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From:   Evgenii Stepanov <eugenis@google.com>
Date:   Mon, 17 Jun 2019 09:57:36 -0700
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Subject: Re: [PATCH v17 03/15] arm64: Introduce prctl() options to control the
 tagged user addresses ABI
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Andrey Konovalov <andreyknvl@google.com>,
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On Mon, Jun 17, 2019 at 6:56 AM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> On Wed, Jun 12, 2019 at 01:43:20PM +0200, Andrey Konovalov wrote:
> > From: Catalin Marinas <catalin.marinas@arm.com>
> >
> > 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@arm.com>
>
> 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.