[PATCH 2/4] objtool: Implement jump_assert for _static_cpu_has()

[Peter Zijlstra]

Unlike the jump_label bits, static_cpu_has is implemented with
alternatives. Sadly it doesn't readily distinguish itself from any
other alternatives.

Use a heuristic to guess at it :/

But like jump_labels, make static_cpu_has set br_static on the
instructions after the static branch such that we can assert on it.

Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 tools/objtool/check.c   |   21 +++++++++++++++++++++
 tools/objtool/special.c |   27 ++++++++++++++++++++++++++-
 tools/objtool/special.h |    1 +
 3 files changed, 48 insertions(+), 1 deletion(-)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -636,6 +636,12 @@ static int handle_group_alt(struct objto
 	fake_jump->ignore = true;
 
 	if (!special_alt->new_len) {
+		/*
+		 * The NOP case for _static_cpu_has()
+		 */
+		if (special_alt->static_feat)
+			fake_jump->jump_dest->br_static = true;
+
 		*new_insn = fake_jump;
 		return 0;
 	}
@@ -664,6 +670,21 @@ static int handle_group_alt(struct objto
 				  insn->sec, insn->offset);
 			return -1;
 		}
+
+		if (special_alt->static_feat) {
+			if (insn->type != INSN_JUMP_UNCONDITIONAL) {
+				WARN_FUNC("not an unconditional jump in _static_cpu_has()",
+					  insn->sec, insn->offset);
+			}
+			if (insn->jump_dest == fake_jump) {
+				WARN_FUNC("jump inside alternative for _static_cpu_has()",
+					  insn->sec, insn->offset);
+			}
+			/*
+			 * The JMP+disp case for _static_cpu_has()
+			 */
+			insn->jump_dest->br_static = true;
+		}
 	}
 
 	if (!last_new_insn) {
--- a/tools/objtool/special.c
+++ b/tools/objtool/special.c
@@ -41,6 +41,7 @@
 #define ALT_ORIG_LEN_OFFSET	10
 #define ALT_NEW_LEN_OFFSET	11
 
+#define X86_FEATURE_ALWAYS (3*32+21)
 #define X86_FEATURE_POPCNT (4*32+23)
 
 struct special_entry {
@@ -110,6 +111,9 @@ static int get_alt_entry(struct elf *elf
 		 */
 		if (feature == X86_FEATURE_POPCNT)
 			alt->skip_orig = true;
+
+		if (feature == X86_FEATURE_ALWAYS)
+			alt->static_feat = true;
 	}
 
 	orig_rela = find_rela_by_dest(sec, offset + entry->orig);
@@ -155,7 +159,7 @@ int special_get_alts(struct elf *elf, st
 	struct special_entry *entry;
 	struct section *sec;
 	unsigned int nr_entries;
-	struct special_alt *alt;
+	struct special_alt *alt, *last = NULL;
 	int idx, ret;
 
 	INIT_LIST_HEAD(alts);
@@ -186,6 +190,27 @@ int special_get_alts(struct elf *elf, st
 				return ret;
 
 			list_add_tail(&alt->list, alts);
+
+			if (last) {
+				/*
+				 * If we have two consecutive alternatives for
+				 * the same location, for which the first has
+				 * FEATURE_ALWAYS set and the second has no
+				 * replacement, then assume this is
+				 * _static_cpu_has() and will have to set
+				 * br_static to make jump_assert work.
+				 */
+				if (alt->orig_sec == last->orig_sec &&
+				    alt->orig_off == last->orig_off &&
+				    alt->orig_len == last->orig_len &&
+				    !alt->new_len && !alt->new_sec && !alt->new_off &&
+				    last->static_feat)
+					alt->static_feat = true;
+				else
+					last->static_feat = false;
+			}
+
+			last = alt;
 		}
 	}
 
--- a/tools/objtool/special.h
+++ b/tools/objtool/special.h
@@ -27,6 +27,7 @@ struct special_alt {
 	bool group;
 	bool skip_orig;
 	bool jump_or_nop;
+	bool static_feat;
 
 	struct section *orig_sec;
 	unsigned long orig_off;