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

3 files changed, 25 insertions, 9 deletions

diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c
index 24fbe803a0d3..9bd347d3c244 100644
--- a/tools/objtool/builtin-check.c
+++ b/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;
 
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index b64518c7c7b4..71cf4b4ba1da 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1256,17 +1256,18 @@ 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) {
-			reloc->type = R_NONE;
-			elf_write_reloc(file->elf, reloc);
-		}
-
-		elf_write_insn(file->elf, insn->sec,
-			       insn->offset, insn->len,
-			       arch_nop_insn(insn->len));
+			elf_write_insn(file->elf, insn->sec,
+				       insn->offset, insn->len,
+				       arch_nop_insn(insn->len));
 
-		insn->type = INSN_NOP;
+			insn->type = INSN_NOP;
+		}
 
 		list_add_tail(&insn->call_node, &file->mcount_loc_list);
 		return;
diff --git a/tools/objtool/include/objtool/builtin.h b/tools/objtool/include/objtool/builtin.h
index 42a52f1a0add..0785707c5a92 100644
--- a/tools/objtool/include/objtool/builtin.h
+++ b/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;