[PATCH 3/6] arm64: Add support for hooks to handle undefined instructions

[Punit Agrawal]

Will Deacon <will.deacon at arm.com> writes:

> Hi Punit,
>
> On Tue, Aug 26, 2014 at 11:28:47AM +0100, Punit Agrawal wrote:
>> Add support to register hooks for undefined instructions. The handlers
>> will be called when the undefined instruction and the processor state
>> (as contained in pstate) match criteria used at registration.
>> 
>> Note: The patch only deals with ARM instruction encodings and needs
>> fixing to handle thumb instructions as well.
>
> [...]
>
>> +static int call_undef_hook(struct pt_regs *regs)
>> +{
>> +	struct undef_hook *hook;
>> +	unsigned long flags;
>> +	u32 instr;
>> +	int (*fn)(struct pt_regs *regs, u32 instr) = NULL;
>> +	void __user *pc = (void __user *)instruction_pointer(regs);
>> +
>> +	/*
>> +	 * Currently, undefined instruction patching is only supported
>> +	 * for user mode. Also, as we're not emulating any thumb
>> +	 * instructions lets not add thumb instruction decoding until
>> +	 * it is needed.
>> +	 */
>> +	if (!compat_user_mode(regs) || compat_thumb_mode(regs))
>> +		return 1;
>
> What do you mean by `undefined instruction patching'? 

That's poorly worded. I'll improve upon it.

> I don't see anything
> in the mechanism that means this can't be reused for kernel code, then we
> just register the SWP emulation hook for userspace only using the mode (like
> we do for kgdb).

There's nothing in the mechanism to prevent it's use for kernel code. I
was erring on the side of caution as I hadn't tested it.

>
>> +	get_user(instr, (u32 __user *)pc);
>> +	instr = le32_to_cpu(instr);
>> +
>> +	raw_spin_lock_irqsave(&undef_lock, flags);
>> +	list_for_each_entry(hook, &undef_hook, node)
>> +		if ((instr & hook->instr_mask) == hook->instr_val &&
>> +			(regs->pstate & hook->pstate_mask) == hook->pstate_val)
>> +			fn = hook->fn;
>> +
>> +	raw_spin_unlock_irqrestore(&undef_lock, flags);
>> +
>> +	return fn ? fn(regs, instr) : 1;
>> +}
>> +
>>  asmlinkage void __exception do_undefinstr(struct pt_regs *regs)
>>  {
>>  	siginfo_t info;
>> @@ -266,6 +329,9 @@ asmlinkage void __exception do_undefinstr(struct pt_regs *regs)
>>  	if (!aarch32_break_handler(regs))
>>  		return;
>>  
>> +	if (call_undef_hook(regs) == 0)
>> +		return;
>
> I'd like to reuse this hook for the aarch32 break hooks (you can see the
> direct call in the context above). That means adding support for thumb
> after all. Is there a reason you've been avoiding that?

None, other than to not add code before it's needed. I've just had a
quick look at break handler and it looks pretty straight forward to fold
that change in this set. Are you OK with that?

The code started out when it wasn't clear if the code would target
upstream kernels. I tried to not enable too many, what was by some
considered, hacks.

>
> Will
>
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