linux-kernel - Re: [PATCH] x86/mm: annotate no_context with UNWIND

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Date:   Sun, 14 Oct 2018 20:43:18 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     nick.desaulniers@...il.com, Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, natechancellor@...il.com,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/mm: annotate no_context with UNWIND_HINTS

On Sun, Oct 14, 2018 at 5:37 PM Nick Desaulniers
<nick.desaulniers@...il.com> wrote:
>
> Fixes the objtool warning:
> arch/x86/mm/fault.o: warning: objtool: no_context()+0x220: unreachable
> instruction
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/204
> Signed-off-by: Nick Desaulniers <nick.desaulniers@...il.com>
> ---
>  arch/x86/mm/fault.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 47bebfe6efa7..057d2178fa19 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -760,9 +760,11 @@ no_context(struct pt_regs *regs, unsigned long error_code,
>                  * and then double-fault, though, because we're likely to
>                  * break the console driver and lose most of the stack dump.
>                  */
> -               asm volatile ("movq %[stack], %%rsp\n\t"
> +               asm volatile (UNWIND_HINT_SAVE
> +                             "movq %[stack], %%rsp\n\t"
>                               "call handle_stack_overflow\n\t"
> -                             "1: jmp 1b"
> +                             "1: jmp 1b\n\t"
> +                             UNWIND_HINT_RESTORE
>                               : ASM_CALL_CONSTRAINT
>                               : "D" ("kernel stack overflow (page fault)"),
>                                 "S" (regs), "d" (address),

NAK.  Just below this snippet is unreachable();

Can you reply with objtool -dr output on a problematic fault.o?  Josh,
it *looks* like annotate_unreachable() should be doing the right
thing, but something is clearly busted.

Also, shouldn't compiler-clang.h contain a reasonable definition of
unreachable()?

--Andy