[PATCH 2/2] arm: Add support for function error injection

[Leo Yan]

This patch implement regs_set_return_value() and
override_function_with_return() to support function error injection
for arm.

In the exception flow, we can update pt_regs::ARM_pc with
pt_regs::ARM_lr so that can override the probed function return.

Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
---
 arch/arm/Kconfig                       |  1 +
 arch/arm/include/asm/error-injection.h | 13 +++++++++++++
 arch/arm/include/asm/ptrace.h          |  5 +++++
 arch/arm/lib/Makefile                  |  2 ++
 arch/arm/lib/error-inject.c            | 19 +++++++++++++++++++
 5 files changed, 40 insertions(+)
 create mode 100644 arch/arm/include/asm/error-injection.h
 create mode 100644 arch/arm/lib/error-inject.c

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 8869742a85df..f7932a5e29ea 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -74,6 +74,7 @@ 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_ERROR_INJECTION 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
diff --git a/arch/arm/include/asm/error-injection.h b/arch/arm/include/asm/error-injection.h
new file mode 100644
index 000000000000..da057e8ed224
--- /dev/null
+++ b/arch/arm/include/asm/error-injection.h
@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+
+#ifndef __ASM_ERROR_INJECTION_H_
+#define __ASM_ERROR_INJECTION_H_
+
+#include <linux/compiler.h>
+#include <linux/linkage.h>
+#include <asm/ptrace.h>
+#include <asm-generic/error-injection.h>
+
+void override_function_with_return(struct pt_regs *regs);
+
+#endif /* __ASM_ERROR_INJECTION_H_ */
diff --git a/arch/arm/include/asm/ptrace.h b/arch/arm/include/asm/ptrace.h
index 91d6b7856be4..3b41f37b361a 100644
--- a/arch/arm/include/asm/ptrace.h
+++ b/arch/arm/include/asm/ptrace.h
@@ -89,6 +89,11 @@ static inline long regs_return_value(struct pt_regs *regs)
 	return regs->ARM_r0;
 }
 
+static inline void regs_set_return_value(struct pt_regs *regs, unsigned long rc)
+{
+	regs->ARM_r0 = rc;
+}
+
 #define instruction_pointer(regs)	(regs)->ARM_pc
 
 #ifdef CONFIG_THUMB2_KERNEL
diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
index 0bff0176db2c..d3d7430ecd76 100644
--- a/arch/arm/lib/Makefile
+++ b/arch/arm/lib/Makefile
@@ -43,3 +43,5 @@ ifeq ($(CONFIG_KERNEL_MODE_NEON),y)
   CFLAGS_xor-neon.o		+= $(NEON_FLAGS)
   obj-$(CONFIG_XOR_BLOCKS)	+= xor-neon.o
 endif
+
+obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o
diff --git a/arch/arm/lib/error-inject.c b/arch/arm/lib/error-inject.c
new file mode 100644
index 000000000000..96319d017114
--- /dev/null
+++ b/arch/arm/lib/error-inject.c
@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/error-injection.h>
+#include <linux/kprobes.h>
+
+void override_function_with_return(struct pt_regs *regs)
+{
+	/*
+	 * 'regs' represents the state on entry of a predefined function in
+	 * the kernel/module and which is captured on a kprobe.
+	 *
+	 * 'regs->ARM_lr' contains the the link register for the probed
+	 * function and assign it to 'regs->ARM_pc', so when kprobe returns
+	 * back from exception it will override the end of probed function
+	 * and drirectly return to the predefined function's caller.
+	 */
+	regs->ARM_pc = regs->ARM_lr;
+}
+NOKPROBE_SYMBOL(override_function_with_return);
-- 
2.17.1