Re: [PATCH v4 2/2] x86/kconfig: make it easier to switch to the new ORC unwinder

From: Josh Poimboeuf
Date: Tue Jul 25 2017 - 09:54:46 EST

On Tue, Jul 25, 2017 at 11:10:59AM +0200, Ingo Molnar wrote:
>
> * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> > A couple of Kconfig changes which make it much easier to switch to the
> > new CONFIG_ORC_UNWINDER:
> >
> > 1) Remove x86 dependencies on CONFIG_FRAME_POINTER for lockdep,
> > latencytop, and fault injection. x86 has a 'guess' unwinder which
> > just scans the stack for kernel text addresses. It's not 100%
> > accurate but in many cases it's good enough. This allows those users
> > who don't want the text overhead of the frame pointer or ORC
> > unwinders to still use these features. More importantly, this also
> > makes it much more straightforward to disable frame pointers.
> >
> > 2) Make CONFIG_ORC_UNWINDER depend on !CONFIG_FRAME_POINTER. While it
> > would be possible to have both enabled, it doesn't really make sense
> > to do so. So enforce a sane configuration to prevent the user from
> > making a dumb mistake.
> >
> > With these changes, when you disable CONFIG_FRAME_POINTER, "make
> > oldconfig" will ask if you want to enable CONFIG_ORC_UNWINDER.
>
> Yeah, so I think this is still suboptimal: the frame pointer and the Orc unwinders
> are configured in different places, and the user won't know about the various
> unwinder options unless stumbling across them by accidentally disabling frame
> pointers ...
>
> Also, the Kconfig help text for frame pointers is now actively misleading:
>
> CONFIG_FRAME_POINTER:
>
> If you say Y here the resulting kernel image will be slightly
> larger and slower, but it gives very useful debugging information
> in case of kernel bugs. (precise oopses/stacktraces/warnings)
>
> Please, as I suggested it before, make it a multiple choice option: frame, Orc, or
> the guess unwinder.
>
> I'd only offer the 'guess' unwinder if EXPERT is selected, because it's a really
> sub-optimal selection all things considered.
>
> Once things are tested and it's all rosy we can change the default x86 unwinder to
> Orc and organize the naming of the config variables to:
>
> CONFIG_UNWINDER_ORC
> CONFIG_UNWINDER_FRAME_POINTER
> CONFIG_UNWINDER_GUESS

How about the below patch? It goes on top of the other two.

----

From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Subject: [PATCH] x86/kconfig: consolidate unwinders into multiple choice selection

There are three mutually exclusive unwinders. Make that more obvious by
combining them into a multiple-choice selection:

CONFIG_FRAME_POINTER_UNWINDER
CONFIG_ORC_UNWINDER
CONFIG_GUESS_UNWINDER (if CONFIG_EXPERT=y)

Frame pointers are still the default (for now).

The old CONFIG_FRAME_POINTER option is still used in some
arch-independent places, so keep it around, but make it invisible to the
user on x86. It's now selected by CONFIG_FRAME_POINTER_UNWINDER.

Suggested-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
arch/x86/Kconfig | 3 +--
arch/x86/Kconfig.debug | 47 ++++++++++++++++++++++++++++++++++++-------
arch/x86/configs/tiny.config | 2 ++
arch/x86/include/asm/unwind.h | 4 ++--
4 files changed, 45 insertions(+), 11 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 21c75a652bb9..1e789ecefc02 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -73,7 +73,6 @@ config X86
select ARCH_USE_QUEUED_RWLOCKS
select ARCH_USE_QUEUED_SPINLOCKS
select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
- select ARCH_WANT_FRAME_POINTERS
select ARCH_WANTS_DYNAMIC_TASK_STRUCT
select ARCH_WANTS_THP_SWAP if X86_64
select BUILDTIME_EXTABLE_SORT
@@ -168,7 +167,7 @@ config X86
select HAVE_PERF_REGS
select HAVE_PERF_USER_STACK_DUMP
select HAVE_REGS_AND_STACK_ACCESS_API
- select HAVE_RELIABLE_STACKTRACE if X86_64 && FRAME_POINTER && STACK_VALIDATION
+ select HAVE_RELIABLE_STACKTRACE if X86_64 && FRAME_POINTER_UNWINDER && STACK_VALIDATION
select HAVE_STACK_VALIDATION if X86_64
select HAVE_SYSCALL_TRACEPOINTS
select HAVE_UNSTABLE_SCHED_CLOCK
diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index e6176007b838..71a48a30fc84 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -356,9 +356,32 @@ config PUNIT_ATOM_DEBUG
The current power state can be read from
/sys/kernel/debug/punit_atom/dev_power_state

+choice
+ prompt "Choose kernel unwinder"
+ default FRAME_POINTER_UNWINDER
+ ---help---
+ This determines which method will be used for unwinding kernel stack
+ traces for panics, oopses, bugs, warnings, perf, /proc/<pid>/stack,
+ livepatch, lockdep, and more.
+
+config FRAME_POINTER_UNWINDER
+ bool "Frame pointer unwinder"
+ select FRAME_POINTER
+ ---help---
+ This option enables the frame pointer unwinder for unwinding kernel
+ stack traces.
+
+ The unwinder itself is fast and it uses less RAM than the ORC
+ unwinder, but the kernel text size will grow by ~3% and the kernel's
+ overall performance will degrade by roughly 5-10%.
+
+ This option is recommended if you want to use the livepatch
+ consistency model, as this is currently the only way to get a
+ reliable stack trace (CONFIG_HAVE_RELIABLE_STACKTRACE).
+
config ORC_UNWINDER
bool "ORC unwinder"
- depends on X86_64 && !FRAME_POINTER
+ depends on X86_64
select STACK_VALIDATION
---help---
This option enables the ORC (Oops Rewind Capability) unwinder for
@@ -372,12 +395,22 @@ config ORC_UNWINDER
Enabling this option will increase the kernel's runtime memory usage
by roughly 2-4MB, depending on your kernel config.

-config FRAME_POINTER_UNWINDER
- def_bool y
- depends on !ORC_UNWINDER && FRAME_POINTER
-
config GUESS_UNWINDER
- def_bool y
- depends on !ORC_UNWINDER && !FRAME_POINTER
+ bool "Guess unwinder"
+ depends on EXPERT
+ ---help---
+ This option enables the "guess" unwinder for unwinding kernel stack
+ traces. It scans the stack and reports every kernel text address it
+ finds. Some of the addresses it reports may be incorrect.
+
+ While this option often produces false positives, it can still be
+ useful in many cases. Unlike the other unwinders, it has no runtime
+ overhead.
+
+endchoice
+
+config FRAME_POINTER
+ depends on !ORC_UNWINDER && !GUESS_UNWINDER
+ bool

endmenu
diff --git a/arch/x86/configs/tiny.config b/arch/x86/configs/tiny.config
index 4b429df40d7a..550cd5012b73 100644
--- a/arch/x86/configs/tiny.config
+++ b/arch/x86/configs/tiny.config
@@ -1,3 +1,5 @@
CONFIG_NOHIGHMEM=y
# CONFIG_HIGHMEM4G is not set
# CONFIG_HIGHMEM64G is not set
+CONFIG_GUESS_UNWINDER=y
+# CONFIG_FRAME_POINTER_UNWINDER is not set
diff --git a/arch/x86/include/asm/unwind.h b/arch/x86/include/asm/unwind.h
index 25b8d31a007d..e9f793e2df7a 100644
--- a/arch/x86/include/asm/unwind.h
+++ b/arch/x86/include/asm/unwind.h
@@ -16,7 +16,7 @@ struct unwind_state {
bool signal, full_regs;
unsigned long sp, bp, ip;
struct pt_regs *regs;
-#elif defined(CONFIG_FRAME_POINTER)
+#elif defined(CONFIG_FRAME_POINTER_UNWINDER)
bool got_irq;
unsigned long *bp, *orig_sp, ip;
struct pt_regs *regs;
@@ -50,7 +50,7 @@ void unwind_start(struct unwind_state *state, struct task_struct *task,
__unwind_start(state, task, regs, first_frame);
}

-#if defined(CONFIG_ORC_UNWINDER) || defined(CONFIG_FRAME_POINTER)
+#if defined(CONFIG_ORC_UNWINDER) || defined(CONFIG_FRAME_POINTER_UNWINDER)
static inline struct pt_regs *unwind_get_entry_regs(struct unwind_state *state)
{
if (unwind_done(state))
--
2.13.3