[PATCH v2 2/3] objtool/klp: Reject changed functions without an ftrace call site

[Puranjay Mohan <puranjay@kernel.org>]

Livepatch redirects a function through its ftrace call site.  If a
changed function doesn't have one, klp-build succeeds but the patch
module fails to load:

  livepatch: failed to find location for function 'nsproxy_free'
  livepatch: failed to patch object 'vmlinux'
  livepatch: failed to enable patch 'livepatch_nsproxy'

This is easy to hit because the kernel's 'inline' implies notrace, so
any inline function which the compiler emits out of line can't be
patched.

Make klp diff fail for any changed function which has no __mcount_loc
entry in the original object.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
---
 tools/objtool/include/objtool/elf.h |  1 +
 tools/objtool/klp-diff.c            | 47 +++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index a82517a76a0f6..f7e6a652ee788 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -98,6 +98,7 @@ struct symbol {
 	u8 klp		     : 1;
 	u8 dont_correlate    : 1;
 	u8 fake		     : 1;
+	u8 traceable	     : 1;
 	struct list_head pv_target;
 	struct reloc *relocs;
 	struct section *group_sec;
diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index 34da55e456097..f8659950ec9a9 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -1098,6 +1098,50 @@ static int mark_changed_functions(struct elfs *e)
 	return !changed ? 1 : 0;
 }
 
+/*
+ * Like ftrace_location(), accept an ftrace call site anywhere in the function.
+ */
+static void mark_traceable_functions(struct elf *elf)
+{
+	struct section *sec;
+	struct reloc *reloc;
+
+	sec = find_section_by_name(elf, "__mcount_loc");
+	if (!sec || !sec->rsec)
+		return;
+
+	for_each_reloc(sec->rsec, reloc) {
+		struct symbol *func;
+
+		func = find_func_containing(reloc->sym->sec,
+					    reloc->sym->offset + reloc_addend(reloc));
+		if (func)
+			func->traceable = 1;
+	}
+}
+
+/*
+ * Livepatch redirects functions through their ftrace call sites, so a function
+ * without one can't be patched.
+ */
+static int validate_traceable_functions(struct elfs *e)
+{
+	struct symbol *patched_sym;
+	bool error = false;
+
+	mark_traceable_functions(e->orig);
+
+	for_each_sym(e->patched, patched_sym) {
+		if (!patched_sym->changed || patched_sym->twin->traceable)
+			continue;
+
+		ERROR("%s: no ftrace call site, can't patch", patched_sym->twin->name);
+		error = true;
+	}
+
+	return error ? -1 : 0;
+}
+
 static int clone_included_functions(struct elfs *e)
 {
 	struct symbol *patched_sym;
@@ -2260,6 +2304,9 @@ int cmd_klp_diff(int argc, const char **argv)
 	if (ret > 0)
 		return 0;
 
+	if (validate_traceable_functions(&e))
+		return -1;
+
 	e.out = elf_create_file(&e.orig->ehdr, argv[2]);
 	if (!e.out)
 		return -1;
-- 
2.53.0-Meta