Re: [PATCH v17 03/15] arm64: Introduce prctl() options to control the tagged user addresses ABI

[Catalin Marinas]

On Mon, Jun 17, 2019 at 09:57:36AM -0700, Evgenii Stepanov wrote:
> On Mon, Jun 17, 2019 at 6:56 AM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> > On Wed, Jun 12, 2019 at 01:43:20PM +0200, Andrey Konovalov wrote:
> > > From: Catalin Marinas <catalin.marinas@xxxxxxx>
> > >
> > > It is not desirable to relax the ABI to allow tagged user addresses into
> > > the kernel indiscriminately. This patch introduces a prctl() interface
> > > for enabling or disabling the tagged ABI with a global sysctl control
> > > for preventing applications from enabling the relaxed ABI (meant for
> > > testing user-space prctl() return error checking without reconfiguring
> > > the kernel). The ABI properties are inherited by threads of the same
> > > application and fork()'ed children but cleared on execve().
> > >
> > > The PR_SET_TAGGED_ADDR_CTRL will be expanded in the future to handle
> > > MTE-specific settings like imprecise vs precise exceptions.
> > >
> > > Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> >
> > A question for the user-space folk: if an application opts in to this
> > ABI, would you want the sigcontext.fault_address and/or siginfo.si_addr
> > to contain the tag? We currently clear it early in the arm64 entry.S but
> > we could find a way to pass it down if needed.
> 
> For HWASan this would not be useful because we instrument memory
> accesses with explicit checks anyway. For MTE, on the other hand, it
> would be very convenient to know the fault address tag without
> disassembling the code.

I could as this differently: does anything break if, once the user
opts in to TBI, fault_address and/or si_addr have non-zero top byte?

Alternatively, we could present the original FAR_EL1 register as a
separate field as we do with ESR_EL1, independently of whether the user
opted in to TBI or not.

-- 
Catalin