Re: Kconfig dependency issue on function-graph tracer and frame pointer on arm

[Masami Hiramatsu <mhiramat@kernel.org>]

On Sun, 14 Apr 2019 14:34:58 +0100
Russell King - ARM Linux admin <linux@armlinux.org.uk> wrote:

> On Sun, Apr 14, 2019 at 07:47:05PM +0900, Masami Hiramatsu wrote:
> > Hello,
> > 
> > Recently, Naresh reported that the function-graph tracer on the latest
> > kernel crashes on arm. I could reproduce it and bisected. I finally found
> > the commit f9b58e8c7d03 ("ARM: 8800/1: use choice for kernel unwinders")
> > was the first bad commit.
> 
> I don't think that littering the rest of the kernel Kconfig with ARM
> specific stuff is really a viable solution to this.
> 
> If we examine the current situation, we have:
> 
> - THUMB2_KERNEL selecting ARM_UNWIND when enabled.
> - UNWINDER_FRAME_POINTER disabled if THUMB2_KERNEL is enabled, provided
>   we're not using Clang.  This leaves UNWINDER_ARM as the only choice,
>   which also selects ARM_UNWIND.
> - The default choice is dependent on the settings of AEABI and
>   FUNCTION_GRAPH_TRACER.
> - HAVE_FUNCTION_GRAPH_TRACER is disabled if THUMB2_KERNEL is enabled.
> 
> which seems to be _way_ too messy.
> 
> Looking back before this commit, the function graph tracer never had a
> strong dependence on frame pointers being enabled in the kernel, but it
> seems the code relies upon them (see ftrace_return_to_handler() in
> kernel/trace/ and return_to_handler in arch/arm/kernel/entry-frace.S).
> There is also the __ftrace_graph_caller macro which seems to rely on it.

Yes, so I think similar bug is hiding in other LTS kernels. It should
have a dependency to FRAME_POINTER on arm.

> Since Clang does not support frame pointers, we shouldn't even offer
> the function graph tracer for Clang compilers, so let's do that with
> the first hunk of the patch below.
> 
> The subsequent hunks remove the defaulting of the choice according to
> the function graph tracer - this is not a "hint" where the user can
> still choose either option irrespective of the state of the function
> graph tracer.  They should only be able to select the frame pointer
> option in that case.

Agreed. Using default for making dependency is wrong.

> 
> Another way forward would be for someone to put the work in to making
> the function graph tracer work without frame pointers.

Yes, we eventually need that. But for fixing current released kernel
(this bug is in v5.0 series), I think Kconfig fix is needed.

> 
> So, how about this:
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 850b4805e2d1..9aed25a6019b 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -73,7 +73,7 @@ config ARM
>  	select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
>  	select HAVE_EXIT_THREAD
>  	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
> -	select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL
> +	select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL && !CC_IS_CLANG
>  	select HAVE_FUNCTION_TRACER if !XIP_KERNEL
>  	select HAVE_GCC_PLUGINS
>  	select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)
> diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
> index 6d6e0330930b..e388af4594a6 100644
> --- a/arch/arm/Kconfig.debug
> +++ b/arch/arm/Kconfig.debug
> @@ -47,8 +47,8 @@ config DEBUG_WX
>  
>  choice
>  	prompt "Choose kernel unwinder"
> -	default UNWINDER_ARM if AEABI && !FUNCTION_GRAPH_TRACER
> -	default UNWINDER_FRAME_POINTER if !AEABI || FUNCTION_GRAPH_TRACER
> +	default UNWINDER_ARM if AEABI
> +	default UNWINDER_FRAME_POINTER if !AEABI

If UNWINDER_ARM depends on ARM EABI, would we really need this "if !AEABI"?
(I doubt we need these default...)

>  	help
>  	  This determines which method will be used for unwinding kernel stack
>  	  traces for panics, oopses, bugs, warnings, perf, /proc/<pid>/stack,
> @@ -65,7 +65,7 @@ config UNWINDER_FRAME_POINTER
>  
>  config UNWINDER_ARM
>  	bool "ARM EABI stack unwinder"
> -	depends on AEABI
> +	depends on AEABI && !FUNCTION_GRAPH_TRACER

Hmm, AFAIK, FUNCTION_GRAPH_TRACER only depends on FRAME_POINTER, but not
UNWINDER_FRAME_POINTER. So can user still choose UNWINDER_ARM even if
FUNCTION_GRAPH_TRACER=y ? (Of course that may not be a meaningful option)

Thank you,

>  	select ARM_UNWIND
>  	help
>  	  This option enables stack unwinding support in the kernel
> 
> -- 
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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-- 
Masami Hiramatsu <mhiramat@kernel.org>