Date:	Fri, 21 Aug 2015 08:32:07 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Michal Marek <mmarek@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Alves <palves@...hat.com>,
	Namhyung Kim <namhyung@...il.com>,
	Bernd Petrovitsch <bernd@...rovitsch.priv.at>,
	Chris J Arges <chris.j.arges@...onical.com>,
	live-patching@...r.kernel.org
Subject: Re: [PATCH v10 03/20] x86/stackvalidate: Compile-time stack
 validation

On Fri, Aug 21, 2015 at 09:54:49AM +0200, Ingo Molnar wrote:
> 
> * Josh Poimboeuf <jpoimboe@...hat.com> 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 <jpoimboe@...hat.com>
Subject: [PATCH] stackvalidate: Add missing frame pointer example

Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 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:
+
+     [<ffffffff81812584>] dump_stack+0x4b/0x63
+     [<ffffffff812d6dc2>] cmdline_proc_show+0x12/0x30
+     [<ffffffff8127f568>] seq_read+0x108/0x3e0
+     [<ffffffff812cce62>] proc_reg_read+0x42/0x70
+     [<ffffffff81256197>] __vfs_read+0x37/0x100
+     [<ffffffff81256b16>] vfs_read+0x86/0x130
+     [<ffffffff81257898>] SyS_read+0x58/0xd0
+     [<ffffffff8181c1f2>] 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:
+
+     [<ffffffff81812584>] dump_stack+0x4b/0x63
+     [<ffffffff812d6dc2>] cmdline_proc_show+0x12/0x30
+     [<ffffffff812cce62>] proc_reg_read+0x42/0x70
+     [<ffffffff81256197>] __vfs_read+0x37/0x100
+     [<ffffffff81256b16>] vfs_read+0x86/0x130
+     [<ffffffff81257898>] SyS_read+0x58/0xd0
+     [<ffffffff8181c1f2>] 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

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