[PATCH v19] arm64: expose FAR_EL1 tag bits in siginfo

[Eric W. Biederman]

Peter Collingbourne <pcc at google.com> writes:

> diff --git a/kernel/signal.c b/kernel/signal.c
> index 8f34819e80de..678cdeb235ae 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2524,6 +2524,26 @@ static int ptrace_signal(int signr, kernel_siginfo_t *info)
>  	return signr;
>  }
>  
> +static void hide_si_addr_tag_bits(struct ksignal *ksig)
> +{
> +	switch (siginfo_layout(ksig->sig, ksig->info.si_code)) {
> +	case SIL_FAULT:
> +	case SIL_FAULT_MCEERR:
> +	case SIL_FAULT_BNDERR:
> +	case SIL_FAULT_PKUERR:
> +		ksig->info.si_addr = arch_untagged_si_addr(
> +			ksig->info.si_addr, ksig->sig, ksig->info.si_code);
> +		break;
> +	case SIL_KILL:
> +	case SIL_TIMER:
> +	case SIL_POLL:
> +	case SIL_CHLD:
> +	case SIL_RT:
> +	case SIL_SYS:
> +		break;
> +	}
> +}
> +
>  bool get_signal(struct ksignal *ksig)
>  {
>  	struct sighand_struct *sighand = current->sighand;
> @@ -2761,6 +2781,10 @@ bool get_signal(struct ksignal *ksig)
>  	spin_unlock_irq(&sighand->siglock);
>  
>  	ksig->sig = signr;
> +
> +	if (!(sighand->action[signr - 1].sa.sa_flags & SA_EXPOSE_TAGBITS))
> +		hide_si_addr_tag_bits(ksig);
> +
>  	return ksig->sig > 0;
>  }

Ok.  Seeing that this code compiles out I don't have any concerns about
it's impact on other architectures.  And I like having it always
present as that makes all of the concerns the code has to deal with
easier to discover.

There is one small issue.  The test should be:
	if (!(ksig->ka.sa.sa_flags & SA_EXPOSE_TAGBITS))
		hide_si_addr_tag_bits(ksig);

Outside of sighand->siglock it is racy to access sighand->action and we
already have an atomic snapshot of the values for exactly this reason.

Is there some reason you didn't use ksig->ka?  Am I missing something?

I agree that our consumption of SA bits is slow enough that my other
concerns are a non-issue.

Eric