Re: [PATCH v6 02/17] arm64: stacktrace: Factor out on_accessible_stack_common()

On Tue, Jul 26, 2022 at 9:01 AM Marc Zyngier [off-list ref] wrote:

On Tue, 26 Jul 2022 08:37:35 +0100,
Kalesh Singh [off-list ref] wrote:

quoted

Move common on_accessible_stack checks to stacktrace/common.h. This is
used in the implementation of the nVHE hypervisor unwinder later in
this series.

Signed-off-by: Kalesh Singh <redacted>
Reviewed-by: Fuad Tabba <redacted>
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Fuad Tabba <redacted>
---

Changes in v6:
  - Add Fuad's Tested-by tag

Changes in v5:
  - Add Reviewed-by tags from Mark Brown and Fuad
  - Remove random whitespace change, per Mark Brown

 arch/arm64/include/asm/stacktrace.h        |  6 ++----
 arch/arm64/include/asm/stacktrace/common.h | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
index 79f455b37c84..43f4b4a6d383 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h

@@ -65,8 +65,8 @@ static inline bool on_accessible_stack(const struct task_struct *tsk,
                                     unsigned long sp, unsigned long size,
                                     struct stack_info *info)
 {
-     if (info)
-             info->type = STACK_TYPE_UNKNOWN;
+     if (on_accessible_stack_common(tsk, sp, size, info))
+             return true;

      if (on_task_stack(tsk, sp, size, info))
              return true;

@@ -74,8 +74,6 @@ static inline bool on_accessible_stack(const struct task_struct *tsk,
              return false;
      if (on_irq_stack(sp, size, info))
              return true;
-     if (on_overflow_stack(sp, size, info))
-             return true;
      if (on_sdei_stack(sp, size, info))
              return true;

diff --git a/arch/arm64/include/asm/stacktrace/common.h b/arch/arm64/include/asm/stacktrace/common.h
index 64ae4f6b06fe..f58b786460d3 100644
--- a/arch/arm64/include/asm/stacktrace/common.h
+++ b/arch/arm64/include/asm/stacktrace/common.h

@@ -62,6 +62,9 @@ struct unwind_state {
      struct task_struct *task;
 };

+static inline bool on_overflow_stack(unsigned long sp, unsigned long size,
+                                  struct stack_info *info);
+
 static inline bool on_stack(unsigned long sp, unsigned long size,
                          unsigned long low, unsigned long high,
                          enum stack_type type, struct stack_info *info)

@@ -80,6 +83,21 @@ static inline bool on_stack(unsigned long sp, unsigned long size,
      return true;
 }

+static inline bool on_accessible_stack_common(const struct task_struct *tsk,
+                                           unsigned long sp,
+                                           unsigned long size,
+                                           struct stack_info *info)
+{
+     if (info)
+             info->type = STACK_TYPE_UNKNOWN;
+
+     /*
+      * Both the kernel and nvhe hypervisor make use of
+      * an overflow_stack
+      */
+     return on_overflow_stack(sp, size, info);
+}

on_accessible_stack has the following comment:

/*
 * We can only safely access per-cpu stacks from current in a non-preemptible
 * context.
 */

With this change, I don't think we satisfy this requirement anymore,
as we're checking the overflow stack *before* the preemptible check,
which is a big change in behaviour.

The hypervisor doesn't have this requirement: the unwinding is either
done out of context (nVHE, where EL1 unwinds EL2) or in a
non-preemptible section (pKVM, where the whole thing is
non-preemptible). But the kernel is usually preemptible, so this patch
needs fixing.

I'll see if I can address it locally (I'm currently moving things
around, stay tuned).

Hi Marc,

You are right it changes the order of the checks. I think the simplest
way then is to drop this patch, and replace uses of on_accessible
_stack_common().

Thanks,
Kalesh

Thanks,

        M.

--
Without deviation from the norm, progress is not possible.

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