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

[Dominique Martinet <asmadeus@codewreck.org>]

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.

OTOH, 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?

 	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
-- 
Dominique