[PATCH v3] powerpc: Add support for function error injection

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

We implement regs_set_return_value() and override_function_with_return()
for this purpose.

On powerpc, a return from a function (blr) just branches to the location
contained in the link register. So, we can just update pt_regs rather
than redirecting execution to a dummy function that returns.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
The only change is to add a comment in override_function_with_return() 
to clarify that we don't need to worry about 32-bit userspace while 
emulating 'blr'.

- Naveen

 arch/powerpc/Kconfig                       |  1 +
 arch/powerpc/include/asm/error-injection.h | 13 +++++++++++++
 arch/powerpc/include/asm/ptrace.h          |  5 +++++
 arch/powerpc/lib/Makefile                  |  2 ++
 arch/powerpc/lib/error-inject.c            | 16 ++++++++++++++++
 5 files changed, 37 insertions(+)
 create mode 100644 arch/powerpc/include/asm/error-injection.h
 create mode 100644 arch/powerpc/lib/error-inject.c

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 076fe3094856..00dad3c759a0 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -187,6 +187,7 @@ config PPC
 	select HAVE_EBPF_JIT			if PPC64
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS	if !(CPU_LITTLE_ENDIAN && POWER7_CPU)
 	select HAVE_FTRACE_MCOUNT_RECORD
+	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER
 	select HAVE_GCC_PLUGINS
diff --git a/arch/powerpc/include/asm/error-injection.h b/arch/powerpc/include/asm/error-injection.h
new file mode 100644
index 000000000000..740c3075bdf4
--- /dev/null
+++ b/arch/powerpc/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/powerpc/include/asm/ptrace.h b/arch/powerpc/include/asm/ptrace.h
index e4923686e43a..c0705296c2f0 100644
--- a/arch/powerpc/include/asm/ptrace.h
+++ b/arch/powerpc/include/asm/ptrace.h
@@ -101,6 +101,11 @@ static inline long regs_return_value(struct pt_regs *regs)
 		return -regs->gpr[3];
 }
 
+static inline void regs_set_return_value(struct pt_regs *regs, unsigned long rc)
+{
+	regs->gpr[3] = rc;
+}
+
 #ifdef __powerpc64__
 #define user_mode(regs) ((((regs)->msr) >> MSR_PR_LG) & 0x1)
 #else
diff --git a/arch/powerpc/lib/Makefile b/arch/powerpc/lib/Makefile
index d0ca13ad8231..dd43c5a53396 100644
--- a/arch/powerpc/lib/Makefile
+++ b/arch/powerpc/lib/Makefile
@@ -14,6 +14,8 @@ obj-y += string.o alloc.o code-patching.o feature-fixups.o
 
 obj-$(CONFIG_PPC32)	+= div64.o copy_32.o crtsavres.o
 
+obj-$(CONFIG_FUNCTION_ERROR_INJECTION)	+= error-inject.o
+
 # See corresponding test in arch/powerpc/Makefile
 # 64-bit linker creates .sfpr on demand for final link (vmlinux),
 # so it is only needed for modules, and only for older linkers which
diff --git a/arch/powerpc/lib/error-inject.c b/arch/powerpc/lib/error-inject.c
new file mode 100644
index 000000000000..407b992fb02f
--- /dev/null
+++ b/arch/powerpc/lib/error-inject.c
@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+#include <linux/error-injection.h>
+#include <linux/kprobes.h>
+#include <linux/uaccess.h>
+
+void override_function_with_return(struct pt_regs *regs)
+{
+	/*
+	 * Emulate 'blr'. 'regs' represents the state on entry of a predefined
+	 * function in the kernel/module, captured on a kprobe. We don't need
+	 * to worry about 32-bit userspace on a 64-bit kernel.
+	 */
+	regs->nip = regs->link;
+}
+NOKPROBE_SYMBOL(override_function_with_return);
-- 
2.17.0