Re: [PATCH v2 1/2] mm: Add an F_SEAL_FS_WRITE seal to memfd

From: Christoph Hellwig <hch@infradead.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Christoph Hellwig <hch@infradead.org>,
	linux-kernel@vger.kernel.org, kernel-team@android.com,
	jreck@google.com, john.stultz@linaro.org, tkjos@google.com,
	gregkh@linuxfoundation.org,
	Andrew Morton <akpm@linux-foundation.org>,
	dancol@google.com, "J. Bruce Fields" <bfields@fieldses.org>,
	Jeff Layton <jlayton@kernel.org>,
	Khalid Aziz <khalid.aziz@oracle.com>,
	linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-mm@kvack.org, Mike Kravetz <mike.kravetz@oracle.com>,
	minchan@google.com, Shuah Khan <shuah@kernel.org>
Subject: Re: [PATCH v2 1/2] mm: Add an F_SEAL_FS_WRITE seal to memfd
Date: Wed, 17 Oct 2018 05:08:29 -0700	[thread overview]
Message-ID: <20181017120829.GA19731@infradead.org> (raw)
In-Reply-To: <20181017103958.GB230639@joelaf.mtv.corp.google.com>

On Wed, Oct 17, 2018 at 03:39:58AM -0700, Joel Fernandes wrote:
> > > This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> > > To support the usecase, this patch adds a new F_SEAL_FS_WRITE seal which
> > > prevents any future mmap and write syscalls from succeeding while
> > > keeping the existing mmap active. The following program shows the seal
> > > working in action:
> > 
> > Where does the FS come from?  I'd rather expect this to be implemented
> > as a 'force' style flag that applies the seal even if the otherwise
> > required precondition is not met.
> 
> The "FS" was meant to convey that the seal is preventing writes at the VFS
> layer itself, for example vfs_write checks FMODE_WRITE and does not proceed,
> it instead returns an error if the flag is not set. I could not find a better
> name for it, I could call it F_SEAL_VFS_WRITE if you prefer?

I don't think there is anything VFS or FS about that - at best that
is an implementation detail.

Either do something like the force flag I suggested in the last mail,
or give it a name that matches the intention, e.g F_SEAL_FUTURE_WRITE.

> I could make it such that this seal would not be allowed unless F_SEAL_SHRINK
> and F_SEAL_GROW are either previously set, or they are passed along with this
> seal. Would that make more sense to you?

Yes.

> > >  static int memfd_add_seals(struct file *file, unsigned int seals)
> > >  {
> > > @@ -219,6 +220,9 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
> > >  		}
> > >  	}
> > >  
> > > +	if ((seals & F_SEAL_FS_WRITE) && !(*file_seals & F_SEAL_FS_WRITE))
> > > +		file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
> > > +
> > 
> > This seems to lack any synchronization for f_mode.
> 
> The f_mode is set when the struct file is first created and then memfd sets
> additional flags in memfd_create. Then later we are changing it here at the
> time of setting the seal. I donot see any possiblity of a race since it is
> impossible to set the seal before memfd_create returns. Could you provide
> more details about what kind of synchronization is needed and what is the
> race condition scenario you were thinking off?

Even if no one changes these specific flags we still need a lock due
to rmw cycles on the field.  For example fadvise can set or clear
FMODE_RANDOM.  It seems to use file->f_lock for synchronization.