---- 在 星期一, 2020-12-07 04:53:18 Dominique Martinet <asmadeus@xxxxxxxxxxxxx> 撰写 ----
> Dominique Martinet wrote on Sun, Dec 06, 2020:
> > Chengguang Xu wrote on Sat, Dec 05, 2020:
> > > If vma is shared and the file was opened for writing,
> > > we should also create writeback fid because vma may be
> > > mprotected writable even if now readonly.
> >
> > Hm, I guess it makes sense.
>
> I had a second look, and generic_file_readonly_mmap uses vma's
> `vma->vm_flags & VM_MAYWRITE` instead (together with VM_SHARED),
> while mapping_writably_mapped ultimately basically only seems to
> validate that the mapping is shared from a look at mapping_map_writable
> callers? It's not very clear to me.
>
> , VM_MAYWRITE is set anytime we have a shared map where file has
> been opened read-write, which seems to be what you want with regards to
> protecting from mprotect calls.
>
> How about simply changing check from WRITE to MAYWRITE?
It would be fine and based on the code in do_mmap(), it seems we even don't
need extra check here. The condition (vma->vm_flags & VM_SHARED) will be enough.
Am I missing something?
Thanks,
Chengguang
>
> v9inode = V9FS_I(inode);
> mutex_lock(&v9inode->v_mutex);
> if (!v9inode->writeback_fid &&
> (vma->vm_flags & VM_SHARED) &&
> - (vma->vm_flags & VM_WRITE)) {
> + (vma->vm_flags & VM_MAYWRITE)) {
> /*
> * clone a fid and add it to writeback_fid
> * we do it during mmap instead of
