[PATCH v2 36/39] objtool: Find unused ENDBR instructions

[Peter Zijlstra <peterz@infradead.org>]

Find all unused ENDBR instructions and stick them in a section such
that the kernel can poison them.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/vmlinux.lds.S           |    9 ++++
 tools/objtool/builtin-check.c           |    3 -
 tools/objtool/check.c                   |   72 +++++++++++++++++++++++++++++++-
 tools/objtool/include/objtool/builtin.h |    2 
 tools/objtool/include/objtool/objtool.h |    1 
 tools/objtool/objtool.c                 |    1 
 6 files changed, 85 insertions(+), 3 deletions(-)

--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -285,6 +285,15 @@ SECTIONS
 	}
 #endif
 
+#ifdef CONFIG_X86_KERNEL_IBT
+	. = ALIGN(8);
+	.ibt_endbr_sites : AT(ADDR(.ibt_endbr_sites) - LOAD_OFFSET) {
+		__ibt_endbr_sites = .;
+		*(.ibt_endbr_sites)
+		__ibt_endbr_sites_end = .;
+	}
+#endif
+
 	/*
 	 * struct alt_inst entries. From the header (alternative.h):
 	 * "Alternative instructions for different CPU types or capabilities"
--- a/tools/objtool/builtin-check.c
+++ b/tools/objtool/builtin-check.c
@@ -21,7 +21,7 @@
 
 bool no_fp, no_unreachable, retpoline, module, backtrace, uaccess, stats,
      lto, vmlinux, mcount, noinstr, backup, sls, dryrun,
-     ibt, ibt_fix_direct;
+     ibt, ibt_fix_direct, ibt_seal;
 
 static const char * const check_usage[] = {
 	"objtool check [<options>] file.o",
@@ -50,6 +50,7 @@ const struct option check_options[] = {
 	OPT_BOOLEAN(0, "dry-run", &dryrun, "don't write the modifications"),
 	OPT_BOOLEAN(0, "ibt", &ibt, "validate ENDBR placement"),
 	OPT_BOOLEAN(0, "ibt-fix-direct", &ibt_fix_direct, "fixup direct jmp/call to ENDBR"),
+	OPT_BOOLEAN(0, "ibt-seal", &ibt_seal, "list superfluous ENDBR instructions"),
 	OPT_END(),
 };
 
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -732,6 +732,58 @@ static int create_retpoline_sites_sectio
 	return 0;
 }
 
+static int create_ibt_endbr_sites_sections(struct objtool_file *file)
+{
+	struct instruction *insn;
+	struct section *sec;
+	int idx;
+
+	sec = find_section_by_name(file->elf, ".ibt_endbr_sites");
+	if (sec) {
+		WARN("file already has .ibt_endbr_sites, skipping");
+		return 0;
+	}
+
+	idx = 0;
+	list_for_each_entry(insn, &file->endbr_list, call_node)
+		idx++;
+
+	if (stats) {
+		printf("ibt: ENDBR at function start: %d\n", file->nr_endbr);
+		printf("ibt: ENDBR inside functions:  %d\n", file->nr_endbr_int);
+		printf("ibt: superfluous ENDBR:       %d\n", idx);
+	}
+
+	if (!idx)
+		return 0;
+
+	sec = elf_create_section(file->elf, ".ibt_endbr_sites", 0,
+				 sizeof(int), idx);
+	if (!sec) {
+		WARN("elf_create_section: .ibt_endbr_sites");
+		return -1;
+	}
+
+	idx = 0;
+	list_for_each_entry(insn, &file->endbr_list, call_node) {
+
+		int *site = (int *)sec->data->d_buf + idx;
+		*site = 0;
+
+		if (elf_add_reloc_to_insn(file->elf, sec,
+					  idx * sizeof(int),
+					  R_X86_64_PC32,
+					  insn->sec, insn->offset)) {
+			WARN("elf_add_reloc_to_insn: .ibt_endbr_sites");
+			return -1;
+		}
+
+		idx++;
+	}
+
+	return 0;
+}
+
 static int create_mcount_loc_sections(struct objtool_file *file)
 {
 	struct section *sec;
@@ -1179,6 +1231,7 @@ static int add_jump_destinations(struct
 	for_each_insn(file, insn) {
 		if (insn->type == INSN_ENDBR && insn->func) {
 			if (insn->offset == insn->func->offset) {
+				list_add_tail(&insn->call_node, &file->endbr_list);
 				file->nr_endbr++;
 			} else {
 				file->nr_endbr_int++;
@@ -3633,8 +3687,12 @@ validate_ibt_reloc(struct objtool_file *
 	if (!dest)
 		return NULL;
 
-	if (dest->type == INSN_ENDBR)
+	if (dest->type == INSN_ENDBR) {
+		if (!list_empty(&dest->call_node))
+			list_del_init(&dest->call_node);
+
 		return NULL;
+	}
 
 	if (reloc->sym->static_call_tramp)
 		return NULL;
@@ -3777,6 +3835,11 @@ int check(struct objtool_file *file)
 		return 1;
 	}
 
+	if (ibt_seal && !ibt_fix_direct) {
+		fprintf(stderr, "--ibt-seal requires: --ibt-fix-direct\n");
+		return 1;
+	}
+
 	arch_initial_func_cfi_state(&initial_func_cfi);
 	init_cfi_state(&init_cfi);
 	init_cfi_state(&func_cfi);
@@ -3854,6 +3917,13 @@ int check(struct objtool_file *file)
 		if (ret < 0)
 			goto out;
 		warnings += ret;
+	}
+
+	if (ibt_seal) {
+		ret = create_ibt_endbr_sites_sections(file);
+		if (ret < 0)
+			goto out;
+		warnings += ret;
 	}
 
 	if (stats) {
--- a/tools/objtool/include/objtool/builtin.h
+++ b/tools/objtool/include/objtool/builtin.h
@@ -10,7 +10,7 @@
 extern const struct option check_options[];
 extern bool no_fp, no_unreachable, retpoline, module, backtrace, uaccess, stats,
 	    lto, vmlinux, mcount, noinstr, backup, sls, dryrun,
-	    ibt, ibt_fix_direct;
+	    ibt, ibt_fix_direct, ibt_seal;
 
 extern int cmd_parse_options(int argc, const char **argv, const char * const usage[]);
 
--- a/tools/objtool/include/objtool/objtool.h
+++ b/tools/objtool/include/objtool/objtool.h
@@ -26,6 +26,7 @@ struct objtool_file {
 	struct list_head retpoline_call_list;
 	struct list_head static_call_list;
 	struct list_head mcount_loc_list;
+	struct list_head endbr_list;
 	bool ignore_unreachables, c_file, hints, rodata;
 
 	unsigned int nr_endbr;
--- a/tools/objtool/objtool.c
+++ b/tools/objtool/objtool.c
@@ -128,6 +128,7 @@ struct objtool_file *objtool_open_read(c
 	INIT_LIST_HEAD(&file.retpoline_call_list);
 	INIT_LIST_HEAD(&file.static_call_list);
 	INIT_LIST_HEAD(&file.mcount_loc_list);
+	INIT_LIST_HEAD(&file.endbr_list);
 	file.c_file = !vmlinux && find_section_by_name(file.elf, ".comment");
 	file.ignore_unreachables = no_unreachable;
 	file.hints = false;