netdev - Re: [PATCH] x86/dumpstack/64: Add guard pages to stack

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Date:   Thu, 16 Sep 2021 10:03:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     王贇 <yun.wang@...ux.alibaba.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "open list:PERFORMANCE EVENTS SUBSYSTEM" 
        <linux-perf-users@...r.kernel.org>,
        "open list:PERFORMANCE EVENTS SUBSYSTEM" 
        <linux-kernel@...r.kernel.org>,
        "open list:BPF (Safe dynamic programs and tools)" 
        <netdev@...r.kernel.org>,
        "open list:BPF (Safe dynamic programs and tools)" 
        <bpf@...r.kernel.org>, jroedel@...e.de, x86@...nel.org
Subject: Re: [PATCH] x86/dumpstack/64: Add guard pages to stack_info

On Thu, Sep 16, 2021 at 10:00:15AM +0200, Peter Zijlstra wrote:
> On Thu, Sep 16, 2021 at 11:47:49AM +0800, 王贇 wrote:
> 
> > I did some debug and found the issue, we are missing:
> > 
> > @@ -122,7 +137,10 @@ static __always_inline bool in_exception_stack(unsigned long *stack, struct stac
> >         info->type      = ep->type;
> >         info->begin     = (unsigned long *)begin;
> >         info->end       = (unsigned long *)end;
> > -       info->next_sp   = (unsigned long *)regs->sp;
> > +
> > +       if (!(ep->type & STACK_TYPE_GUARD))
> > +               info->next_sp   = (unsigned long *)regs->sp;
> > +
> >         return true;
> >  }
> > 
> > as the guard page are not working as real stack I guess?
> 
> Correct, but I thought I put if (type & GUARD) terminators in all paths
> that ended up caring about ->next_sp. Clearly I seem to have missed one
> :/
> 
> Let me try and figure out where that happens.

Oh, I'm an idiot... yes it tries to read regs the stack, but clearly
that won't work for the guard page.