[RFC][PATCH 3/7] objtool: Allow branches within the same alternative.

[Peter Zijlstra <peterz@infradead.org>]

From: Alexandre Chartre <alexandre.chartre@oracle.com>

Currently objtool prevents any branch to an alternative. While preventing
branching from the outside to the middle of an alternative makes perfect
sense, branching within the same alternative should be allowed. To do so,
identify each alternative and check that a branch to an alternative comes
from the same alternative.

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200414103618.12657-3-alexandre.chartre@oracle.com
---
 tools/objtool/check.c |   27 +++++++++++++++++++++------
 tools/objtool/check.h |    3 ++-
 2 files changed, 23 insertions(+), 7 deletions(-)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -768,7 +768,9 @@ static int handle_group_alt(struct objto
 			    struct instruction *orig_insn,
 			    struct instruction **new_insn)
 {
+	static unsigned int alt_group_next_index = 1;
 	struct instruction *last_orig_insn, *last_new_insn, *insn, *fake_jump = NULL;
+	unsigned int alt_group = alt_group_next_index++;
 	unsigned long dest_off;
 
 	last_orig_insn = NULL;
@@ -777,7 +779,7 @@ static int handle_group_alt(struct objto
 		if (insn->offset >= special_alt->orig_off + special_alt->orig_len)
 			break;
 
-		insn->alt_group = true;
+		insn->alt_group = alt_group;
 		last_orig_insn = insn;
 	}
 
@@ -811,6 +813,7 @@ static int handle_group_alt(struct objto
 	}
 
 	last_new_insn = NULL;
+	alt_group = alt_group_next_index++;
 	insn = *new_insn;
 	sec_for_each_insn_from(file, insn) {
 		if (insn->offset >= special_alt->new_off + special_alt->new_len)
@@ -820,6 +823,7 @@ static int handle_group_alt(struct objto
 
 		insn->ignore = orig_insn->ignore_alts;
 		insn->func = orig_insn->func;
+		insn->alt_group = alt_group;
 
 		/*
 		 * Since alternative replacement code is copy/pasted by the
@@ -2157,6 +2161,15 @@ static int validate_return(struct symbol
 	return 0;
 }
 
+static bool is_branch_to_alternative(struct instruction *from,
+				     struct instruction *to)
+{
+	if (!from || !to->alt_group || !list_empty(&to->alts))
+		return false;
+
+	return (from->alt_group != to->alt_group);
+}
+
 /*
  * Follow the branch starting at the given instruction, and recursively follow
  * any other branches (jumps).  Meanwhile, track the frame pointer state at
@@ -2164,6 +2177,7 @@ static int validate_return(struct symbol
  * tools/objtool/Documentation/stack-validation.txt.
  */
 static int validate_branch(struct objtool_file *file, struct symbol *func,
+			   struct instruction *from,
 			   struct instruction *insn, struct insn_state state)
 {
 	struct alternative *alt;
@@ -2174,7 +2188,7 @@ static int validate_branch(struct objtoo
 
 	sec = insn->sec;
 
-	if (insn->alt_group && list_empty(&insn->alts)) {
+	if (is_branch_to_alternative(from, insn)) {
 		WARN_FUNC("don't know how to handle branch to middle of alternative instruction group",
 			  sec, insn->offset);
 		return 1;
@@ -2221,7 +2235,8 @@ static int validate_branch(struct objtoo
 				if (alt->skip_orig)
 					skip_orig = true;
 
-				ret = validate_branch(file, func, alt->insn, state);
+				ret = validate_branch(file, func,
+						      NULL, alt->insn, state);
 				if (ret) {
 					if (backtrace)
 						BT_FUNC("(alt)", insn);
@@ -2264,7 +2279,7 @@ static int validate_branch(struct objtoo
 					return ret;
 
 			} else if (insn->jump_dest) {
-				ret = validate_branch(file, func,
+				ret = validate_branch(file, func, insn,
 						      insn->jump_dest, state);
 				if (ret) {
 					if (backtrace)
@@ -2396,7 +2411,7 @@ static int validate_unwind_hints(struct
 
 	while (&insn->list != &file->insn_list && (!sec || insn->sec == sec)) {
 		if (insn->hint && !insn->visited) {
-			ret = validate_branch(file, insn->func, insn, state);
+			ret = validate_branch(file, insn->func, NULL, insn, state);
 			if (ret && backtrace)
 				BT_FUNC("<=== (hint)", insn);
 			warnings += ret;
@@ -2537,7 +2552,7 @@ static int validate_symbol(struct objtoo
 
 	state->uaccess = sym->uaccess_safe;
 
-	ret = validate_branch(file, insn->func, insn, *state);
+	ret = validate_branch(file, insn->func, NULL, insn, *state);
 	if (ret && backtrace)
 		BT_FUNC("<=== (sym)", insn);
 	return ret;
--- a/tools/objtool/check.h
+++ b/tools/objtool/check.h
@@ -30,7 +30,8 @@ struct instruction {
 	unsigned int len;
 	enum insn_type type;
 	unsigned long immediate;
-	bool alt_group, dead_end, ignore, ignore_alts;
+	int alt_group;
+	bool dead_end, ignore, ignore_alts;
 	bool hint;
 	bool retpoline_safe;
 	s8 instr;