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SubjectRe: Kconfig dependency issue on function-graph tracer and frame pointer on arm
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/
> FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
> According to speedtest.net: 11.9Mbps down 500kbps up


--
Masami Hiramatsu <mhiramat@kernel.org>

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