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

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Message-ID: <CALCETrWtgjxSZOg=hqGe3hDDvpuBDZtjptHFDAuHYOaxs1CO_w@mail.gmail.com>
Date:   Mon, 15 Oct 2018 09:03:40 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     natechancellor@...il.com, Andrew Lutomirski <luto@...nel.org>,
        nick.desaulniers@...il.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 Mon, Oct 15, 2018 at 8:31 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> On Mon, Oct 15, 2018 at 08:22:21AM -0700, Nathan Chancellor wrote:
> > > >>> @@ -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
> > > >
> > > >
> > >
> > > Hmm, -dr wasn’t quite enough to dump the .discard bits, assuming they’re there at all. Can you just put the whole .o file somewhere?
> >
> > Here you go: https://nathanchance.me/downloads/.tmp/fault.o
>
> $ eu-readelf -S /tmp/fault.o  |grep reachable
> [12] .discard.reachable   PROGBITS     0000000000000000 00002bc0 00000014  0        0   0  1
> [13] .rela.discard.reachable RELA         0000000000000000 00002bd8 00000078 24 I     32  12  8
>
> That confirms that you need a clang version of the unreachable() macro.
>

Duh.

That being said, the generic macro is:

# define unreachable() do { annotate_reachable(); do { } while (1); } while (0)

I'm probably missing some subtlety here, but shouldn't that be
annotate_*un*reachable()?

Of course, there are any number of reasons why there should be a real
definition.  Nathan and Nick, does adding something like:

#define unreachable() \
        do {                                    \
                annotate_unreachable();         \
                __builtin_unreachable();        \
        } while (0)

to compiler-clang.h fix the problem?

--Andy