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