Subject: [RFC 6/8] ARM64: Handle TRAP_HWBRKPT for user mode as well

From: [email-masked] (Will Deacon)
Date: Thu, 8 Jan 2015 17:01:52 +0000

On Fri, Jan 02, 2015 at 06:05:23PM +0000, Oleg Nesterov wrote:
> Let me repeat once again that I know absolutely nothing about arm* ;)
> 
> On 12/31, Pratyush Anand wrote:
> >
> > uprobe registers a handler at step_hook. So, single_step_handler now
> > checks for user mode as well if there is a valid hook.
> >
> > Signed-off-by: Pratyush Anand <[email-masked]>
> > ---
> >  arch/arm64/kernel/debug-monitors.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> > index b056369fd47d..2676b8655241 100644
> > --- a/arch/arm64/kernel/debug-monitors.c
> > +++ b/arch/arm64/kernel/debug-monitors.c
> > @@ -236,6 +236,9 @@ static int single_step_handler(unsigned long addr, unsigned int esr,
> >  	if (!reinstall_suspended_bps(regs))
> >  		return 0;
> >
> > +	if (call_step_hook(regs, esr) == DBG_HOOK_HANDLED)
> > +		return 0;
> > +
> >  	if (user_mode(regs)) {
> >  		info.si_signo = SIGTRAP;
> >  		info.si_errno = 0;
> > @@ -251,9 +254,6 @@ static int single_step_handler(unsigned long addr, unsigned int esr,
> >  		 */
> >  		user_rewind_single_step(current);
> >  	} else {
> > -		if (call_step_hook(regs, esr) == DBG_HOOK_HANDLED)
> > -			return 0;
> > -
> 
> Agreed, we need something like this change...
> 
> But did you verify that it can't break other users of register_step_hook() ?
> The current handlers do not check user_mode() == F, they assume that they
> can't be called otherwise.
> 
> If this all is correct, please explain why in the changelog.

I think you're right, and kgdb will need fixing with this change.

Will