powerpc: Add support for function error injection

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 <[e-mail]>
Reviewed-by: Samuel Mendoza-Jonas <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>

arch/powerpc/Kconfig

···
 	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			if GCC_VERSION >= 50200   # plugin support on gcc <= 5.1 is buggy on PPC

arch/powerpc/include/asm/error-injection.h

···
+/* 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 */

arch/powerpc/include/asm/ptrace.h

···
 		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

arch/powerpc/lib/Makefile

···
 
 obj-$(CONFIG_PPC32)	+= div64.o copy_32.o crtsavres.o strlen_32.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

arch/powerpc/lib/error-inject.c

···
+// 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);