Re: [PATCH 03/14] arm: Add restartable sequences support

[Mathieu Desnoyers]

----- On May 16, 2018, at 12:18 PM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:

> On Mon, Apr 30, 2018 at 06:44:22PM -0400, Mathieu Desnoyers wrote:
>> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
>> index a7f8e7f4b88f..4f5c386631d4 100644
>> --- a/arch/arm/Kconfig
>> +++ b/arch/arm/Kconfig
>> @@ -91,6 +91,7 @@ config ARM
>>  	select HAVE_PERF_USER_STACK_DUMP
>>  	select HAVE_RCU_TABLE_FREE if (SMP && ARM_LPAE)
>>  	select HAVE_REGS_AND_STACK_ACCESS_API
>> +	select HAVE_RSEQ
>>  	select HAVE_SYSCALL_TRACEPOINTS
>>  	select HAVE_UID16
>>  	select HAVE_VIRT_CPU_ACCOUNTING_GEN
>> diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
>> index bd8810d4acb3..5879ab3f53c1 100644
>> --- a/arch/arm/kernel/signal.c
>> +++ b/arch/arm/kernel/signal.c
>> @@ -541,6 +541,12 @@ static void handle_signal(struct ksignal *ksig, struct
>> pt_regs *regs)
>>  	int ret;
>>  
>>  	/*
>> +	 * Increment event counter and perform fixup for the pre-signal
>> +	 * frame.
>> +	 */
>> +	rseq_signal_deliver(regs);
>> +
>> +	/*
>>  	 * Set up the stack frame
>>  	 */
>>  	if (ksig->ka.sa.sa_flags & SA_SIGINFO)
>> @@ -660,6 +666,7 @@ do_work_pending(struct pt_regs *regs, unsigned int
>> thread_flags, int syscall)
>>  			} else {
>>  				clear_thread_flag(TIF_NOTIFY_RESUME);
>>  				tracehook_notify_resume(regs);
>> +				rseq_handle_notify_resume(regs);
>>  			}
>>  		}
>>  		local_irq_disable();
> 
> I think you forgot to hook up rseq_syscall() checking.

Considering that rseq_syscall is implemented as follows:

+void rseq_syscall(struct pt_regs *regs)
+{
+       unsigned long ip = instruction_pointer(regs);
+       struct task_struct *t = current;
+       struct rseq_cs rseq_cs;
+
+       if (!t->rseq)
+               return;
+       if (!access_ok(VERIFY_READ, t->rseq, sizeof(*t->rseq)) ||
+           rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
+               force_sig(SIGSEGV, t);
+}

and that x86 calls it from syscall_return_slowpath() (which AFAIU is
now used in the fast-path since KPTI), I wonder where we should call
this on ARM ? I was under the impression that ARM return to userspace
fast-path was not calling C code unless work flags were set, but I might
be wrong.

Thoughts ?

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com