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

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Message-ID: <20181015051705.GA20687@flashbox>
Date:   Sun, 14 Oct 2018 22:17:05 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     nick.desaulniers@...il.com, Josh Poimboeuf <jpoimboe@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        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>, 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 08:43:18PM -0700, Andy Lutomirski wrote:
> 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

Hi Andy,

Did you mean 'objdump -dr'? If so, here you go (rather long, sorry if I
should have pasted it here instead):
https://gist.github.com/nathanchance/f038bb0a6653b975bb8a4e64fcd5503e

Nathan