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    SubjectRe: [PATCH 01/21] x86/unwinder/orc: Don't bail on stack overflow
    On Mon, Nov 27, 2017 at 11:45:09AM +0100, Ingo Molnar wrote:
    > From: Andy Lutomirski <luto@kernel.org>
    >
    > If we overflow the stack into a guard page and then try to unwind it
    > with ORC, it should work well: by construction, there can't be any
    > meaningful data in the guard page because no writes to the guard page
    > will have succeeded.
    >
    > This patch fixes a bug that unwinding from working correctly: if the
    ^
    prevents

    > starting register state has RSP pointing into a stack guard page, the
    > ORC unwinder bails out immediately. This patch fixes that: the ORC

    I believe here we can kill the second "This patch" :)

    > unwinder will start the unwind.
    >
    > I tested this by intentionally overflowing the task stack. The
    > result is an accurate call trace instead of a trace consisting
    > purely of '?' entries.
    >
    > There are a few other bugs that are triggered if the unwinder
    > encounters a stack overflow after the first step, and Josh has WIP
    > patches to fix those as well.

    I guess we don't need that paragraph.

    > Signed-off-by: Andy Lutomirski <luto@kernel.org>
    > Cc: Borislav Petkov <bpetkov@suse.de>
    > Cc: Brian Gerst <brgerst@gmail.com>
    > Cc: Dave Hansen <dave.hansen@intel.com>
    > Cc: Josh Poimboeuf <jpoimboe@redhat.com>
    > Cc: Linus Torvalds <torvalds@linux-foundation.org>
    > Cc: Peter Zijlstra <peterz@infradead.org>
    > Cc: Thomas Gleixner <tglx@linutronix.de>
    > Link: http://lkml.kernel.org/r/927042950d7f1a7007dd0f58538966a593508f8b.1511715954.git.luto@kernel.org
    > Signed-off-by: Ingo Molnar <mingo@kernel.org>
    > ---
    > arch/x86/kernel/unwind_orc.c | 14 ++++++++++++--
    > 1 file changed, 12 insertions(+), 2 deletions(-)
    >
    > diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
    > index a3f973b2c97a..7f6e3935666b 100644
    > --- a/arch/x86/kernel/unwind_orc.c
    > +++ b/arch/x86/kernel/unwind_orc.c
    > @@ -553,8 +553,18 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
    > }
    >
    > if (get_stack_info((unsigned long *)state->sp, state->task,
    > - &state->stack_info, &state->stack_mask))
    > - return;
    > + &state->stack_info, &state->stack_mask)) {
    > + /*
    > + * We weren't on a valid stack. It's possible that
    > + * we overflowed a valid stack into a guard page.
    > + * See if the next page up is valid so that we can
    > + * generate some kind of backtrace if this happens.
    > + */

    Right, should we issue a marker or somesuch here to denote that we somehow
    walked into the guard page?

    It might be helpful when debugging issues, to see the big picture...

    > + void *next_page = (void *)PAGE_ALIGN((unsigned long)regs->sp);
    > + if (get_stack_info(next_page, state->task, &state->stack_info,
    > + &state->stack_mask))
    > + return;
    > + }
    >
    > /*
    > * The caller can provide the address of the first frame directly
    > --

    --
    Regards/Gruss,
    Boris.

    Good mailing practices for 400: avoid top-posting and trim the reply.

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