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Commit bc01bdf6 authored by Ravi Bangoria's avatar Ravi Bangoria Committed by Michael Ellerman
Browse files

powerpc/watchpoint: Disable watchpoint hit by larx/stcx instructions 



If watchpoint exception is generated by larx/stcx instructions, the
reservation created by larx gets lost while handling exception, and
thus stcx instruction always fails. Generally these instructions are
used in a while(1) loop, for example spinlocks. And because stcx
never succeeds, it loops forever and ultimately hangs the system.

Note that ptrace anyway works in one-shot mode and thus for ptrace
we don't change the behaviour. It's up to ptrace user to take care
of this.

Signed-off-by: default avatarRavi Bangoria <ravi.bangoria@linux.ibm.com>
Acked-by: default avatarNaveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190910131513.30499-1-ravi.bangoria@linux.ibm.com
parent 587164cd

arch/powerpc/kernel/hw_breakpoint.c

+33 −16
Original line number Diff line number Diff line
@@ -195,14 +195,32 @@ void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs) 
	tsk->thread.last_hit_ubp = NULL;

}



static bool is_larx_stcx_instr(struct pt_regs *regs, unsigned int instr)

{

	int ret, type;

	struct instruction_op op;



	ret = analyse_instr(&op, regs, instr);

	type = GETTYPE(op.type);

	return (!ret && (type == LARX || type == STCX));

}



/*

 * Handle debug exception notifications.

 */

static bool stepping_handler(struct pt_regs *regs, struct perf_event *bp,

			     unsigned long addr)

{

	int stepped;

	unsigned int instr;

	unsigned int instr = 0;



	if (__get_user_inatomic(instr, (unsigned int *)regs->nip))

		goto fail;



	if (is_larx_stcx_instr(regs, instr)) {

		printk_ratelimited("Breakpoint hit on instruction that can't be emulated."

				   " Breakpoint at 0x%lx will be disabled.\n", addr);

		goto disable;

	}



	/* Do not emulate user-space instructions, instead single-step them */

	if (user_mode(regs)) {
@@ -211,24 +229,23 @@ static bool stepping_handler(struct pt_regs *regs, struct perf_event *bp, 
		return false;

	}



	stepped = 0;

	instr = 0;

	if (!__get_user_inatomic(instr, (unsigned int *)regs->nip))

		stepped = emulate_step(regs, instr);

	if (!emulate_step(regs, instr))

		goto fail;



	return true;



fail:

	/*

	 * emulate_step() could not execute it. We've failed in reliably

	 * handling the hw-breakpoint. Unregister it and throw a warning

	 * message to let the user know about it.

	 * We've failed in reliably handling the hw-breakpoint. Unregister

	 * it and throw a warning message to let the user know about it.

	 */

	if (!stepped) {

	WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "

		"0x%lx will be disabled.", addr);



disable:

	perf_event_disable_inatomic(bp);

	return false;

}

	return true;

}



int hw_breakpoint_handler(struct die_args *args)

{