objtool: Add --mnop as an option to --mcount

Some architectures (powerpc) may not support ftrace locations being nop'ed
out at build time. Introduce CONFIG_HAVE_OBJTOOL_NOP_MCOUNT for objtool, as
a means for architectures to enable nop'ing of ftrace locations. Add --mnop
as an option to objtool --mcount, to indicate support for the same.

Also, make sure that --mnop can be passed as an option to objtool only when
--mcount is passed.

Tested-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221114175754.1131267-12-sv@linux.ibm.com

Makefile

@@ -933,7 +933,9 @@ ifdef CONFIG_FTRACE_MCOUNT_USE_CC
   endif
 endif
 ifdef CONFIG_FTRACE_MCOUNT_USE_OBJTOOL
-  CC_FLAGS_USING	+= -DCC_USING_NOP_MCOUNT
+  ifdef CONFIG_HAVE_OBJTOOL_NOP_MCOUNT
+    CC_FLAGS_USING	+= -DCC_USING_NOP_MCOUNT
+  endif
 endif
 ifdef CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
   ifdef CONFIG_HAVE_C_RECORDMCOUNT

arch/x86/Kconfig

@@ -195,6 +195,7 @@ config X86
 	select HAVE_CONTEXT_TRACKING_USER_OFFSTACK	if HAVE_CONTEXT_TRACKING_USER
 	select HAVE_C_RECORDMCOUNT
 	select HAVE_OBJTOOL_MCOUNT		if HAVE_OBJTOOL
+	select HAVE_OBJTOOL_NOP_MCOUNT		if HAVE_OBJTOOL_MCOUNT
 	select HAVE_BUILDTIME_MCOUNT_SORT
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DMA_CONTIGUOUS

kernel/trace/Kconfig

@@ -82,6 +82,13 @@ config HAVE_OBJTOOL_MCOUNT
 	help
 	  Arch supports objtool --mcount
 
+config HAVE_OBJTOOL_NOP_MCOUNT
+	bool
+	help
+	  Arch supports the objtool options --mcount with --mnop.
+	  An architecture can select this if it wants to enable nop'ing
+	  of ftrace locations.
+
 config HAVE_C_RECORDMCOUNT
 	bool
 	help

scripts/Makefile.lib

@@ -256,6 +256,9 @@ objtool-args-$(CONFIG_HAVE_JUMP_LABEL_HACK)		+= --hacks=jump_label
 objtool-args-$(CONFIG_HAVE_NOINSTR_HACK)		+= --hacks=noinstr
 objtool-args-$(CONFIG_X86_KERNEL_IBT)			+= --ibt
 objtool-args-$(CONFIG_FTRACE_MCOUNT_USE_OBJTOOL)	+= --mcount
+ifdef CONFIG_FTRACE_MCOUNT_USE_OBJTOOL
+objtool-args-$(CONFIG_HAVE_OBJTOOL_NOP_MCOUNT)		+= --mnop
+endif
 objtool-args-$(CONFIG_UNWINDER_ORC)			+= --orc
 objtool-args-$(CONFIG_RETPOLINE)			+= --retpoline
 objtool-args-$(CONFIG_RETHUNK)				+= --rethunk

tools/objtool/builtin-check.c

@@ -82,6 +82,7 @@ const struct option check_options[] = {
 	OPT_BOOLEAN(0, "dry-run", &opts.dryrun, "don't write modifications"),
 	OPT_BOOLEAN(0, "link", &opts.link, "object is a linked object"),
 	OPT_BOOLEAN(0, "module", &opts.module, "object is part of a kernel module"),
+	OPT_BOOLEAN(0, "mnop", &opts.mnop, "nop out mcount call sites"),
 	OPT_BOOLEAN(0, "no-unreachable", &opts.no_unreachable, "skip 'unreachable instruction' warnings"),
 	OPT_BOOLEAN(0, "sec-address", &opts.sec_address, "print section addresses in warnings"),
 	OPT_BOOLEAN(0, "stats", &opts.stats, "print statistics"),
@@ -150,6 +151,16 @@ static bool opts_valid(void)
 	return false;
 }
 
+static bool mnop_opts_valid(void)
+{
+	if (opts.mnop && !opts.mcount) {
+		ERROR("--mnop requires --mcount");
+		return false;
+	}
+
+	return true;
+}
+
 static bool link_opts_valid(struct objtool_file *file)
 {
 	if (opts.link)
@@ -198,6 +209,9 @@ int objtool_run(int argc, const char **argv)
 	if (!file)
 		return 1;
 
+	if (!mnop_opts_valid())
+		return 1;
+
 	if (!link_opts_valid(file))
 		return 1;
 

tools/objtool/check.c

@@ -1256,18 +1256,19 @@ static void annotate_call_site(struct objtool_file *file,
 	if (opts.mcount && sym->fentry) {
 		if (sibling)
 			WARN_FUNC("Tail call to __fentry__ !?!?", insn->sec, insn->offset);
+		if (opts.mnop) {
+			if (reloc) {
+				reloc->type = R_NONE;
+				elf_write_reloc(file->elf, reloc);
+			}
 
-		if (reloc) {
+			elf_write_insn(file->elf, insn->sec,
-			reloc->type = R_NONE;
+				       insn->offset, insn->len,
-			elf_write_reloc(file->elf, reloc);
+				       arch_nop_insn(insn->len));
+
+			insn->type = INSN_NOP;
 		}
 
-		elf_write_insn(file->elf, insn->sec,
-			       insn->offset, insn->len,
-			       arch_nop_insn(insn->len));
-
-		insn->type = INSN_NOP;
-
 		list_add_tail(&insn->call_node, &file->mcount_loc_list);
 		return;
 	}

tools/objtool/include/objtool/builtin.h

@@ -31,6 +31,7 @@ struct opts {
 	bool backup;
 	bool dryrun;
 	bool link;
+	bool mnop;
 	bool module;
 	bool no_unreachable;
 	bool sec_address;