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 537A7C433F5 for ; Thu, 14 Oct 2021 10:02:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 37BC46109E for ; Thu, 14 Oct 2021 10:02:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230097AbhJNKEE (ORCPT ); Thu, 14 Oct 2021 06:04:04 -0400 Received: from mail.kernel.org ([198.145.29.99]:44348 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229468AbhJNKED (ORCPT ); Thu, 14 Oct 2021 06:04:03 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 661C76101E; Thu, 14 Oct 2021 10:01:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1634205719; bh=LQXws2m2KNS8oaYOQqJFpw4WI2EHw9lQOWuJTqzRGng=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=Dhva+lcu+iWoZoHCwkWrlrYDu7Y9TiJQGtgip+8YkQhgdedHZLJRj8cp9TsdCt5LI u8YOMwMLQO2Wm8AqytfUrPT+6Z5TOLeEfkxclXonNmsC29jG4IcqfNLSOipJePYT8P xgEd/xPuBRiNeqWi6BpniWwfVoGK+AZW/SsxMeDRn/1g60vFkAW+jmwu9Wzouul/It s8QzrruJyeWjXdHL3JxeVEB0uuTrs3vtGAhcigYPnSKWVh7u44y8qWb7Sq560sjniP DPizE6uchDG0u5KeQ8X9EauoT4be8DuE6G1Typ1+Y6MtNtJlTgf0XYGZBCazvZ3K2E uaymTzQJaMz3w== Date: Thu, 14 Oct 2021 19:01:55 +0900 From: Masami Hiramatsu To: Mark Rutland Cc: Steven Rostedt , "Naveen N . Rao" , Ananth N Mavinakayanahalli , Ingo Molnar , linux-kernel@vger.kernel.org, Sven Schnelle , Catalin Marinas , Will Deacon , Russell King , Nathan Chancellor , Nick Desaulniers , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH 3/8] arm64: kprobes: Record frame pointer with kretprobe instance Message-Id: <20211014190155.3fdc7cf7c42e44ee75c43a9d@kernel.org> In-Reply-To: <20211014091332.GA13770@C02TD0UTHF1T.local> References: <163369609308.636038.15295764725220907794.stgit@devnote2> <163369611948.636038.11552166777773804729.stgit@devnote2> <20211013100126.GA3187@C02TD0UTHF1T.local> <20211014170405.f59d287b30086efe7dd7f4d9@kernel.org> <20211014091332.GA13770@C02TD0UTHF1T.local> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 14 Oct 2021 10:13:32 +0100 Mark Rutland wrote: > On Thu, Oct 14, 2021 at 05:04:05PM +0900, Masami Hiramatsu wrote: > > On Wed, 13 Oct 2021 11:01:39 +0100 > > Mark Rutland wrote: > > > > > On Fri, Oct 08, 2021 at 09:28:39PM +0900, Masami Hiramatsu wrote: > > > > Record the frame pointer instead of stack address with kretprobe > > > > instance as the identifier on the instance list. > > > > Since arm64 always enable CONFIG_FRAME_POINTER, we can use the > > > > actual frame pointer (x29). > > > > > > Just to check, why do we need to use the FP rather than SP? It wasn't > > > clear to me if that's necessary later in the series, or if I'm missing > > > something here. > > > > Actually, this is for finding correct return address from the per-task > > kretprobe instruction list (suppose it as a shadow stack) when it will > > be searched in stack-backtracing. At that point, the framepointer will > > be a reliable key. > > Sure, my question was more "why isn't the SP a reliable key?", because both > the SP and FP should be balanced at function-entry and function-return > time. I'm asking because I think I'm missing a subtlety. Ah, because SP is not used while unwinding frame. For the kretprobe, either FP or SP is OK. But for the stacktrace.c, I can not use SP and is easy to change to use FP. :) So, when we introduce ORC unwinder on arm64, I think I have to reconsider using SP based on the configuration. Thank you, > > I'm perfectly happy to use the FP even if they're equivalent; I just > want to make sure there's not some issue I'm unaware of that could > affect unwinding. > > Thanks, > Mark. > > > > FWIW, I plan to rework arm64's ftrace bits to use FP for > > > HAVE_FUNCTION_GRAPH_RET_ADDR_PTR, so I'm happy to do likewise here. > > > > Yes, I think you can use FP for that too. > > > > > > > > > Signed-off-by: Masami Hiramatsu > > > > > > Regardless of the above: > > > > > > Acked-by: Mark Rutland > > > > Thank you! > > > > > > > > Mark. > > > > > > > --- > > > > arch/arm64/kernel/probes/kprobes.c | 4 ++-- > > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c > > > > index e7ad6da980e8..d9dfa82c1f18 100644 > > > > --- a/arch/arm64/kernel/probes/kprobes.c > > > > +++ b/arch/arm64/kernel/probes/kprobes.c > > > > @@ -401,14 +401,14 @@ int __init arch_populate_kprobe_blacklist(void) > > > > > > > > void __kprobes __used *trampoline_probe_handler(struct pt_regs *regs) > > > > { > > > > - return (void *)kretprobe_trampoline_handler(regs, (void *)kernel_stack_pointer(regs)); > > > > + return (void *)kretprobe_trampoline_handler(regs, (void *)regs->regs[29]); > > > > } > > > > > > > > void __kprobes arch_prepare_kretprobe(struct kretprobe_instance *ri, > > > > struct pt_regs *regs) > > > > { > > > > ri->ret_addr = (kprobe_opcode_t *)regs->regs[30]; > > > > - ri->fp = (void *)kernel_stack_pointer(regs); > > > > + ri->fp = (void *)regs->regs[29]; > > > > > > > > /* replace return addr (x30) with trampoline */ > > > > regs->regs[30] = (long)&__kretprobe_trampoline; > > > > > > > > > > -- > > Masami Hiramatsu -- Masami Hiramatsu