From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id EFF8F30DD10; Mon, 1 Sep 2025 09:56:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756720616; cv=none; b=iIA8Q6Mh3WSikvzJQX8tUE/nsWXRZZurkYAf7csTDGq4F9P1UEITvTesJ0rdbgFPstOoqR0vljFZ8OR43pLcfvo6Coz72Ywh4LXjcKJBLjvZQTReg14B/vNIvB0LMpHsEZIg0uITjunVYP1UuU2ZyDzcRAbuhHsnITKJP9S8W/o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756720616; c=relaxed/simple; bh=/hqCkOwtBOhHQ1dFLUCN9G4AgIEKlLyHRGqnlAn23so=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Tnn7gHygqoMOuUBC7PQdyMCyHf0ncGbaJBN54CNwIZ3Da/zgC5FsLn7+2d6cd1njuIR03xwX3QtdsCZkQFf3PndVYeWXFt20YZi+MUGSS3UAEqnH63PnJNygjl5raTRfVDPjPsC6wepiKkBZmj4x7kD0vL+9/fLMQ47m0RFmxWk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 00C0C1A25; Mon, 1 Sep 2025 02:56:45 -0700 (PDT) Received: from J2N7QTR9R3 (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 308D43F6A8; Mon, 1 Sep 2025 02:56:50 -0700 (PDT) Date: Mon, 1 Sep 2025 10:56:47 +0100 From: Mark Rutland To: Catalin Marinas Cc: Steven Rostedt , Luo Gengkun , mhiramat@kernel.org, mathieu.desnoyers@efficios.com, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Will Deacon , linux-arm-kernel@lists.infradead.org, Al Viro Subject: Re: [PATCH] tracing: Fix tracing_marker may trigger page fault during preempt_disable Message-ID: References: <20250819105152.2766363-1-luogengkun@huaweicloud.com> <20250819135008.5f1ba00e@gandalf.local.home> <436e4fa7-f8c7-4c23-a28a-4e5eebe2f854@huaweicloud.com> <20250829082604.1e3fd06e@gandalf.local.home> <20250829083655.3d38d02b@gandalf.local.home> <20250829181311.079f33bf@gandalf.local.home> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Sat, Aug 30, 2025 at 11:22:51AM +0100, Catalin Marinas wrote: > On Fri, Aug 29, 2025 at 06:13:11PM -0400, Steven Rostedt wrote: > > On Fri, 29 Aug 2025 20:53:40 +0100 > > Catalin Marinas wrote: > > valid user address. > > > BTW, arm64 also bails out early in do_page_fault() if in_atomic() but I > > > suspect that's not the case here. > > > > > > Adding Al Viro since since he wrote a large part of uaccess.h. > > > > So, __copy_from_user_inatomic() is supposed to be called if > > pagefault_disable() has already been called? If this is the case, can we > > add more comments to this code? I've been using the inatomic() version this > > way in preempt disabled locations since 2016. > > This should work as long as in_atomic() returns true as it's checked in > the arm64 fault code. With PREEMPT_NONE, however, I don't think this > works. Sorry, what exactly breaks for the PREEMPT_NONE case? > __copy_from_user_inatomic() could be changed to call > pagefault_disable() if !in_atomic() but you might as well call > copy_from_user_nofault() in the trace code directly as per Luo's patch. That makes sense to me. I'll go check the arm64 users of __copy_from_user_inatomic(), in case they're doing something dodgy. > > I just wanted to figure out why __copy_from_user_inatomic() wasn't atomic. > > If anything, it needs to be better documented. > > Yeah, I had no idea until I looked at the code. I guess it means it can > be safely used if in_atomic() == true (well, making it up, not sure what > the intention was). I think that was the intention -- it's the caller asserting that they know the access won't fault (and hence won't sleep), and that's why __copy_to_user_inatomic() and __copy_to_user() only differ by the latter calling might_sleep(). It looks like other inconsistencies have crept in by accident. AFAICT the should_fail_usercopy() check in __copy_from_user() was accidentally missed from __copy_from_user_inatomic() when it was introduced in commit: 4d0e9df5e43dba52 ("lib, uaccess: add failure injection to usercopy functions") ... and the instrumentation is ordered inconsistently w.r.t. should_fail_usercopy() since commit: 33b75c1d884e81ec ("instrumented.h: allow instrumenting both sides of copy_from_user()") ... so there's a bunch of scope for cleanup, and we could probably have: /* * TODO: comment saying to only call this directly when you know * that the fault handler won't sleep. */ static __always_inline __must_check unsigned long __copy_from_user_inatomic(void *to, const void __user *from, unsigned long n) { ... } static __always_inline __must_check unsigned long __copy_from_user(void *to, const void __user *from, unsigned long n) { might_fault(); return __copy_from_user_inatomic(); } ... to avoid the inconsistency. Mark.