[PATCH v4 5/9] ARM: unwind: dump exception stack from calling frame

[Ard Biesheuvel]

The existing code that dumps the contents of the pt_regs structure
passed to __entry routines does so while unwinding the callee frame, and
dereferences the stack pointer as a struct pt_regs*. This will no longer
work when we enable support for IRQ or overflow stacks, because the
struct pt_regs may live on the task stack, while we are executing from
another stack.

The unwinder has access to this information, but only while unwinding
the calling frame. So let's combine the exception stack dumping code
with the handling of the calling frame as well. By printing it before
dumping the caller/callee addresses, the output order is preserved.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
Reviewed-by: Arnd Bergmann <arnd at arndb.de>
Acked-by: Linus Walleij <linus.walleij at linaro.org>
Tested-by: Keith Packard <keithpac at amazon.com>
---
 arch/arm/include/asm/stacktrace.h | 10 ++++++++++
 arch/arm/kernel/traps.c           |  3 ++-
 arch/arm/kernel/unwind.c          |  8 +++++++-
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/stacktrace.h b/arch/arm/include/asm/stacktrace.h
index 33ee1aa4b8c0..d87d60532b86 100644
--- a/arch/arm/include/asm/stacktrace.h
+++ b/arch/arm/include/asm/stacktrace.h
@@ -18,6 +18,16 @@ struct stackframe {
 	struct llist_node *kr_cur;
 	struct task_struct *tsk;
 #endif
+#ifdef CONFIG_ARM_UNWIND
+	/*
+	 * This field is used to track the stack pointer value when calling
+	 * __entry routines. This is needed when IRQ stacks and overflow stacks
+	 * are used, because in that case, the struct pt_regs passed to these
+	 * __entry routines may be at the top of the task stack, while we are
+	 * executing from another stack.
+	 */
+	unsigned long sp_low;
+#endif
 };
 
 static __always_inline
diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
index e4698605bb72..89be21ec3b52 100644
--- a/arch/arm/kernel/traps.c
+++ b/arch/arm/kernel/traps.c
@@ -74,7 +74,8 @@ void dump_backtrace_entry(unsigned long where, unsigned long from,
 		loglvl, where, from);
 #endif
 
-	if (in_entry_text(from) && end <= ALIGN(frame, THREAD_SIZE))
+	if (!IS_ENABLED(CONFIG_UNWINDER_ARM) &&
+	    in_entry_text(from) && end <= ALIGN(frame, THREAD_SIZE))
 		dump_mem(loglvl, "Exception stack", frame + 4, end);
 }
 
diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
index 9cb9af3fc433..b7a6141c342f 100644
--- a/arch/arm/kernel/unwind.c
+++ b/arch/arm/kernel/unwind.c
@@ -29,6 +29,7 @@
 #include <linux/spinlock.h>
 #include <linux/list.h>
 
+#include <asm/sections.h>
 #include <asm/stacktrace.h>
 #include <asm/traps.h>
 #include <asm/unwind.h>
@@ -459,6 +460,7 @@ int unwind_frame(struct stackframe *frame)
 	frame->sp = ctrl.vrs[SP];
 	frame->lr = ctrl.vrs[LR];
 	frame->pc = ctrl.vrs[PC];
+	frame->sp_low = ctrl.sp_low;
 
 	return URC_OK;
 }
@@ -502,7 +504,11 @@ void unwind_backtrace(struct pt_regs *regs, struct task_struct *tsk,
 		urc = unwind_frame(&frame);
 		if (urc < 0)
 			break;
-		dump_backtrace_entry(where, frame.pc, frame.sp - 4, loglvl);
+		if (in_entry_text(where))
+			dump_mem(loglvl, "Exception stack", frame.sp_low,
+				 frame.sp_low + sizeof(struct pt_regs));
+
+		dump_backtrace_entry(where, frame.pc, 0, loglvl);
 	}
 }
 
-- 
2.30.2