Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Sat, 2 Nov 2019 18:55:36 +0300
From:   Alexey Dobriyan <adobriyan@gmail.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Chris Down <chris@chrisdown.name>,
        Johannes Weiner <hannes@cmpxchg.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH] kernel: sysctl: make drop_caches write-only
Message-ID: <20191102155536.GA10251@avx2>
References: <20191031221602.9375-1-hannes@cmpxchg.org>
 <20191031162825.a545a5d4d8567368501769bd@linux-foundation.org>
 <20191101110901.GB690103@chrisdown.name>
 <20191101144540.GA12808@cmpxchg.org>
 <20191101115950.bb88d49849bfecb1af0a88bf@linux-foundation.org>
 <20191101192405.GA866154@chrisdown.name>
 <20191101122920.798a6d61b2725da8cfe80549@linux-foundation.org>
 <20191101123544.c9b0024a1e8f5ddf63148b48@linux-foundation.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20191101123544.c9b0024a1e8f5ddf63148b48@linux-foundation.org>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Nov 01, 2019 at 12:35:44PM -0700, Andrew Morton wrote:
> On Fri, 1 Nov 2019 12:29:20 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> > > Either change is an upgrade from the current situation, at least. I prefer 
> > > towards whatever makes the API the least confusing, which appears to be 
> > > Johannes' original change, but I'd support a patch which always set it to 
> > > 0 instead if it was deemed safer.
> > 
> > On the other hand..  As I mentioned earlier, if someone's code is
> > failing because of the permissions change, they can chmod
> > /proc/sys/vm/drop_caches at boot time and be happy.  They have no such
> > workaround if their software misbehaves due to a read always returning
> > "0".
> 
> I lied.  I can chmod things in /proc but I can't chmod things in
> /proc/sys/vm.  Huh, why did we do that?

To conserve memory! It was in 2007.
For the record I support 0200 on vm.drop_caches.

	commit 77b14db502cb85a031fe8fde6c85d52f3e0acb63
	[PATCH] sysctl: reimplement the sysctl proc support

	+static int proc_sys_setattr(struct dentry *dentry, struct iattr *attr)
	+{
	+       struct inode *inode = dentry->d_inode;
	+       int error;
	+
	+       if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
	+               return -EPERM;