From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5B97AC43460 for ; Tue, 4 May 2021 08:39:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1DD3661004 for ; Tue, 4 May 2021 08:39:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230156AbhEDIke (ORCPT ); Tue, 4 May 2021 04:40:34 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37186 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230122AbhEDIk2 (ORCPT ); Tue, 4 May 2021 04:40:28 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 149BFC06174A; Tue, 4 May 2021 01:39:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=jHj0U851XAR8dHLCObZjvsMxkhCDRfe/hly+/JhNV6I=; b=i58/6+bhGSraSN+KcG3X89rTmB fhTwqUp8GO7z5KV9/cXXwo6ImpdHjCATymAK8Ue9U0dVBfSj1CLBLUMc5kETh3WNRiIOBLrxsfdXs ujSOJWFg0jIArJcvyOYUzLQYiE/qI2pyNISmOZEvUGx3YmgYp1IWCgmYdLJdcFMs8pyKVAmMs+mIY b+WI2qaeBU3bQv+Sonsso7Y1wLYCCF6G3JdAWTUSff0UrDccxrwcHrgEZc9nT2cVWs+SPjy25YGS0 nwzG4eG2t5AU+NvsJAG+aCx+dVqFPIXHqyEq14SjsNIt04zVYAtlI+e1qM4GWCm/CYOWmBiVowWSE unLQnalg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94 #2 (Red Hat Linux)) id 1ldqaT-00Floj-Gh; Tue, 04 May 2021 08:39:25 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id BDE55300036; Tue, 4 May 2021 10:39:23 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id ACADB20D77EF8; Tue, 4 May 2021 10:39:23 +0200 (CEST) Date: Tue, 4 May 2021 10:39:23 +0200 From: Peter Zijlstra To: Linus Torvalds Cc: Andy Lutomirski , Thomas Gleixner , Stefan Metzmacher , Jens Axboe , Linux Kernel Mailing List , io-uring , the arch/x86 maintainers , linux-toolchains@vger.kernel.org Subject: Re: [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es' registers for io_threads Message-ID: References: <8735v3ex3h.ffs@nanos.tec.linutronix.de> <3C41339D-29A2-4AB1-958F-19DB0A92D8D7@amacapital.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org + linux-toolchains On Mon, May 03, 2021 at 12:14:45PM -0700, Linus Torvalds wrote: > On Mon, May 3, 2021 at 9:05 AM Andy Lutomirski wrote: > > > > Linus, what is the actual effect of allowing gdb to attach these threads? Can we instead make all the regset ops do: > > > > if (not actually a user thread) return -EINVAL; > > I don't think it matters - the end result ends up being the same, ie > gdb gets confused about whether the (parent) thread is a 32-bit or > 64-bit one. > > So the basic issue is > > (a) we want the IO threads to look exactly like normal user threads > as far as the kernel is concerned, because we had way too many bugs > due to special cases. > > (b) but that means that they are also visible to user space, and then > gdb has this odd thing where it takes the 64-bit vs 32-bit data for > the whole process from one thread, and picks the worst possible thread > to do it (ie explicitly not even the main thread, so usually the IO > thread!) > > That (a) ended up really being critical. The issues with special cases > were just horrendous, both for security issues (ie "make them kernel > threads but carry user credentials" just caused lots of problems) but > also for various just random other state handling issues (signal state > in particular). > > So generally, the IO threads are now 100% normal threads - it's > literally just that they never return to user space because they are > always just doing the IO offload on the kernel side. > > That part is lovely, but part of the "100% IO threads" really is that > they share the signal struct too, which in turn means that they very > much show up as normal threads. Again, not a problem: they really > _are_ normal threads for all intents and purposes. > > But then that (b) issue means that gdb gets confused by them. I > personally think that's just a pure gdb mis-feature, but I also think > that "hey, if we just make the register state look like the main > thread, and unconfuse gdb that way, problem solved". > > So I'd actually rather not make these non-special threads any more > special at all. And I strongly suspect that making ptrace() not work > on them will just confuse gdb even more - so it would make them just > unnecessarily special in the kernel, for no actual gain. > > Is the right thing to do to fix gdb to not look at irrelevant thread B > when deciding whether thread A is 64-bit or not? Yeah, that seems like > obviously the RightThing(tm) to me. > > But at the same time, this is arguably about "regression", although at > the same time it's "gdb doesn't understand new user programs that use > new features, film at 11", so I think that argument is partly bogus > too. > > So my personal preference would be: > > - make those threads look even more like user threads, even if that > means giving them pointless user segment data that the threads > themselves will never use > > So I think Stefan's patch is reasonable, if not pretty. Literally > becasue of that "make these threads look even more normal" > > - ALSO fix gdb that is doing obviously garbage stupid things > > But I'm obviously not involved in that "ALSO fix gdb" part, and > arguably the kernel hack then makes it more likely that gdb will > continue doing its insane broken thing. Anybody on toolchains that can help get GDB fixed?