Implement a jump_label assertion that asserts that the code location
is indeed only reachable through a static_branch. Because if GCC is
absolutely retaded it could generate code like:

	xor rax,rax
	NOP/JMP 1f
	mov $1, rax
1:
	test rax,rax
	jz 2f
	<do-code>
2:

instead of the sensible:

	NOP/JMP 1f
	<do-code>
1:

This implements objtool infrastructure for ensuring the code ends up
sane, since we'll rely on that for correctness and security.

We tag the instructions after the static branch with static_jump_dest=true;
that is the instruction after the NOP and the instruction at the
JMP+disp site.

Then, when we read the .discard.jump_assert section, we assert that
each entry points to an instruction that has static_jump_dest set.

With this we can assert that the code emitted for the if statement
ends up at the static jump location and nothing untowards happened.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 tools/objtool/check.c |   70 ++++++++++++++++++++++++++++++++++++++++++++++++--
 tools/objtool/check.h |    2 -
 2 files changed, 69 insertions(+), 3 deletions(-)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -686,8 +686,17 @@ static int handle_jump_alt(struct objtoo
 			   struct instruction *orig_insn,
 			   struct instruction **new_insn)
 {
-	if (orig_insn->type == INSN_NOP)
+	struct instruction *next_insn = list_next_entry(orig_insn, list);
+
+	if (orig_insn->type == INSN_NOP) {
+		/*
+		 * If orig_insn is a NOP, then new_insn is the branch target
+		 * for when it would've been a JMP.
+		 */
+		next_insn->static_jump_dest = true;
+		(*new_insn)->static_jump_dest = true;
 		return 0;
+	}
 
 	if (orig_insn->type != INSN_JUMP_UNCONDITIONAL) {
 		WARN_FUNC("unsupported instruction at jump label",
@@ -695,7 +704,16 @@ static int handle_jump_alt(struct objtoo
 		return -1;
 	}
 
-	*new_insn = list_next_entry(orig_insn, list);
+	/*
+	 * Otherwise, orig_insn is a JMP and it will have orig_insn->jump_dest.
+	 * In this case we'll effectively NOP the alt by pointing new_insn at
+	 * next_insn.
+	 */
+	orig_insn->jump_dest->static_jump_dest = true;
+	next_insn->static_jump_dest = true;
+
+	*new_insn = next_insn;
+
 	return 0;
 }
 
@@ -1114,6 +1132,50 @@ static int read_retpoline_hints(struct o
 	return 0;
 }
 
+static int assert_static_jumps(struct objtool_file *file)
+{
+	struct section *sec, *relasec;
+	struct instruction *insn;
+	struct rela *rela;
+	int i;
+
+	sec = find_section_by_name(file->elf, ".discard.jump_assert");
+	if (!sec)
+		return 0;
+
+	relasec = sec->rela;
+	if (!relasec) {
+		WARN("missing .rela.discard.jump_assert section");
+		return -1;
+	}
+
+	if (sec->len % sizeof(unsigned long)) {
+		WARN("jump_assert size mismatch: %d %ld", sec->len, sizeof(unsigned long));
+		return -1;
+	}
+
+	for (i = 0; i < sec->len / sizeof(unsigned long); i++) {
+		rela = find_rela_by_dest(sec, i * sizeof(unsigned long));
+		if (!rela) {
+			WARN("can't find rela for jump_assert[%d]", i);
+			return -1;
+		}
+
+		insn = find_insn(file, rela->sym->sec, rela->addend);
+		if (!insn) {
+			WARN("can't find insn for jump_assert[%d]", i);
+			return -1;
+		}
+
+		if (!insn->static_jump_dest) {
+			WARN_FUNC("static assert FAIL", insn->sec, insn->offset);
+			return -1;
+		}
+	}
+
+	return 0;
+}
+
 static int decode_sections(struct objtool_file *file)
 {
 	int ret;
@@ -2073,6 +2135,10 @@ int check(const char *_objname, bool orc
 		goto out;
 	warnings += ret;
 
+	ret = assert_static_jumps(&file);
+	if (ret < 0)
+		return ret;
+
 	if (list_empty(&file.insn_list))
 		goto out;
 
--- a/tools/objtool/check.h
+++ b/tools/objtool/check.h
@@ -45,7 +45,7 @@ struct instruction {
 	unsigned char type;
 	unsigned long immediate;
 	bool alt_group, visited, dead_end, ignore, hint, save, restore, ignore_alts;
-	bool retpoline_safe;
+	bool retpoline_safe, static_jump_dest;
 	struct symbol *call_dest;
 	struct instruction *jump_dest;
 	struct list_head alts;