On Wed, Oct 07, 2020 at 12:33:06PM -0700, Dan Williams wrote:
On Wed, Oct 7, 2020 at 11:11 AM Daniel Vetter daniel.vetter@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.
The inode does not come from the filesystem char/mem.c creates a singular anon inode in devmem_init_inode()
Seems OK to use this more widely, but it feels a bit weird to live in char/memory.c.
This is what got me thinking maybe this needs to be a bit bigger generic infrastructure - eg enter this scheme from fops mmap and everything else is in mm/user_iomem.c
Jason