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

On 26 August 2014 15:13, Will Deacon [off-list ref] wrote:

Hi Punit,

On Tue, Aug 26, 2014 at 11:28:47AM +0100, Punit Agrawal wrote:

quoted

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.

[...]

quoted

+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'? 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).

You need this patch in order to be able to return from an undef
exception taken in EL1:

--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S

@@ -287,7 +287,9 @@ el1_undef:
         */
        enable_dbg
        mov     x0, sp
-       b       do_undefinstr
+       bl      do_undefinstr
+
+       kernel_exit 1
 el1_dbg:
        /*
         * Debug exception handling

-- 
Ard.


>> +     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?
>
> Will
>
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