Re: [PATCH] arm64: dovetail: Fix undefinstr/break trap handling

[Jan Kiszka <jan.kiszka@siemens.com>]

On 28.08.23 14:52, Florian Bezdeka wrote:
> On Mon, 2023-08-28 at 14:33 +0200, Jan Kiszka wrote:
>> On 25.08.23 14:58, Florian Bezdeka wrote:
>>> When running an compat RT application on arm64 the break trap is
>>> handled via the undefined instruction trap.
>>>
>>> A possible call stack looks like this:
>>>
>>> Call trace:
>>>   handle_inband_event+0x2d0/0x320
>>>   inband_event_notify+0x28/0x50
>>>   signal_wake_up_state+0x7c/0xa4
>>>   complete_signal+0x104/0x2d0
>>>   __send_signal_locked+0x1d0/0x3e4
>>>   send_signal_locked+0xf0/0x140
>>>   force_sig_info_to_task+0xa0/0x164
>>>   force_sig_fault+0x64/0x94
>>>   arm64_force_sig_fault+0x48/0x80
>>>   send_user_sigtrap+0x50/0x8c
>>>   aarch32_break_handler+0xac/0x1d0
>>>   do_undefinstr+0x6c/0x360
>>>   el0_undef+0x4c/0xd0
>>>   el0t_32_sync_handler+0xd0/0x140
>>>   el0t_32_sync+0x190/0x194
>>>
>>> The trap is never reported to the companion core at that stage so
>>> running_oob() in do_undefinstr() will always return true. As the
>>> following bailout happens before calling the compat breakpoint
>>> detection (aarch32_break_handler()) debugging the compat 
>>> application does not work.
>>>
>>> In addition aarch32_break_handler() has to report the trap entry to the
>>> companion core.
>>>
>>> Reported-by: Clara Kowalsky <clara.kowalsky@siemens.com>
>>> Tested-by: Clara Kowalsky <clara.kowalsky@siemens.com>
>>> Signed-off-by: Florian Bezdeka <florian.bezdeka@siemens.com>
>>> ---
>>>  arch/arm64/kernel/debug-monitors.c | 3 +++
>>>  arch/arm64/kernel/traps.c          | 7 -------
>>>  2 files changed, 3 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
>>> index 32271ed24ef5..ef7ac042a0a6 100644
>>> --- a/arch/arm64/kernel/debug-monitors.c
>>> +++ b/arch/arm64/kernel/debug-monitors.c
>>> @@ -373,7 +373,10 @@ int aarch32_break_handler(struct pt_regs *regs)
>>>  	if (!bp)
>>>  		return -EFAULT;
>>>  
>>> +	mark_trap_entry(ARM64_TRAP_UNDI, regs);
>>>  	send_user_sigtrap(TRAP_BRKPT);
>>> +	mark_trap_exit(ARM64_TRAP_UNDI, regs);
>>> +
>>>  	return 0;
>>>  }
>>>  NOKPROBE_SYMBOL(aarch32_break_handler);
>>> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
>>> index cc68be400244..9bf2f309248f 100644
>>> --- a/arch/arm64/kernel/traps.c
>>> +++ b/arch/arm64/kernel/traps.c
>>> @@ -489,13 +489,6 @@ void arm64_notify_segfault(unsigned long addr)
>>>  
>>>  void do_undefinstr(struct pt_regs *regs, unsigned long esr)
>>>  {
>>> -	/*
>>> -	 * If the companion core did not switched us to in-band
>>> -	 * context, we may assume that it has handled the trap.
>>> -	 */
>>> -	if (running_oob())
>>> -		return;
>>> -
>>>  	/* check for AArch32 breakpoint instructions */
>>>  	if (!aarch32_break_handler(regs))
>>>  		return;
>>
>> This is not against v6.5-dovetail-rebase, right? We likely need to start
>> from the top, then backport to stable.
> 
> That applied for 6.1 and all other (lower) versions that are currently
> covered by our CI. Might need some lifting for 6.5, didn't check yet.
> 
>>
>> Also note that this change came in via
>> https://source.denx.de/Xenomai/linux-dovetail/-/commit/2b2ccdaeb8116727cf4076960d664a3cedff0ac6,
>> so just dropping it will likely cause problems elsewhere. Should we
>> rather move that down in the handler?
> 
> Well, as written in the commit message running_oob() will always return
> true for RT tasks so I'm quite sure that the invalid instruction
> handling was broken on arm64 for some time now. We bailed out even
> before sending the notification to the companion core.
> 
> Moving it down might fix the compat case but native arm64 would stay
> broken. No?
> 

Something looks still fishy, either in the original patch that
introduced the condition (I still don't get that special case) or now
with this change trying to restore things. I agree that also the
original change needed the notification to be delivered.

Philippe, can you help clarifying the logic behind
do_undefinstr/do_el0_undef?

Jan

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