Re: [PATCH v10 03/20] x86/stackvalidate: Compile-time stack validation

[Ingo Molnar <mingo@kernel.org>]

* Josh Poimboeuf <jpoimboe@redhat.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?

Thanks,

	Ingo