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Date: Fri, 9 Mar 2012 08:33:48 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
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        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Anton Arapov <anton@redhat.com>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Jim Keniston <jkenisto@linux.vnet.ibm.com>,
        Jiri Olsa <jolsa@redhat.com>, Josh Stone <jistone@redhat.com>
Subject: Re: [PATCH] uprobes/core: handle breakpoint and signal step
 exception.
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* Srikar Dronamraju <srikar@linux.vnet.ibm.com> wrote:

> * Ingo Molnar <mingo@elte.hu> [2012-03-08 14:48:09]:
> 
> > 
> > * Srikar Dronamraju <srikar@linux.vnet.ibm.com> wrote:
> > 
> > > @@ -233,9 +233,11 @@ static inline void __user *arch_compat_alloc_user_space(long len)
> > >  
> > >  	if (test_thread_flag(TIF_IA32)) {
> > >  		sp = task_pt_regs(current)->sp;
> > > +#ifdef CONFIG_X86_64
> > >  	} else {
> > >  		/* -128 for the x32 ABI redzone */
> > >  		sp = __this_cpu_read(old_rsp) - 128;
> > > +#endif
> > >  	}
> > >  
> > >  	return (void __user *)round_down(sp - len, 16);
> > 
> > So 'sp' is undefined if that TIF check fails?
> > 
> > Also, on a 32-bit kernel the TIF check probably fails all the 
> > time, because we don't set TIF_IA32 (and don't know that flag).
> 
> > 
> > It would probably be better to make the whole helper inline 
> > #ifdef 64-bit, it does not look very useful on 32-bit.
> > 
> 
> arch_compat_alloc_user_space gets called from compat_alloc_user_space
> which is arch agnostic and exported too.
> 
> So I will change this to
> 
> void __user *arch_compat_alloc_user_space(long len)
> {
>   	if (is_ia32_compat_task(current))
>   		sp = task_pt_regs(current)->sp;
> #ifdef CONFIG_X86_64
>   	else
>   		/* -128 for the x32 ABI redzone */
>   		sp = __this_cpu_read(old_rsp) - 128;
> #endif
>   
>   	return (void __user *)round_down(sp - len, 16);
> }
> 
> where is_ia32_compat_task() is the new macro that you 
> suggested we put in compat.h which would return true if the 
> task is 32 bit emulated on x86_64 or running on i386 machine.
> 
> Hence we can avoid the case where sp is not set.

Ok - looks good at first glance.

Thanks,

	Ingo