Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Sun, 6 Dec 2020 21:53:18 +0100
From:   Dominique Martinet <asmadeus@codewreck.org>
To:     Chengguang Xu <cgxu519@mykernel.net>
Cc:     ericvh@gmail.com, lucho@ionkov.net, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, v9fs-developer@lists.sourceforge.net
Subject: Re: [V9fs-developer] [RFC PATCH] 9p: create writeback fid on shared
 mmap
Message-ID: <20201206205318.GA25257@nautica>
References: <20201205130904.518104-1-cgxu519@mykernel.net>
 <20201206091618.GA22629@nautica>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20201206091618.GA22629@nautica>
User-Agent: Mutt/1.5.21 (2010-09-15)
Precedence: bulk

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