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

[Al Viro]

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

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: