Re: [PATCH 5.15 000/923] 5.15.3-rc3 review

From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Linus Torvalds" <torvalds@linux-foundation.org>,
	"Holger Hoffstätte" <holger@applied-asynchrony.com>,
	"Qi Zheng" <zhengqi.arch@bytedance.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Justin Forbes" <jmforbes@linuxtx.org>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Guenter Roeck" <linux@roeck-us.net>,
	"Shuah Khan" <shuah@kernel.org>,
	patches@kernelci.org, lkft-triage@lists.linaro.org,
	"Pavel Machek" <pavel@denx.de>,
	"Jon Hunter" <jonathanh@nvidia.com>,
	"Florian Fainelli" <f.fainelli@gmail.com>,
	stable <stable@vger.kernel.org>
Subject: Re: [PATCH 5.15 000/923] 5.15.3-rc3 review
Date: Thu, 18 Nov 2021 18:04:27 -0800	[thread overview]
Message-ID: <20211119020427.2y5esq2czquwmvwc@treble> (raw)
In-Reply-To: <YZZC3Shc0XA/gHK9@hirez.programming.kicks-ass.net>

On Thu, Nov 18, 2021 at 01:11:09PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 18, 2021 at 10:39:44AM +0100, Peter Zijlstra wrote:
> > On Thu, Nov 18, 2021 at 09:18:52AM +0100, Peter Zijlstra wrote:
> > > On Thu, Nov 18, 2021 at 09:06:27AM +0100, Peter Zijlstra wrote:
> > > > On Wed, Nov 17, 2021 at 03:50:17PM -0800, Linus Torvalds wrote:
> > > > 
> > > > > I really don't think the WCHAN code should use unwinders at all. It's
> > > > > too damn fragile, and it's too easily triggered from user space.
> > > > 
> > > > On x86, esp. with ORC, it pretty much has to. The thing is, the ORC
> > > > unwinder has been very stable so far. I'm guessing there's some really
> > > > stupid thing going on, like for example trying to unwind a freed stack.
> > > > 
> > > > I *just* managed to reproduce, so let me go have a poke.
> > > 
> > > Confirmed, with the below it no longer reproduces. Now, let me go undo
> > > that and fix the unwinder to not explode while trying to unwind nothing.
> > 
> > OK, so the bug is firmly with 5d1ceb3969b6 ("x86: Fix __get_wchan() for
> > !STACKTRACE") which lost the try_get_task_stack() that stack_trace_*()
> > does.
> > 
> > We can ofc trivially re-instate that, but I'm now running with the
> > below which I suppose is a better fix, hmm?
> > 
> > (obv I still need to look a the other two unwinders)
> 
> I now have the below, the only thing missing is that there's a
> user_mode() call on a stack based regs. Now on x86_64 we can
> __get_kernel_nofault() regs->cs and call it a day, but on i386 we have
> to also fetch regs->flags.
> 
> Is this really the way to go?

Please no.  Can we just add a check in unwind_start() to ensure the
caller did try_get_task_stack()?

-- 
Josh