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

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.

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

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
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
select ARM_UNWIND
help
This option enables stack unwinding support in the kernel
--
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