[PATCH v2 10/10] objtool: More complex static jump implementation

[Peter Zijlstra]

When using something like:

  -#define sched_feat(x) (static_branch_##x(&sched_feat_keys[__SCHED_FEAT_##x]))
  +#define sched_feat(x) (static_branch_##x(&sched_feat_keys[__SCHED_FEAT_##x]) && \
  +			(arch_static_assert(), true))

we get an objtool assertion fail like:

kernel/sched/fair.o: warning: objtool: hrtick_update()+0xd: static assert FAIL

where:

0000000000001140 <hrtick_update>:
    1140:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
    1145:       c3                      retq
    1146:       48 8b b7 30 09 00 00    mov    0x930(%rdi),%rsi
    114d:       8b 87 d8 09 00 00       mov    0x9d8(%rdi),%eax
    1153:       48 0f a3 05 00 00 00    bt     %rax,0x0(%rip)        # 115b <hrtick_update+0x1b>
    115a:       00
                        1157: R_X86_64_PC32     __cpu_active_mask-0x4

and:

RELOCATION RECORDS FOR [__jump_table]:
0000000000000150 R_X86_64_64       .text+0x0000000000001140
0000000000000158 R_X86_64_64       .text+0x0000000000001146

RELOCATION RECORDS FOR [.discard.jump_assert]:
0000000000000028 R_X86_64_64       .text+0x000000000000114d

IOW, GCC managed to place the assertion 1 instruction _after_ the
static jump target.

So while the code generation is fine, the assertion gets placed wrong.
We can 'fix' this by not only considering the immediate static jump
locations but also all the unconditional code after it, terminating
the basic block on any unconditional instruction or branch entry
point.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 tools/objtool/check.c |   40 ++++++++++++++++++++++++++++++++++++++++
 tools/objtool/check.h |    2 +-
 2 files changed, 41 insertions(+), 1 deletion(-)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -520,6 +520,8 @@ static int add_jump_destinations(struct
 				  dest_off);
 			return -1;
 		}
+
+		insn->jump_dest->branch_target = true;
 	}
 
 	return 0;
@@ -1197,6 +1199,40 @@ static int read_retpoline_hints(struct o
 	return 0;
 }
 
+static int grow_static_blocks(struct objtool_file *file)
+{
+	struct instruction *insn;
+	bool static_block = false;
+
+	for_each_insn(file, insn) {
+		if (!static_block && !insn->static_jump_dest)
+			continue;
+
+		if (insn->static_jump_dest) {
+			static_block = true;
+			continue;
+		}
+
+		if (insn->branch_target) {
+			static_block = false;
+			continue;
+		} else switch (insn->type) {
+		case INSN_JUMP_CONDITIONAL:
+		case INSN_JUMP_DYNAMIC:
+		case INSN_CALL:
+		case INSN_CALL_DYNAMIC:
+		case INSN_RETURN:
+		case INSN_BUG:
+			static_block = false;
+			continue;
+		}
+
+		insn->static_jump_dest = static_block;
+	}
+
+	return 0;
+}
+
 static int decode_sections(struct objtool_file *file)
 {
 	int ret;
@@ -1239,6 +1275,10 @@ static int decode_sections(struct objtoo
 	if (ret)
 		return ret;
 
+	ret = grow_static_blocks(file);
+	if (ret)
+		return ret;
+
 	return 0;
 }
 
--- 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 static_jump_dest, retpoline_safe;
+	bool static_jump_dest, retpoline_safe, branch_target;
 	struct symbol *call_dest;
 	struct instruction *jump_dest;
 	struct list_head alts;