linux-kernel - Re: [BUGFIX][PATCH -rc/-mm] prevent kprobes from catching spurious page faults

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Message-ID: <20090205235727.GA16040@elte.hu>
Date:	Fri, 6 Feb 2009 00:57:27 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <greg@...ah.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Nick Piggin <npiggin@...e.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ibm.com>,
	systemtap-ml <systemtap@...rces.redhat.com>,
	"Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [BUGFIX][PATCH -rc/-mm] prevent kprobes from catching spurious
	page  faults


* Masami Hiramatsu <mhiramat@...hat.com> wrote:

> -	if (notify_page_fault(regs))
> -		return;
>  	if (unlikely(kmmio_fault(regs, address)))
>  		return;
> 
> @@ -634,6 +632,9 @@ void __kprobes do_page_fault(struct pt_r
>  		if (spurious_fault(address, error_code))
>  			return;
> 
> +		/* kprobes don't want to hook the spurious faults. */
> +		if (notify_page_fault(regs))
> +			return;
>  		/*
>  		 * Don't take the mm semaphore here. If we fixup a prefetch
>  		 * fault we could otherwise deadlock.
> @@ -641,6 +642,9 @@ void __kprobes do_page_fault(struct pt_r
>  		goto bad_area_nosemaphore;
>  	}
> 
> +	/* kprobes don't want to hook the spurious faults. */
> +	if (notify_page_fault(regs))
> +		return;

I dont know - this spreads that callback to two places now. Any
reason why kprobes cannot call spurious_fault(), if there's a
probe active?

Also, moving that would remove the planned cleanup of merging these
two into one call:

 	if (notify_page_fault(regs))
 		return;
  	if (unlikely(kmmio_fault(regs, address)))
  		return;

We should reduce the probing cross section, not increase it,
especially in such a critical codepath as the pagefault handler.

Btw., why cannot kprobes install a dynamic probe to the fault
handler itself? That way the default path would have no such
callbacks and checks at all.

	Ingo
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