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

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Date:   Mon, 7 Dec 2020 12:24:10 +0100
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Chengguang Xu <cgxu519@...ernel.net>
Cc:     ericvh <ericvh@...il.com>, lucho <lucho@...kov.net>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        v9fs-developer <v9fs-developer@...ts.sourceforge.net>
Subject: Re: [V9fs-developer] [RFC PATCH] 9p: create writeback fid on shared
 mmap

Chengguang Xu wrote on Mon, Dec 07, 2020:
>  > , 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?

VM_MAYWRITE is unset if the file hasn't been open for writing (in which
case the mapping can't be mprotect()ed to writable map), so checking it
is a bit more efficient.

Anyway I'd like to obsolete the writeback fid uses now that fids have a
refcount (this usecase can be a simple refcount increase), in which case
efficiency is less of a problem, but we're not there yet...

Please resend with MAYWRITE if you want authorship and I'll try to take
some time to test incl. the mprotect usecase.

-- 
Dominique