Re: [PATCH v9 3/3] arm64: Relax Documentation/arm64/tagged-pointers.rst

Will Deacon
2019-08-21T17:33:53Z

On Wed, Aug 21, 2019 at 05:47:30PM +0100, Catalin Marinas wrote:
> From: Vincenzo Frascino <vincenzo.frascino@arm.com>
>
> On AArch64 the TCR_EL1.TBI0 bit is set by default, allowing userspace
> (EL0) to perform memory accesses through 64-bit pointers with a non-zero
> top byte. However, such pointers were not allowed at the user-kernel
> syscall ABI boundary.
>
> With the Tagged Address ABI patchset, it is now possible to pass tagged
> pointers to the syscalls. Relax the requirements described in
> tagged-pointers.rst to be compliant with the behaviours guaranteed by
> the AArch64 Tagged Address ABI.
>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>
> Cc: Kevin Brodsky <kevin.brodsky@arm.com>
> Acked-by: Andrey Konovalov <andreyknvl@google.com>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Co-developed-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> ---
> Documentation/arm64/tagged-pointers.rst | 23 ++++++++++++++++-------
> 1 file changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/arm64/tagged-pointers.rst b/Documentation/arm64/tagged-pointers.rst
> index 2acdec3ebbeb..04f2ba9b779e 100644
> --- a/Documentation/arm64/tagged-pointers.rst
> +++ b/Documentation/arm64/tagged-pointers.rst
> @@ -20,7 +20,9 @@ Passing tagged addresses to the kernel
> --------------------------------------
>
> All interpretation of userspace memory addresses by the kernel assumes
> -an address tag of 0x00.
> +an address tag of 0x00, unless the application enables the AArch64
> +Tagged Address ABI explicitly
> +(Documentation/arm64/tagged-address-abi.rst).
>
> This includes, but is not limited to, addresses found in:
>
> @@ -33,13 +35,15 @@ This includes, but is not limited to, addresses found in:
> - the frame pointer (x29) and frame records, e.g. when interpreting
> them to generate a backtrace or call graph.
>
> -Using non-zero address tags in any of these locations may result in an
> -error code being returned, a (fatal) signal being raised, or other modes
> -of failure.
> +Using non-zero address tags in any of these locations when the
> +userspace application did not enable the AArch64 Tagged Address ABI may
> +result in an error code being returned, a (fatal) signal being raised,
> +or other modes of failure.
>
> -For these reasons, passing non-zero address tags to the kernel via
> -system calls is forbidden, and using a non-zero address tag for sp is
> -strongly discouraged.
> +For these reasons, when the AArch64 Tagged Address ABI is disabled,
> +passing non-zero address tags to the kernel via system calls is
> +forbidden, and using a non-zero address tag for sp is strongly
> +discouraged.
>
> Programs maintaining a frame pointer and frame records that use non-zero
> address tags may suffer impaired or inaccurate debug and profiling
> @@ -59,6 +63,11 @@ be preserved.
> The architecture prevents the use of a tagged PC, so the upper byte will
> be set to a sign-extension of bit 55 on exception return.
>
> +This behaviour is maintained when the AArch64 Tagged Address ABI is
> +enabled. In addition, with the exceptions above, the kernel will
> +preserve any non-zero tags passed by the user via syscalls and stored in
> +kernel data structures (e.g. ``set_robust_list()``, ``sigaltstack()``).

Hmm. I can see the need to provide this guarantee for things like
set_robust_list(), but the problem is that the statement above is too broad
and isn't strictly true: for example, mmap() doesn't propagate the tag of
its address parameter into the VMA.

So I think we need to nail this down a bit more, but I'm having a really
hard time coming up with some wording :(

Will