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

Dave Martin
2020-04-29T10:26:32Z

On Tue, Apr 28, 2020 at 02:43:01PM +0100, Catalin Marinas wrote:
> 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 <vincenzo.frascino@arm.com>
> > >
> > > 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.

Agreed, that precedent is good enough for me. I hadn't refreshed my
memory of how sparc was using these signals.

>
> > > 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).

Sounds fair.

Cheers
---Dave