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From:   Alexandre Chartre <alexandre.chartre@oracle.com>
To:     x86@kernel.org
Cc:     linux-kernel@vger.kernel.org, jpoimboe@redhat.com,
        peterz@infradead.org, jthierry@redhat.com, tglx@linutronix.de,
        alexandre.chartre@oracle.com
Subject: [PATCH 2/7] objtool: Allow branches within the same alternative.
Date:   Thu,  2 Apr 2020 10:22:15 +0200
Message-Id: <20200402082220.808-3-alexandre.chartre@oracle.com>
In-Reply-To: <20200402082220.808-1-alexandre.chartre@oracle.com>
References: <20200402082220.808-1-alexandre.chartre@oracle.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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>
---
 tools/objtool/check.c | 19 +++++++++++++------
 tools/objtool/check.h |  3 ++-
 2 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 708562fb89e1..214809ac2776 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -712,7 +712,9 @@ static int handle_group_alt(struct objtool_file *file,
 			    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;
@@ -721,7 +723,7 @@ static int handle_group_alt(struct objtool_file *file,
 		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;
 	}
 
@@ -1942,6 +1944,7 @@ static int validate_sibling_call(struct instruction *insn, struct insn_state *st
  * tools/objtool/Documentation/stack-validation.txt.
  */
 static int validate_branch(struct objtool_file *file, struct symbol *func,
+			   struct instruction *from,
 			   struct instruction *first, struct insn_state state)
 {
 	struct alternative *alt;
@@ -1953,7 +1956,9 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 	insn = first;
 	sec = insn->sec;
 
-	if (insn->alt_group && list_empty(&insn->alts)) {
+	if (insn->alt_group &&
+	    (!from || from->alt_group != insn->alt_group) &&
+	    list_empty(&insn->alts)) {
 		WARN_FUNC("don't know how to handle branch to middle of alternative instruction group",
 			  sec, insn->offset);
 		return 1;
@@ -2035,7 +2040,8 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 				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);
@@ -2105,7 +2111,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 					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)
@@ -2236,7 +2242,8 @@ static int validate_unwind_hints(struct objtool_file *file)
 
 	for_each_insn(file, insn) {
 		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;
@@ -2377,7 +2384,7 @@ static int validate_functions(struct objtool_file *file)
 
 			state.uaccess = func->uaccess_safe;
 
-			ret = validate_branch(file, func, insn, state);
+			ret = validate_branch(file, func, NULL, insn, state);
 			if (ret && backtrace)
 				BT_FUNC("<=== (func)", insn);
 			warnings += ret;
diff --git a/tools/objtool/check.h b/tools/objtool/check.h
index 6d875ca6fce0..cffb23d81782 100644
--- a/tools/objtool/check.h
+++ b/tools/objtool/check.h
@@ -33,7 +33,8 @@ struct instruction {
 	unsigned int len;
 	enum insn_type type;
 	unsigned long immediate;
-	bool alt_group, dead_end, ignore, hint, save, restore, ignore_alts;
+	unsigned int alt_group;
+	bool dead_end, ignore, hint, save, restore, ignore_alts;
 	bool retpoline_safe;
 	u8 visited;
 	struct symbol *call_dest;
-- 
2.18.2