Subject: [PATCH 7/8] ARM: unwind: track location of LR value in stack frame

From: Ard Biesheuvel <[email-masked]>
Date: Tue, 25 Jan 2022 16:36:55 +0100

The ftrace graph tracer needs to override the return address of an
instrumented function, in order to install a hook that gets invoked when
the function returns again.

Currently, we only support this when building for ARM using GCC with
frame pointers, as in this case, it is guaranteed that the function will
reload LR from [FP, #-4] in all cases, and we can simply pass that
address to the ftrace code.

In order to support this for configurations that rely on the EABI
unwinder, such as Thumb2 builds, make the unwinder keep track of the
address from which LR was unwound, permitting ftrace to make use of this
in a subsequent patch.

Drop the call to is_kernel_text_address(), which is problematic in terms
of ftrace recursion, given that it may be instrumented itself. The call
is redundant anyway, as no unwind directives will be found unless the PC
points to memory that is known to contain executable code.

Signed-off-by: Ard Biesheuvel <[email-masked]>
---
 arch/arm/include/asm/stacktrace.h | 2 ++
 arch/arm/kernel/Makefile          | 1 +
 arch/arm/kernel/unwind.c          | 7 ++++---
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/arm/include/asm/stacktrace.h b/arch/arm/include/asm/stacktrace.h
index d87d60532b86..e5ce7440cba8 100644
--- a/arch/arm/include/asm/stacktrace.h
+++ b/arch/arm/include/asm/stacktrace.h
@@ -27,6 +27,8 @@ struct stackframe {
 	 * executing from another stack.
 	 */
 	unsigned long sp_low;
+	/* address of the LR value on the stack */
+	unsigned long *lr_addr;
 #endif
 };
 
diff --git a/arch/arm/kernel/Makefile b/arch/arm/kernel/Makefile
index ae295a3bcfef..56511856ff9d 100644
--- a/arch/arm/kernel/Makefile
+++ b/arch/arm/kernel/Makefile
@@ -10,6 +10,7 @@ ifdef CONFIG_FUNCTION_TRACER
 CFLAGS_REMOVE_ftrace.o = -pg
 CFLAGS_REMOVE_insn.o = -pg
 CFLAGS_REMOVE_patch.o = -pg
+CFLAGS_REMOVE_unwind.o = -pg
 endif
 
 CFLAGS_REMOVE_return_address.o = -pg
diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
index c5ea328c428d..b4e468a7674b 100644
--- a/arch/arm/kernel/unwind.c
+++ b/arch/arm/kernel/unwind.c
@@ -55,6 +55,7 @@ struct unwind_ctrl_block {
 	const unsigned long *insn;	/* pointer to the current instructions word */
 	unsigned long sp_low;		/* lowest value of sp allowed */
 	unsigned long sp_high;		/* highest value of sp allowed */
+	unsigned long *lr_addr;		/* address of LR value on the stack */
 	/*
 	 * 1 : check for stack overflow for each register pop.
 	 * 0 : save overhead if there is plenty of stack remaining.
@@ -239,6 +240,8 @@ static int unwind_pop_register(struct unwind_ctrl_block *ctrl,
 	 * from being tracked by KASAN.
 	 */
 	ctrl->vrs[reg] = READ_ONCE_NOCHECK(*(*vsp));
+	if (reg == 14)
+		ctrl->lr_addr = *vsp;
 	(*vsp)++;
 	return URC_OK;
 }
@@ -395,9 +398,6 @@ int unwind_frame(struct stackframe *frame)
 	pr_debug("%s(pc = %08lx lr = %08lx sp = %08lx)\n", __func__,
 		 frame->pc, frame->lr, frame->sp);
 
-	if (!kernel_text_address(frame->pc))
-		return -URC_FAILURE;
-
 	idx = unwind_find_idx(frame->pc);
 	if (!idx) {
 		pr_warn("unwind: Index not found %08lx\n", frame->pc);
@@ -476,6 +476,7 @@ int unwind_frame(struct stackframe *frame)
 	frame->lr = ctrl.vrs[LR];
 	frame->pc = ctrl.vrs[PC];
 	frame->sp_low = ctrl.sp_low;
+	frame->lr_addr = ctrl.lr_addr;
 
 	return URC_OK;
 }
-- 
2.30.2

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