Re: [PATCH] objtool: ignore unreachable trap after call to noreturn functions

[Nick Desaulniers <ndesaulniers@google.com>]

On Thu, Sep 17, 2020 at 1:49 AM Ilie Halip <ilie.halip@gmail.com> wrote:
>
> 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.
>
> objtool silences similar warnings (trap after dead end instructions), so
> expand that check to include dead end functions.

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reported-by: kbuild test robot <lkp@intel.com>

Thanks for the patch, Ilie!  With this, a
defconfig+CONFIG_UBSAN+CONFIG_UBSAN_TRAP goes from 82 unreachable
instruction warnings from objtool to 0.

$ make LLVM=1 -j71 defconfig
$ make LLVM=1 -j71 menuconfig
<enable UBSAN and UBSAN_TRAP>
$ make LLVM=1 -j71 2> log.txt
$ grep warning: log.txt | cut -d ' ' -f 2- | sort | uniq -c | wc -l
82
$ b4 am https://lore.kernel.org/lkml/20200917084905.1647262-1-ilie.halip@gmail.com/
-o - | git am
$ make LLVM=1 -j71 clean
$ make LLVM=1 -j71 2> log.txt
$ grep warning: log.txt | cut -d ' ' -f 2- | sort | uniq -c | wc -l
0

>
> 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>
> Signed-off-by: Ilie Halip <ilie.halip@gmail.com>
> ---
>  tools/objtool/check.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index e034a8f24f46..eddf8bf16b05 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -2612,9 +2612,10 @@ 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;
> @@ -2639,8 +2640,11 @@ 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).
> +        * CONFIG_UBSAN_TRAP 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)))
> @@ -2767,7 +2771,7 @@ 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);
> --
> 2.25.1
>


-- 
Thanks,
~Nick Desaulniers