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 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 46A4BC433FE for ; Mon, 25 Oct 2021 20:42:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 25AE760EBC for ; Mon, 25 Oct 2021 20:42:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232596AbhJYUpO (ORCPT ); Mon, 25 Oct 2021 16:45:14 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41442 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230024AbhJYUpN (ORCPT ); Mon, 25 Oct 2021 16:45:13 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B77C1C061745; Mon, 25 Oct 2021 13:42:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; 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=OZ3sXa5W5LKoK68CV6cLI60WYGmCrZAiYPql/iGcsZk=; b=Rw/uWqthQJhk7xelE+mjEOhsxY mXW3XpEPfjFPib8sFG6dgHGK4g0BR2lGgNL7ydITKuwjAAhjL6JGvJNIGlGQoih/9iTpuE7OCZrjm wEQFMMnM8ifnvOhnUHdSa/qD4urDCSMIp8HGTbR9DZbiKa5b6Ig0oO6KSBM/ffITgyRUWoDWeocsC 7XbY9Kv9tGz3gXRnK1+PzFrgibUAMbmn7E4f8s2nfTjrRkYxf8wj6My02cFKOKS32kVyx9JS+mlc5 jcfHECVc0zRgn8zoSnnFn2zZuwyr93PpsHAYkFn9dF84DjFgRVnMcXcnxUnAM+UqA1+A3xsJeylSP WkGKjUzg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1mf6jr-00GQxi-23; Mon, 25 Oct 2021 20:38:51 +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 AAE2F3002AE; Mon, 25 Oct 2021 22:38:33 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 6A8582CCDBA64; Mon, 25 Oct 2021 22:38:33 +0200 (CEST) Date: Mon, 25 Oct 2021 22:38:33 +0200 From: Peter Zijlstra To: Mark Rutland Cc: Kees Cook , x86@kernel.org, linux-kernel@vger.kernel.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, akpm@linux-foundation.org, zhengqi.arch@bytedance.com, linux@armlinux.org.uk, catalin.marinas@arm.com, will@kernel.org, mpe@ellerman.id.au, paul.walmsley@sifive.com, palmer@dabbelt.com, hca@linux.ibm.com, gor@linux.ibm.com, borntraeger@de.ibm.com, linux-arch@vger.kernel.org, ardb@kernel.org Subject: Re: [PATCH 2/7] stacktrace,sched: Make stack_trace_save_tsk() more robust Message-ID: References: <20211022150933.883959987@infradead.org> <20211022152104.215612498@infradead.org> <202110220919.46F58199D@keescook> <20211022165431.GF86184@C02TD0UTHF1T.local> <20211022170135.GF174703@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211022170135.GF174703@worktop.programming.kicks-ass.net> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 22, 2021 at 07:01:35PM +0200, Peter Zijlstra wrote: > On Fri, Oct 22, 2021 at 05:54:31PM +0100, Mark Rutland wrote: > > > > Pardon my thin understanding of the scheduler, but I assume this change > > > doesn't mean stack_trace_save_tsk() stops working for "current", right? > > > In trying to answer this for myself, I couldn't convince myself what value > > > current->__state have here. Is it one of TASK_(UN)INTERRUPTIBLE ? > > > > Regardless of that, current->on_rq will be non-zero, so you're right that this > > causes stack_trace_save_tsk() to not work for current, e.g. > > > > | # cat /proc/self/stack > > | # wc /proc/self/stack > > | 0 0 0 /proc/self/stack > > > > TBH, I think that (taking a step back from this issue in particular) > > stack_trace_save_tsk() *shouldn't* work for current, and callers *should* be > > forced to explicitly handle current separately from blocked tasks. > > That.. So I think I'd prefer the following approach to that (and i'm not currently volunteering for it): - convert all archs to ARCH_STACKWALK; this gets the semantics out of arch code and into the single kernel/stacktrace.c file. - bike-shed a new/improved stack_trace_save*() API and implement it *once* in generic code based on arch_stack_walk(). - convert users; delete old etc.. For now, current users of stack_trace_save_tsk() very much expect tsk==current to work. > > So we could fix this in the stacktrace code with: > > > > | diff --git a/kernel/stacktrace.c b/kernel/stacktrace.c > > | index a1cdbf8c3ef8..327af9ff2c55 100644 > > | --- a/kernel/stacktrace.c > > | +++ b/kernel/stacktrace.c > > | @@ -149,7 +149,10 @@ unsigned int stack_trace_save_tsk(struct task_struct *tsk, unsigned long *store, > > | .skip = skipnr + (current == tsk), > > | }; > > | > > | - task_try_func(tsk, try_arch_stack_walk_tsk, &c); > > | + if (tsk == current) > > | + try_arch_stack_walk_tsk(tsk, &c); > > | + else > > | + task_try_func(tsk, try_arch_stack_walk_tsk, &c); > > | > > | return c.len; > > | } > > > > ... and we could rename task_try_func() to blocked_task_try_func(), and > > later push the distinction into higher-level callers. > > I think I favour this fix if we have to. But that's for next week :-) I ended up with the below delta to this patch. --- a/kernel/stacktrace.c +++ b/kernel/stacktrace.c @@ -101,7 +101,7 @@ static bool stack_trace_consume_entry_no } /** - * stack_trace_save - Save a stack trace into a storage array + * stack_trace_save - Save a stack trace (of current) into a storage array * @store: Pointer to storage array * @size: Size of the storage array * @skipnr: Number of entries to skip at the start of the stack trace @@ -132,7 +132,7 @@ static int try_arch_stack_walk_tsk(struc /** * stack_trace_save_tsk - Save a task stack trace into a storage array - * @task: The task to examine + * @task: The task to examine (current allowed) * @store: Pointer to storage array * @size: Size of the storage array * @skipnr: Number of entries to skip at the start of the stack trace @@ -149,13 +149,25 @@ unsigned int stack_trace_save_tsk(struct .skip = skipnr + (current == tsk), }; - task_try_func(tsk, try_arch_stack_walk_tsk, &c); + /* + * If the task doesn't have a stack (e.g., a zombie), the stack is + * empty. + */ + if (!try_get_task_stack(tsk)) + return 0; + + if (tsk == current) + try_arch_stack_walk_tsk(tsk, &c); + else + task_try_func(tsk, try_arch_stack_walk_tsk, &c); + + put_task_stack(tsk); return c.len; } /** - * stack_trace_save_regs - Save a stack trace based on pt_regs into a storage array + * stack_trace_save_regs - Save a stack trace (of current) based on pt_regs into a storage array * @regs: Pointer to pt_regs to examine * @store: Pointer to storage array * @size: Size of the storage array