powerpc/module_64: Fix "expected nop" error on module re-patching

When a module with a livepatched function is unloaded and then reloaded,
klp attempts to dynamically re-patch it.  On ppc64, that fails with the
following error:

  module_64: livepatch_nfsd: Expected nop after call, got e8410018 at e_show+0x60/0x548 [livepatch_nfsd]
  livepatch: failed to initialize patch 'livepatch_nfsd' for module 'nfsd' (-8)
  livepatch: patch 'livepatch_nfsd' failed for module 'nfsd', refusing to load module 'nfsd'

The error happens because the restore r2 instruction had already
previously been written into the klp module's replacement function when
the original function was patched the first time.  So the instruction
wasn't a nop as expected.

When the restore r2 instruction has already been patched in, detect that
and skip the warning and the instruction write.
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/2f6329ffd9674df6ff57e03edeb2ca54414770ab.1674617130.git.jpoimboe@kernel.org

arch/powerpc/kernel/module_64.c

@@ -502,6 +502,7 @@ static unsigned long stub_for_addr(const Elf64_Shdr *sechdrs,
 static int restore_r2(const char *name, u32 *instruction, struct module *me)
 {
 	u32 *prev_insn = instruction - 1;
+	u32 insn_val = *instruction;
 
 	if (is_mprofile_ftrace_call(name))
 		return 0;
@@ -514,9 +515,18 @@ static int restore_r2(const char *name, u32 *instruction, struct module *me)
 	if (!instr_is_relative_link_branch(ppc_inst(*prev_insn)))
 		return 0;
 
-	if (*instruction != PPC_RAW_NOP()) {
+	/*
+	 * For livepatch, the restore r2 instruction might have already been
+	 * written previously, if the referenced symbol is in a previously
+	 * unloaded module which is now being loaded again.  In that case, skip
+	 * the warning and the instruction write.
+	 */
+	if (insn_val == PPC_INST_LD_TOC)
+		return 0;
+
+	if (insn_val != PPC_RAW_NOP()) {
 		pr_err("%s: Expected nop after call, got %08x at %pS\n",
-			me->name, *instruction, instruction);
+			me->name, insn_val, instruction);
 		return -ENOEXEC;
 	}