Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753502AbbHUNcM (ORCPT ); Fri, 21 Aug 2015 09:32:12 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55844 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752613AbbHUNcK (ORCPT ); Fri, 21 Aug 2015 09:32:10 -0400 Date: Fri, 21 Aug 2015 08:32:07 -0500 From: Josh Poimboeuf To: Ingo Molnar Cc: Andrew Morton , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , x86@kernel.org, linux-kernel@vger.kernel.org, Michal Marek , Peter Zijlstra , Andy Lutomirski , Borislav Petkov , Linus Torvalds , Andi Kleen , Pedro Alves , Namhyung Kim , Bernd Petrovitsch , Chris J Arges , live-patching@vger.kernel.org Subject: Re: [PATCH v10 03/20] x86/stackvalidate: Compile-time stack validation Message-ID: <20150821133207.GB14134@treble.redhat.com> References: <07bf51833b5e1c52bfd9a4dbda41b80c508dffff.1439521412.git.jpoimboe@redhat.com> <20150815002354.7fb2f21e.akpm@linux-foundation.org> <20150815124913.GB3254@treble.hsd1.ky.comcast.net> <20150819100138.GA10504@gmail.com> <20150820040050.GC2944@treble.redhat.com> <20150821075449.GA10443@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20150821075449.GA10443@gmail.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5064 Lines: 114 On Fri, Aug 21, 2015 at 09:54:49AM +0200, Ingo Molnar wrote: > > * Josh Poimboeuf wrote: > > > +Why do we need stack validation? > > +-------------------------------- > > + > > +Here are some of the benefits of validating stack metadata: > > + > > +a) More reliable stack traces for frame pointer enabled kernels > > + > > + Frame pointers are used for debugging purposes. They allow runtime > > + code and debug tools to be able to walk the stack to determine the > > + chain of function call sites that led to the currently executing > > + code. > > + > > + For some architectures, frame pointers are enabled by > > + CONFIG_FRAME_POINTER. For some other architectures they may be > > + required by the ABI (sometimes referred to as "backchain pointers"). > > + > > + For C code, gcc automatically generates instructions for setting up > > + frame pointers when the -fno-omit-frame-pointer option is used. > > + > > + But for asm code, the frame setup instructions have to be written by > > + hand, which most people don't do. So the end result is that > > + CONFIG_FRAME_POINTER is honored for C code but not for most asm code. > > + > > + For stack traces based on frame pointers to be reliable, all > > + functions which call other functions must first create a stack frame > > + and update the frame pointer. If a first function doesn't properly > > + create a stack frame before calling a second function, the *caller* > > + of the first function will be skipped on the stack trace. > > + > > + The benefit of stackvalidate here is that it ensures that *all* > > + functions honor CONFIG_FRAME_POINTER. As a result, no functions will > > + ever [*] be skipped on a stack trace. > > + > > + [*] unless an interrupt or exception has occurred at the very > > + beginning of a function before the stack frame has been created, > > + or at the very end of the function after the stack frame has been > > + destroyed. This is an inherent limitation of frame pointers. > > What this section does not point out is the actual effects of missing frame > pointer annotations. I.e. how about quoting a before/after stack backtrace to > demonstrate it? How about this (on top of the last one): ---8<--- From: Josh Poimboeuf Subject: [PATCH] stackvalidate: Add missing frame pointer example Signed-off-by: Josh Poimboeuf --- Documentation/stack-validation.txt | 37 ++++++++++++++++++++++++++++++++++--- 1 file changed, 34 insertions(+), 3 deletions(-) diff --git a/Documentation/stack-validation.txt b/Documentation/stack-validation.txt index 94dad40..87a5ab8 100644 --- a/Documentation/stack-validation.txt +++ b/Documentation/stack-validation.txt @@ -53,9 +53,40 @@ a) More reliable stack traces for frame pointer enabled kernels create a stack frame before calling a second function, the *caller* of the first function will be skipped on the stack trace. - The benefit of stackvalidate here is that it ensures that *all* - functions honor CONFIG_FRAME_POINTER. As a result, no functions will - ever [*] be skipped on a stack trace. + For example, consider the following example backtrace with frame + pointers enabled: + + [] dump_stack+0x4b/0x63 + [] cmdline_proc_show+0x12/0x30 + [] seq_read+0x108/0x3e0 + [] proc_reg_read+0x42/0x70 + [] __vfs_read+0x37/0x100 + [] vfs_read+0x86/0x130 + [] SyS_read+0x58/0xd0 + [] entry_SYSCALL_64_fastpath+0x12/0x76 + + It correctly shows that the caller of cmdline_proc_show() is + seq_read(). + + If we remove the frame pointer logic from cmdline_proc_show() by + replacing the frame pointer related instructions with nops, here's + what it looks like instead: + + [] dump_stack+0x4b/0x63 + [] cmdline_proc_show+0x12/0x30 + [] proc_reg_read+0x42/0x70 + [] __vfs_read+0x37/0x100 + [] vfs_read+0x86/0x130 + [] SyS_read+0x58/0xd0 + [] entry_SYSCALL_64_fastpath+0x12/0x76 + + Notice that cmdline_proc_show()'s caller, seq_read(), has been + skipped. Instead the stack trace seems to show that + cmdline_proc_show() was called by proc_reg_read(). + + The benefit of stackvalidate here is that because it ensures that + *all* functions honor CONFIG_FRAME_POINTER, no functions will ever[*] + be skipped on a stack trace. [*] unless an interrupt or exception has occurred at the very beginning of a function before the stack frame has been created, -- 2.4.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/