[PATCH] riscv: fix a nasty sigreturn bug...

[Palmer Dabbelt]

> Ping?  Does anybody have objections?  AFAICS, the bug is still
> there...

Sorry, something's gone off the rails with email and this thread doesn't 
show up in my inbox (not even any of the replies).  I tried to patch 
together this reply manually so hopefully it works.

This is on fixes, thanks -- trying to debug this one would have been a 
nightmare.

> On Fri, Sep 24, 2021 at 01:55:27AM +0000, Al Viro wrote:
>> riscv has an equivalent of arm bug fixed by 653d48b22166; if signal
>> gets caught by an interrupt that hits when we have the right value
>> in a0 (-513), *and* another signal gets delivered upon sigreturn()
>> (e.g. included into the blocked mask for the first signal and posted
>> while the handler had been running), the syscall restart logics will
>> see regs->cause equal to EXC_SYSCALL (we are in a syscall, after all)
>> and a0 already restored to its original value (-513, which happens to
>> be -ERESTARTNOINTR) and assume that we need to apply the usual
>> syscall restart logics.
>>     
>> Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
>> ---
>> diff --git a/arch/riscv/kernel/signal.c b/arch/riscv/kernel/signal.c
>> index c2d5ecbe55264..f8fb85dc94b7a 100644
>> --- a/arch/riscv/kernel/signal.c
>> +++ b/arch/riscv/kernel/signal.c
>> @@ -121,6 +121,8 @@ SYSCALL_DEFINE0(rt_sigreturn)
>>  	if (restore_altstack(&frame->uc.uc_stack))
>>  		goto badframe;
>>  
>> +	regs->cause = -1UL;
>> +
>>  	return regs->a0;
>>  
>>  badframe: