linux-kernel - [PATCH 01/21] objtool: Fix seg fault in BT

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Message-Id: <c6bd154e55739c332c21ca4a91a66787cc3e104c.1610652862.git.jpoimboe@redhat.com>
Date:   Thu, 14 Jan 2021 13:39:57 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com, Miroslav Benes <mbenes@...e.cz>
Subject: [PATCH 01/21] objtool: Fix seg fault in BT_FUNC() with fake jump

Objtool appends a temporary fake jump at the end of alternative
replacement instructions.  If the replacement code is empty -- resulting
in patched nops -- the fake jump doesn't have a section.  When running
objtool with '--backtrace', the fake jump's missing section can cause
BT_FUNC() to trigger a seg fault when the NULL insn->sec is passed to
offstr().

Fix it by ensuring fake jumps always have a section.

Fixes: 7697eee3ddd7 ("objtool: Add --backtrace support")
Reported-by: Sami Tolvanen <samitolvanen@...gle.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 tools/objtool/check.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 5f8d3eed78a1..ed26c22c8244 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1017,7 +1017,7 @@ static int handle_group_alt(struct objtool_file *file,
 		INIT_LIST_HEAD(&fake_jump->stack_ops);
 		init_cfi_state(&fake_jump->cfi);
 
-		fake_jump->sec = special_alt->new_sec;
+		fake_jump->sec = special_alt->new_sec ? : orig_insn->sec;
 		fake_jump->offset = FAKE_JUMP_OFFSET;
 		fake_jump->type = INSN_JUMP_UNCONDITIONAL;
 		fake_jump->jump_dest = list_next_entry(last_orig_insn, list);
-- 
2.29.2