objtool: Ignore unreachable trap after call to noreturn functions

With CONFIG_UBSAN_TRAP enabled, the compiler may insert a trap
instruction after a call to a noreturn function. In this case, objtool
warns that the UD2 instruction is unreachable.

This is a behavior seen with Clang, from the oldest version capable of
building the mainline x64_64 kernel (9.0), to the latest experimental
version (12.0).

Objtool silences similar warnings (trap after dead end instructions), so
so expand that check to include dead end functions.

Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Rong Chen <rong.a.chen@intel.com>
Cc: Marco Elver <elver@google.com>
Cc: Philip Li <philip.li@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: kasan-dev@googlegroups.com
Cc: x86@kernel.org
Cc: clang-built-linux@googlegroups.com
BugLink: https://github.com/ClangBuiltLinux/linux/issues/1148
Link: https://lore.kernel.org/lkml/CAKwvOdmptEpi8fiOyWUo=AiZJiX+Z+VHJOM2buLPrWsMTwLnyw@mail.gmail.com
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Ilie Halip <ilie.halip@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index a4796e32bbd191fa75438463aaeaa22064247264..2df9f769412e96a304e5795b5c2c71511dcfbd4a 100644 (file)
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2638,9 +2638,10 @@ static bool is_ubsan_insn(struct instruction *insn)
                        "__ubsan_handle_builtin_unreachable"));
 }
 
-static bool ignore_unreachable_insn(struct instruction *insn)
+static bool ignore_unreachable_insn(struct objtool_file *file, struct instruction *insn)
 {
        int i;
+       struct instruction *prev_insn;
 
        if (insn->ignore || insn->type == INSN_NOP)
                return true;
@@ -2668,8 +2669,11 @@ static bool ignore_unreachable_insn(struct instruction *insn)
         * __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).
+        *
+        * It may also insert a UD2 after calling a __noreturn function.
         */
-       if (list_prev_entry(insn, list)->dead_end &&
+       prev_insn = list_prev_entry(insn, list);
+       if ((prev_insn->dead_end || dead_end_function(file, prev_insn->call_dest)) &&
            (insn->type == INSN_BUG ||
             (insn->type == INSN_JUMP_UNCONDITIONAL &&
              insn->jump_dest && insn->jump_dest->type == INSN_BUG)))
@@ -2796,7 +2800,7 @@ static int validate_reachable_instructions(struct objtool_file *file)
                return 0;
 
        for_each_insn(file, insn) {
-               if (insn->visited || ignore_unreachable_insn(insn))
+               if (insn->visited || ignore_unreachable_insn(file, insn))
                        continue;
 
                WARN_FUNC("unreachable instruction", insn->sec, insn->offset);