Subject: Re: [PATCH v3 10/23] arm64: mte: Handle synchronous and asynchronous tag check faults

From: Catalin Marinas <[email-masked]>
Date: Tue, 28 Apr 2020 14:43:01 +0100

On Mon, Apr 27, 2020 at 05:58:22PM +0100, Dave P Martin wrote:
> On Tue, Apr 21, 2020 at 03:25:50PM +0100, Catalin Marinas wrote:
> > From: Vincenzo Frascino <[email-masked]>
> > 
> > The Memory Tagging Extension has two modes of notifying a tag check
> > fault at EL0, configurable through the SCTLR_EL1.TCF0 field:
> > 
> > 1. Synchronous raising of a Data Abort exception with DFSC 17.
> > 2. Asynchronous setting of a cumulative bit in TFSRE0_EL1.
> > 
> > Add the exception handler for the synchronous exception and handling of
> > the asynchronous TFSRE0_EL1.TF0 bit setting via a new TIF flag in
> > do_notify_resume().
> > 
> > On a tag check failure in user-space, whether synchronous or
> > asynchronous, a SIGSEGV will be raised on the faulting thread.
> 
> Has there been any discussion on whether this should be SIGSEGV or
> SIGBUS?
> 
> Probably neither is much more appropriate than the other.

You could argue either way. I don't recall a firm conclusion on this, so
I picked one that follows SPARC ADI.

> > diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> > index 339882db5a91..e377d77c065e 100644
> > --- a/arch/arm64/kernel/signal.c
> > +++ b/arch/arm64/kernel/signal.c
> > @@ -732,6 +732,9 @@ static void setup_return(struct pt_regs *regs, struct k_sigaction *ka,
> >  	regs->regs[29] = (unsigned long)&user->next_frame->fp;
> >  	regs->pc = (unsigned long)ka->sa.sa_handler;
> >  
> > +	/* TCO (Tag Check Override) always cleared for signal handlers */
> > +	regs->pstate &= ~PSR_TCO_BIT;
> > +
> >  	if (ka->sa.sa_flags & SA_RESTORER)
> >  		sigtramp = ka->sa.sa_restorer;
> >  	else
> > @@ -923,6 +926,11 @@ asmlinkage void do_notify_resume(struct pt_regs *regs,
> >  			if (thread_flags & _TIF_UPROBE)
> >  				uprobe_notify_resume(regs);
> >  
> > +			if (thread_flags & _TIF_MTE_ASYNC_FAULT) {
> > +				clear_thread_flag(TIF_MTE_ASYNC_FAULT);
> > +				force_signal_inject(SIGSEGV, SEGV_MTEAERR, 0);
> > +			}
> > +
> 
> Should this definitely be a force_signal_inject()?
> 
> SEGV_MTEAERR is not intrinsically fatal: it must be possible to run past
> the error, because that's the whole point -- chances are we already did.
> 
> Compare this with MTESERR where running past the signal would lead to a
> spin.

Good point. This can be a send_sig_fault() (I need to check the right
API).

Thanks.

-- 
Catalin

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