linux-kernel - Re: [PATCH 3/5] arm64: Handle TRAP

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Date:   Wed, 7 Sep 2016 10:17:52 +0530
From:   Pratyush Anand <panand@...hat.com>
To:     David Long <dave.long@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux@....linux.org.uk,
        will.deacon@....com, Yang Shi <yang.shi@...aro.org>,
        steve.capper@...aro.org, srikar@...ux.vnet.ibm.com,
        Suzuki K Poulose <suzuki.poulose@....com>,
        vijaya.kumar@...iumnetworks.com, linux-kernel@...r.kernel.org,
        oleg@...hat.com, Sandeepa Prabhu <sandeepa.s.prabhu@...il.com>,
        wcohen@...hat.com, Anna-Maria Gleixner <anna-maria@...utronix.de>
Subject: Re: [PATCH 3/5] arm64: Handle TRAP_HWBRKPT for user mode as well

On 06/09/2016:05:36:18 PM, David Long wrote:
> On 09/06/2016 12:11 PM, Catalin Marinas wrote:
> > On Tue, Aug 02, 2016 at 11:00:07AM +0530, Pratyush Anand wrote:
> > > --- a/arch/arm64/kernel/debug-monitors.c
> > > +++ b/arch/arm64/kernel/debug-monitors.c
> > > @@ -246,6 +246,8 @@ static void send_user_sigtrap(int si_code)
> > >   static int single_step_handler(unsigned long addr, unsigned int esr,
> > >   			       struct pt_regs *regs)
> > >   {
> > > +	bool handler_found = false;
> > > +
> > >   	/*
> > >   	 * If we are stepping a pending breakpoint, call the hw_breakpoint
> > >   	 * handler first.
> > > @@ -253,7 +255,14 @@ static int single_step_handler(unsigned long addr, unsigned int esr,
> > >   	if (!reinstall_suspended_bps(regs))
> > >   		return 0;
> > > 
> > > -	if (user_mode(regs)) {
> > > +#ifdef	CONFIG_KPROBES
> > > +	if (kprobe_single_step_handler(regs, esr) == DBG_HOOK_HANDLED)
> > > +		handler_found = true;
> > > +#endif
> > > +	if (!handler_found && call_step_hook(regs, esr) == DBG_HOOK_HANDLED)
> > > +		handler_found = true;
> > > +
> > > +	if (!handler_found && user_mode(regs)) {
> > >   		send_user_sigtrap(TRAP_HWBKPT);
> > 
> > Could we register kprobe_single_step_handler() via register_set_hook()
> > and only invoke call_step_hook() above?
> > 
> 
> I seem to recall a criticism of doing that in a much earlier kprobes64 patch
> of mine.  The concern was that it would cause unnecessarily more kernel
> functions to be kprobes-blacklisted.  Hence the hardcoded check and call.

Yes, all the code regions are kprobe unsafe which lie within the moment we
receive a break/single step exception to the point where it is handled for
kprobe. Therefore we must call kprobe_single_step/breakpoint_handler() before
other handlers. Otherwise, we would not be able to trace other handlers and the
functions called from those handlers.

~Pratyush