[PATCH 1/5] objtool: Fix CONFIG_UBSAN_TRAP unreachable warnings

Josh Poimboeuf
2020-04-01T18:23:43Z

CONFIG_UBSAN_TRAP causes GCC to emit a UD2 whenever it encounters an
unreachable code path. This includes __builtin_unreachable(). Because
the BUG() macro uses __builtin_unreachable() after it emits its own UD2,
this results in a double UD2. In this case objtool rightfully detects
that the second UD2 is unreachable:

init/main.o: warning: objtool: repair_env_string()+0x1c8: unreachable instruction

We weren't able to figure out a way to get rid of the double UD2s, so
just silence the warning.

Cc: Kees Cook <keescook@chromium.org>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
tools/objtool/check.c | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index e3bb76358148..aaec5e1277ea 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2382,14 +2382,27 @@ static bool ignore_unreachable_insn(struct instruction *insn)
!strcmp(insn->sec->name, ".altinstr_aux"))
return true;

+ if (!insn->func)
+ return false;
+
+ /*
+ * CONFIG_UBSAN_TRAP inserts a UD2 when it sees
+ * __builtin_unreachable(). The BUG() macro has an unreachable() after
+ * the UD2, which causes GCC's undefined trap logic to emit another UD2
+ * (or occasionally a JMP to UD2).
+ */
+ if (list_prev_entry(insn, list)->dead_end &&
+ (insn->type == INSN_BUG ||
+ (insn->type == INSN_JUMP_UNCONDITIONAL &&
+ insn->jump_dest && insn->jump_dest->type == INSN_BUG)))
+ return true;
+
/*
* Check if this (or a subsequent) instruction is related to
* CONFIG_UBSAN or CONFIG_KASAN.
*
* End the search at 5 instructions to avoid going into the weeds.
*/
- if (!insn->func)
- return false;
for (i = 0; i < 5; i++) {

if (is_kasan_insn(insn) || is_ubsan_insn(insn))
--
2.21.1