Subject: Re: [PATCH 1/7] powerpc/entry: convert to common and generic entry

From: "虞陆铭" <[email-masked]>
Date: Tue, 22 Oct 2024 12:50:47 +0800

>Le 12/10/2024 à 05:56, Luming Yu a écrit :
>> convert powerpc entry code in syscall and fault to use syscall_work
>> and irqentry_state as well as common calls implemented in generic
>> entry infrastructure.
>> 
>> Signed-off-by: Luming Yu <[email-masked]>
>> ---
>>   arch/powerpc/Kconfig                   | 1 +
>>   arch/powerpc/include/asm/hw_irq.h      | 5 +++++
>>   arch/powerpc/include/asm/processor.h   | 6 ++++++
>>   arch/powerpc/include/asm/syscall.h     | 5 +++++
>>   arch/powerpc/include/asm/thread_info.h | 1 +
>>   arch/powerpc/kernel/syscall.c          | 5 ++++-
>>   arch/powerpc/mm/fault.c                | 3 +++
>>   7 files changed, 25 insertions(+), 1 deletion(-)
>> 
>
>...
>
>> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
>> index 77fedb190c93..e0338bd8d383 100644
>> --- a/arch/powerpc/kernel/syscall.c
>> +++ b/arch/powerpc/kernel/syscall.c
>> @@ -3,6 +3,7 @@
>>   #include <linux/compat.h>
>>   #include <linux/context_tracking.h>
>>   #include <linux/randomize_kstack.h>
>> +#include <linux/entry-common.h>
>>   
>>   #include <asm/interrupt.h>
>>   #include <asm/kup.h>
>> @@ -131,7 +132,7 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
>>   		 * and the test against NR_syscalls will fail and the return
>>   		 * value to be used is in regs->gpr[3].
>>   		 */
>> -		r0 = do_syscall_trace_enter(regs);
>> +		r0 = syscall_enter_from_user_mode(regs, r0);
>
>Can you provide details on how this works ?
I assume the common entry would take over th details.
So I just made the switch from the high level call.

As you said as the subtle ABI requirement about regs->r3 needs to
be restored, I'm wondering which test can capture the lost
ABI feature. As simple Boot test is insufficient, what is the test set
that can capture it?

I will look into the details late this week.

>
>For instance, do_syscall_trace_enter() was calling do_seccomp().
>do_seccomp() sets regs->r3 to -ENOSYS then calls __secure_computing().
>
>Now you call syscall_enter_from_user_mode() instead which calls 
>syscall_enter_from_user_mode_work() which calls syscall_trace_enter() 
>which calls  __secure_computing() but without seting regs->r3 to -ENOSYS.
>
>So what will be returned by the below return when 
>syscall_enter_from_user_mode returns -1 ?
>
>>   		if (unlikely(r0 >= NR_syscalls))
>>   			return regs->gpr[3];
>>   
>> @@ -184,6 +185,8 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
>>   	 * So the resulting 6 or 7 bits of entropy is seen in SP[9:4] or SP[9:3].
>>   	 */
>>   	choose_random_kstack_offset(mftb());
>> +	/*common entry*/
>> +	syscall_exit_to_user_mode(regs);
>>   
>>   	return ret;
>>   }