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

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Date:   Thu, 16 Sep 2021 10:00:15 +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 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.

> With that one things going on correctly, and some trivials below.

> >  enum stack_type {
> > -	STACK_TYPE_UNKNOWN,
> > +	STACK_TYPE_UNKNOWN = 0,
> 
> Is this necessary?

No, but it makes it more explicit we care about the value.

> >  	STACK_TYPE_TASK,
> >  	STACK_TYPE_IRQ,
> >  	STACK_TYPE_SOFTIRQ,
> >  	STACK_TYPE_ENTRY,
> >  	STACK_TYPE_EXCEPTION,
> >  	STACK_TYPE_EXCEPTION_LAST = STACK_TYPE_EXCEPTION + N_EXCEPTION_STACKS-1,
> > +	STACK_TYPE_GUARD = 0x80,

Note that this is a flag.

> >  };
> >  
> >  struct stack_info {
> > --- a/arch/x86/kernel/dumpstack_64.c
> > +++ b/arch/x86/kernel/dumpstack_64.c
> > @@ -32,9 +32,15 @@ const char *stack_type_name(enum stack_t
> >  {
> >  	BUILD_BUG_ON(N_EXCEPTION_STACKS != 6);
> >  
> > +	if (type == STACK_TYPE_TASK)
> > +		return "TASK";
> > +
> >  	if (type == STACK_TYPE_IRQ)
> >  		return "IRQ";
> >  
> > +	if (type == STACK_TYPE_SOFTIRQ)
> > +		return "SOFTIRQ";
> > +
> 
> Do we need one for GUARD too?

No, GUARD is not a single type but a flag. The caller can trivially do
something like:

	"%s %s", stack_type_name(type & ~GUARD),
	         (type & GUARD) ?  "GUARD" : ""

> >  	if (type == STACK_TYPE_ENTRY) {
> >  		/*
> >  		 * On 64-bit, we have a generic entry stack that we

> > @@ -111,10 +122,11 @@ static __always_inline bool in_exception
> >  	k = (stk - begin) >> PAGE_SHIFT;
> >  	/* Lookup the page descriptor */
> >  	ep = &estack_pages[k];
> > -	/* Guard page? */
> > +	/* unknown entry */
> >  	if (!ep->size)
> >  		return false;
> >  
> > +
> 
> Extra line?

Gone now, thanks!