/mem: Revoke mappings when a driver claims
> the region") /dev/kmem zaps ptes when the kernel requests exclusive
> acccess to an iomem region. And with CONFIG_IO_STRICT_DEVMEM, this is
> the default for all driver uses.
>
> Except there's two more ways to access pci bars: sysfs and proc mmap
> support. Let's plug that hole.

Ooh, yes, lets.

>
> For revoke_devmem() to work we need to link our vma into the same
> address_space, with consistent vma->vm_pgoff. ->pgoff is already
> adjusted, because that's how (io_)remap_pfn_range works, but for the
> mapping we need to adjust vma->vm_file->f_mapping. Usually that's done
> at ->open time, but that's a bit tricky here with all the entry points
> and arch code. So instead create a fake file and adjust vma->vm_file.

I don't think you want to share the devmem inode for this, this should
be based off the sysfs inode which I believe there is already only one
instance per resource. In contrast /dev/mem can have multiple inodes
because anyone can just mknod a new character device file, the same
problem does not exist for sysfs.