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 <naveen.n.rao@linux.vnet.ibm.com>
Reviewed-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/lib/Makefile

@@ -12,6 +12,8 @@ obj-y += string.o alloc.o code-patching.o feature-fixups.o
 
 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

@@ -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);