Re: [PATCH v5 19/30] arm64: add POE signal support

[Kevin Brodsky <kevin.brodsky@arm.com>]

On 15/10/2024 14:25, Joey Gouly wrote:
> On Tue, Oct 15, 2024 at 12:41:16PM +0100, Will Deacon wrote:
>> On Tue, Oct 15, 2024 at 10:59:11AM +0100, Joey Gouly wrote:
>>> On Mon, Oct 14, 2024 at 06:10:23PM +0100, Will Deacon wrote:
>>>> Kevin, Joey,
>>>>
>>>> On Wed, Oct 09, 2024 at 03:43:01PM +0100, Will Deacon wrote:
>>>>> On Tue, Sep 24, 2024 at 01:27:58PM +0200, Kevin Brodsky wrote:
>>>>>> On 22/08/2024 17:11, Joey Gouly wrote:
>>>>>>> @@ -1178,6 +1237,9 @@ static void setup_return(struct pt_regs *regs, struct k_sigaction *ka,
>>>>>>>  		sme_smstop();
>>>>>>>  	}
>>>>>>>  
>>>>>>> +	if (system_supports_poe())
>>>>>>> +		write_sysreg_s(POR_EL0_INIT, SYS_POR_EL0);
>>>>>> At the point where setup_return() is called, the signal frame has
>>>>>> already been written to the user stack. In other words, we write to the
>>>>>> user stack first, and then reset POR_EL0. This may be problematic,
>>>>>> especially if we are using the alternate signal stack, which the
>>>>>> interrupted POR_EL0 may not grant access to. In that situation uaccess
>>>>>> will fail and we'll end up with a SIGSEGV.
>>>>>>
>>>>>> This issue has already been discussed on the x86 side, and as it happens
>>>>>> patches to reset PKRU early [1] have just landed. I don't think this is
>>>>>> a blocker for getting this series landed, but we should try and align
>>>>>> with x86. If there's no objection, I'm planning to work on a counterpart
>>>>>> to the x86 series (resetting POR_EL0 early during signal delivery).
>>>>> Did you get a chance to work on that? It would be great to land the
>>>>> fixes for 6.12, if possible, so that the first kernel release with POE
>>>>> support doesn't land with known issues.
>>>> Looking a little more at this, I think we have quite a weird behaviour
>>>> on arm64 as it stands. It looks like we rely on the signal frame to hold
>>>> the original POR_EL0 so, if for some reason we fail to allocate space
>>>> for the POR context, I think we'll return back from the signal with
>>>> POR_EL0_INIT. That seems bad?
>>> If we don't allocate space for POR_EL0, I think the program recieves SIGSGEV?
>>>
>>> setup_sigframe_layout()
>>>         if (system_supports_poe()) {
>>>                 err = sigframe_alloc(user, &user->poe_offset,
>>>                                      sizeof(struct poe_context));
>>>                 if (err)
>>>                         return err;
>>>         }
>>>
>>> Through get_sigframe() and setup_rt_frame(), that eventually hets here:
>>>
>>> handle_signal()
>>> 	ret = setup_rt_frame(usig, ksig, oldset, regs);
>>>
>>> 	[..]
>>>
>>>         signal_setup_done(ret, ksig, test_thread_flag(TIF_SINGLESTEP));
>>>
>>> void signal_setup_done(int failed, struct ksignal *ksig, int stepping)                                                                                                                         
>>> {                                                                                                                                                                                              
>>>         if (failed)                                                                                                                                                                            
>>>                 force_sigsegv(ksig->sig);                                                                                                                                                      
>>>         else                                                                                                                                                                                   
>>>                 signal_delivered(ksig, stepping);                                                                                                                                              
>>> }  
>>>
>>> So I think it's "fine"?
>> Ah, yes, sorry about that. I got confused by the conditional push in
>> setup_sigframe():
>>
>> 	if (system_supports_poe() && err == 0 && user->poe_offset) {
>> 		...
>>
>> which gives the wrong impression that the POR is somehow optional, even
>> if the CPU supports POE. So we should drop that check of
>> 'user->poe_offset' as it cannot be NULL here.
>>
>> We also still need to resolve Kevin's concern, which probably means
>> keeping the thread's original POR around someplace.
> That was cargo culted (by me) from the rest of the function (apart from TPIDR2
> and FPMR). I think Kevin is planning on sending his signal changes still, but
> is on holiday, maybe he can remove the last part of the condition as part of
> his series.

Indeed just got back from holiday. I've got the series ready, about to
send it. I will add a clean-up patch removing this check on poe_offset.

Kevin