Re: [V9fs-developer] [RFC PATCH] 9p: create writeback fid on shared mmap

[Chengguang Xu <cgxu519@mykernel.net>]

 ---- 在 星期一, 2020-12-07 04:53:18 Dominique Martinet <asmadeus@codewreck.org> 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