Subject: Re: [PATCH v3 23/23] arm64: mte: Add Memory Tagging Extension documentation

From: Szabolcs Nagy <[email-masked]>
Date: Tue, 5 May 2020 11:32:33 +0100

The 04/21/2020 15:26, Catalin Marinas wrote:
> diff --git a/Documentation/arm64/memory-tagging-extension.rst b/Documentation/arm64/memory-tagging-extension.rst
> new file mode 100644
> index 000000000000..f82dfbd70061
> --- /dev/null
> +++ b/Documentation/arm64/memory-tagging-extension.rst
> @@ -0,0 +1,260 @@
> +===============================================
> +Memory Tagging Extension (MTE) in AArch64 Linux
> +===============================================
> +
> +Authors: Vincenzo Frascino <[email-masked]>
> +         Catalin Marinas <[email-masked]>
> +
> +Date: 2020-02-25
> +
> +This document describes the provision of the Memory Tagging Extension
> +functionality in AArch64 Linux.
> +
> +Introduction
> +============
> +
> +ARMv8.5 based processors introduce the Memory Tagging Extension (MTE)
> +feature. MTE is built on top of the ARMv8.0 virtual address tagging TBI
> +(Top Byte Ignore) feature and allows software to access a 4-bit
> +allocation tag for each 16-byte granule in the physical address space.
> +Such memory range must be mapped with the Normal-Tagged memory
> +attribute. A logical tag is derived from bits 59-56 of the virtual
> +address used for the memory access. A CPU with MTE enabled will compare
> +the logical tag against the allocation tag and potentially raise an
> +exception on mismatch, subject to system registers configuration.
> +
> +Userspace Support
> +=================
> +
> +When ``CONFIG_ARM64_MTE`` is selected and Memory Tagging Extension is
> +supported by the hardware, the kernel advertises the feature to
> +userspace via ``HWCAP2_MTE``.
> +
> +PROT_MTE
> +--------
> +
> +To access the allocation tags, a user process must enable the Tagged
> +memory attribute on an address range using a new ``prot`` flag for
> +``mmap()`` and ``mprotect()``:
> +
> +``PROT_MTE`` - Pages allow access to the MTE allocation tags.
> +
> +The allocation tag is set to 0 when such pages are first mapped in the
> +user address space and preserved on copy-on-write. ``MAP_SHARED`` is
> +supported and the allocation tags can be shared between processes.
> +
> +**Note**: ``PROT_MTE`` is only supported on ``MAP_ANONYMOUS`` and
> +RAM-based file mappings (``tmpfs``, ``memfd``). Passing it to other
> +types of mapping will result in ``-EINVAL`` returned by these system
> +calls.
> +
> +**Note**: The ``PROT_MTE`` flag (and corresponding memory type) cannot
> +be cleared by ``mprotect()``.

i think there are some non-obvious madvise operations that may
be worth documenting too for mte specific semantics.

e.g. MADV_DONTNEED or MADV_FREE can presumably drop tags which
means that existing pointers can no longer write to the memory
which is a change of behaviour compared to the non-mte case.
(affects most malloc implementations that will have to deal
with this when implementing heap coloring) there might be other
similar problems like MADV_WIPEONFORK that wont work as
currently expected when mte is enabled.

if such behaviour changes cause serious problems to existing
software there may be a need to have a way to opt out from
these changes (e.g. MADV_ flag variant that only affects the
memory content but not the tags) or to make that the default
behaviour. (but i can't tell how widely these are used in
ways that can be expected to work with PROT_MTE)

> +Tag Check Faults
> +----------------
> +
> +When ``PROT_MTE`` is enabled on an address range and a mismatch between
> +the logical and allocation tags occurs on access, there are three
> +configurable behaviours:
> +
> +- *Ignore* - This is the default mode. The CPU (and kernel) ignores the
> +  tag check fault.
> +
> +- *Synchronous* - The kernel raises a ``SIGSEGV`` synchronously, with
> +  ``.si_code = SEGV_MTESERR`` and ``.si_addr = <fault-address>``. The
> +  memory access is not performed.
> +
> +- *Asynchronous* - The kernel raises a ``SIGSEGV``, in the current
> +  thread, asynchronously following one or multiple tag check faults,
> +  with ``.si_code = SEGV_MTEAERR`` and ``.si_addr = 0``.
> +
> +**Note**: There are no *match-all* logical tags available for user
> +applications.
> +
> +The user can select the above modes, per thread, using the
> +``prctl(PR_SET_TAGGED_ADDR_CTRL, flags, 0, 0, 0)`` system call where
> +``flags`` contain one of the following values in the ``PR_MTE_TCF_MASK``
> +bit-field:
> +
> +- ``PR_MTE_TCF_NONE``  - *Ignore* tag check faults
> +- ``PR_MTE_TCF_SYNC``  - *Synchronous* tag check fault mode
> +- ``PR_MTE_TCF_ASYNC`` - *Asynchronous* tag check fault mode
> +
> +Tag checking can also be disabled for a user thread by setting the
> +``PSTATE.TCO`` bit with ``MSR TCO, #1``.
> +
> +**Note**: Signal handlers are always invoked with ``PSTATE.TCO = 0``,
> +irrespective of the interrupted context.
> +
> +**Note**: Kernel accesses to user memory (e.g. ``read()`` system call)
> +are only checked if the current thread tag checking mode is
> +PR_MTE_TCF_SYNC.

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