From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Brian Gerst <brgerst@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
Thomas Gleixner <tglx@linutronix.de>,
Denys Vlasenko <dvlasenk@redhat.com>,
Ingo Molnar <mingo@kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Borislav Petkov <bp@suse.de>, "H. Peter Anvin" <hpa@zytor.com>,
X86 ML <x86@kernel.org>
Subject: Re: [PATCH 3/4] x86: Rewrite switch_to() code
Date: Mon, 23 May 2016 06:47:22 -0500 [thread overview]
Message-ID: <20160523114722.4nv3dpb2e5xr7ku7@treble> (raw)
In-Reply-To: <CAMzpN2hG3N5RguPb6uj=Eau8Kdo8492L9L6k9s3921S2-XDQAg@mail.gmail.com>
On Mon, May 23, 2016 at 07:14:14AM -0400, Brian Gerst wrote:
> On Sun, May 22, 2016 at 10:34 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > On Sun, May 22, 2016 at 10:59:38AM -0700, Andy Lutomirski wrote:
> >> cc: Josh Poimboeuf: do you care about the exact stack layout of the
> >> bottom of the stack of an inactive task?
> >
> > So there's one minor issue with this patch, relating to unwinding the
> > stack of a newly forked task. For detecting reliable stacks, the
> > unwinder needs to unwind all the way to the syscall pt_regs to make sure
> > the stack is sane. But for newly forked tasks, that won't be possible
> > here because the unwinding will stop at the fork_frame instead.
> >
> > So from an unwinder standpoint it might be nice for copy_thread_tls() to
> > place a frame pointer on the stack next to the ret_from_fork return
> > address, so that it would resemble an actual stack frame. The frame
> > pointer could probably just be hard-coded to zero. And then the first
> > bp in fork_frame would need to be a pointer to it instead of zero. That
> > would make it nicely resemble the stack of any other task.
> >
> > Alternatively I could teach the unwinder that if the unwinding starts at
> > the fork_frame offset from the end of the stack page, and the saved rbp
> > is zero, it can assume that it's a newly forked task. But that seems a
> > little more brittle to me, as it requires the unwinder to understand
> > more of the internal workings of the fork code.
> >
> > But overall I think this patch is a really nice cleanup, and other than
> > the above minor issue it should be fine with my reliable unwinder, since
> > rbp is still at the top of the stack.
>
> Ok, how about if it pushed RBP first, then we teach get_wchan() to add
> the fixed offset from thread.sp to get bp? that way it don't have to
> push it twice.
In theory I like the idea, and it would work: the unwinder could just
use the inactive_task_frame struct (as Andy suggested) to find the frame
pointer.
But I suspect it would break all existing unwinders, both in-tree and
out-of-tree. The only out-of-tree one I know of is crash, not sure if
there are more out there.
--
Josh
next prev parent reply other threads:[~2016-05-23 11:47 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-21 16:04 [PATCH 0/4] x86: Rewrite switch_to() Brian Gerst
2016-05-21 16:04 ` [PATCH 1/4] x86: Save return value from kernel_thread Brian Gerst
2016-05-22 1:44 ` Andy Lutomirski
2016-05-22 2:19 ` Brian Gerst
2016-05-21 16:04 ` [PATCH 2/4] x86-32, kgdb: Don't use thread.ip in sleeping_thread_to_gdb_regs() Brian Gerst
2016-05-23 17:05 ` Andy Lutomirski
2016-05-21 16:04 ` [PATCH 3/4] x86: Rewrite switch_to() code Brian Gerst
2016-05-22 17:59 ` Andy Lutomirski
2016-05-22 19:31 ` Brian Gerst
2016-05-22 21:07 ` Andy Lutomirski
2016-05-23 2:34 ` Josh Poimboeuf
2016-05-23 4:47 ` Andy Lutomirski
2016-05-23 11:40 ` Josh Poimboeuf
2016-05-23 11:49 ` Brian Gerst
2016-05-23 12:05 ` Josh Poimboeuf
2016-05-23 11:14 ` Brian Gerst
2016-05-23 11:47 ` Josh Poimboeuf [this message]
2016-05-23 11:49 ` Josh Poimboeuf
2016-05-23 16:46 ` Josh Poimboeuf
2016-05-23 17:03 ` Andy Lutomirski
2016-05-23 18:44 ` Josh Poimboeuf
2016-07-12 14:16 ` Josh Poimboeuf
2016-06-15 1:31 ` Andy Lutomirski
2016-06-15 8:03 ` Ingo Molnar
2016-06-15 11:52 ` Brian Gerst
2016-05-21 16:04 ` [PATCH 4/4] x86: Pass kernel thread parameters in fork_frame Brian Gerst
2016-05-22 18:01 ` Andy Lutomirski
2016-05-22 19:21 ` Brian Gerst
2016-05-23 15:23 ` Josh Poimboeuf
2016-05-23 15:36 ` Andy Lutomirski
2016-05-23 21:04 ` Brian Gerst
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