[RFC][PATCH 5/6] objtool: Allow archs to rewrite retpolines

[Peter Zijlstra <peterz@infradead.org>]

When retpolines are employed, compilers typically emit calls to
retpoline thunks. Objtool recognises these calls and marks them as
dynamic calls.

Provide infrastructure for architectures to rewrite/augment what the
compiler wrote for us.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 tools/objtool/check.c                |   23 +++++++++++++++++++++--
 tools/objtool/include/objtool/arch.h |    3 +++
 2 files changed, 24 insertions(+), 2 deletions(-)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -775,6 +775,14 @@ static int add_ignore_alternatives(struc
 	return 0;
 }
 
+
+__weak int arch_rewrite_retpoline(struct objtool_file *file,
+				  struct instruction *insn,
+				  struct reloc *reloc)
+{
+	return 0;
+}
+
 /*
  * Find the destination instructions for all jumps.
  */
@@ -808,6 +816,9 @@ static int add_jump_destinations(struct
 				insn->type = INSN_JUMP_DYNAMIC_CONDITIONAL;
 
 			insn->retpoline_safe = true;
+
+			arch_rewrite_retpoline(file, insn, reloc);
+
 			continue;
 		} else if (insn->func) {
 			/* internal or external sibling call (with reloc) */
@@ -959,6 +970,8 @@ static int add_call_destinations(struct
 			insn->type = INSN_CALL_DYNAMIC;
 			insn->retpoline_safe = true;
 
+			arch_rewrite_retpoline(file, insn, reloc);
+
 			remove_insn_ops(insn);
 			continue;
 
@@ -1119,6 +1132,8 @@ static int handle_group_alt(struct objto
 		dest_off = arch_jump_destination(insn);
 		if (dest_off == special_alt->new_off + special_alt->new_len)
 			insn->jump_dest = next_insn_same_sec(file, last_orig_insn);
+		else
+			insn->jump_dest = find_insn(file, insn->sec, dest_off);
 
 		if (!insn->jump_dest) {
 			WARN_FUNC("can't find alternative jump destination",
@@ -1704,11 +1719,15 @@ static int decode_sections(struct objtoo
 	if (ret)
 		return ret;
 
-	ret = add_jump_destinations(file);
+	/*
+	 * Must be before add_{jump,call}_destination; for they can add
+	 * magic alternatives we can't actually parse.
+	 */
+	ret = add_special_section_alts(file);
 	if (ret)
 		return ret;
 
-	ret = add_special_section_alts(file);
+	ret = add_jump_destinations(file);
 	if (ret)
 		return ret;
 
--- a/tools/objtool/include/objtool/arch.h
+++ b/tools/objtool/include/objtool/arch.h
@@ -82,6 +82,9 @@ unsigned long arch_jump_destination(stru
 unsigned long arch_dest_reloc_offset(int addend);
 
 const char *arch_nop_insn(int len);
+int arch_rewrite_retpoline(struct objtool_file *file,
+			   struct instruction *insn,
+			   struct reloc *reloc);
 
 int arch_decode_hint_reg(struct instruction *insn, u8 sp_reg);