From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756042AbeEHWjX (ORCPT ); Tue, 8 May 2018 18:39:23 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:57790 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754148AbeEHWjW (ORCPT ); Tue, 8 May 2018 18:39:22 -0400 Date: Tue, 8 May 2018 15:39:20 -0700 From: Andrew Morton To: Alexey Dobriyan Cc: linux-kernel@vger.kernel.org Subject: Re: [PATCH] proc: test /proc/*/fd a bit (+ PF_KTHREAD is ABI!) Message-Id: <20180508153920.9e74e1ab86a8b10e8ddf09a0@linux-foundation.org> In-Reply-To: <20180505000414.GA15090@avx2> References: <20180505000414.GA15090@avx2> X-Mailer: Sylpheed 3.6.0 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 5 May 2018 03:04:14 +0300 Alexey Dobriyan wrote: > * Test lookup in /proc/self/fd. > "map_files" lookup story showed that lookup is not that simple. > > * Test that all those symlinks open the same file. > Check with (st_dev, st_info). > > * Test that kernel threads do not have anything in their /proc/*/fd/ > directory. > > Now this is where things get interesting. > > First, kernel threads aren't pinned by /proc/self or equivalent, > thus some "atomicity" is required. > > Second, ->comm can contain whitespace and ')'. > No, they are not escaped. > > Third, the only reliable way to check if process is kernel thread > appears to be field #9 in /proc/*/stat. > > This field is struct task_struct::flags in decimal! > Check is done by testing PF_KTHREAD flags like we do in kernel. > > PF_KTREAD value is a part of userspace ABI !!! erk. Well if there's a need the we could export and support some stable interface. I wonder how ps determines this. > Other methods for determining kernel threadness are not reliable: > * RSS can be 0 if everything is swapped, even while reading > from /proc/self. > > * ->total_vm CAN BE ZERO if process is finishing > > munmap(NULL, whole address space); > > * /proc/*/maps and similar files can be empty because unmapping > everything works. Read returning 0 can't distinguish between > kernel thread and such suicide process. >