Re: [PATCH v9 2/3] arm64: Define Documentation/arm64/tagged-address-abi.rst

Kevin Brodsky
2019-08-22T09:38:37Z

On 21/08/2019 17:57, Andrey Konovalov wrote:
> On Wed, Aug 21, 2019 at 6:47 PM Catalin Marinas <catalin.marinas@arm.com> 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. Introduce the document describing the relaxation of the
>> syscall ABI that allows userspace to pass certain tagged pointers to
>> kernel syscalls.
>>
>> Cc: Will Deacon <will.deacon@arm.com>
>> Cc: Andrey Konovalov <andreyknvl@google.com>
>> Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>
>> Cc: Kevin Brodsky <kevin.brodsky@arm.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>
> Acked-by: Andrey Konovalov <andreyknvl@google.com>

Acked-by: Kevin Brodsky <kevin.brodsky@arm.com>

>> ---
>> Documentation/arm64/tagged-address-abi.rst | 156 +++++++++++++++++++++
>> 1 file changed, 156 insertions(+)
>> create mode 100644 Documentation/arm64/tagged-address-abi.rst
>>
>> diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst
>> new file mode 100644
>> index 000000000000..d4a85d535bf9
>> --- /dev/null
>> +++ b/Documentation/arm64/tagged-address-abi.rst
>> @@ -0,0 +1,156 @@
>> +==========================
>> +AArch64 TAGGED ADDRESS ABI
>> +==========================
>> +
>> +Authors: Vincenzo Frascino <vincenzo.frascino@arm.com>
>> + Catalin Marinas <catalin.marinas@arm.com>
>> +
>> +Date: 21 August 2019
>> +
>> +This document describes the usage and semantics of the Tagged Address
>> +ABI on AArch64 Linux.
>> +
>> +1. Introduction
>> +---------------
>> +
>> +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. This document describes the relaxation of the
>> +syscall ABI that allows userspace to pass certain tagged pointers to
>> +kernel syscalls.
>> +
>> +2. AArch64 Tagged Address ABI
>> +-----------------------------
>> +
>> +From the kernel syscall interface perspective and for the purposes of
>> +this document, a "valid tagged pointer" is a pointer with a potentially
>> +non-zero top-byte that references an address in the user process address
>> +space obtained in one of the following ways:
>> +
>> +- ``mmap()`` syscall where either:
>> +
>> + - flags have the ``MAP_ANONYMOUS`` bit set or
>> + - the file descriptor refers to a regular file (including those
>> + returned by ``memfd_create()``) or ``/dev/zero``
>> +
>> +- ``brk()`` syscall (i.e. the heap area between the initial location of
>> + the program break at process creation and its current location).
>> +
>> +- any memory mapped by the kernel in the address space of the process
>> + during creation and with the same restrictions as for ``mmap()`` above
>> + (e.g. data, bss, stack).
>> +
>> +The AArch64 Tagged Address ABI has two stages of relaxation depending
>> +how the user addresses are used by the kernel:
>> +
>> +1. User addresses not accessed by the kernel but used for address space
>> + management (e.g. ``mmap()``, ``mprotect()``, ``madvise()``). The use
>> + of valid tagged pointers in this context is always allowed.
>> +
>> +2. User addresses accessed by the kernel (e.g. ``write()``). This ABI
>> + relaxation is disabled by default and the application thread needs to
>> + explicitly enable it via ``prctl()`` as follows:
>> +
>> + - ``PR_SET_TAGGED_ADDR_CTRL``: enable or disable the AArch64 Tagged
>> + Address ABI for the calling thread.
>> +
>> + The ``(unsigned int) arg2`` argument is a bit mask describing the
>> + control mode used:
>> +
>> + - ``PR_TAGGED_ADDR_ENABLE``: enable AArch64 Tagged Address ABI.
>> + Default status is disabled.
>> +
>> + Arguments ``arg3``, ``arg4``, and ``arg5`` must be 0.
>> +
>> + - ``PR_GET_TAGGED_ADDR_CTRL``: get the status of the AArch64 Tagged
>> + Address ABI for the calling thread.
>> +
>> + Arguments ``arg2``, ``arg3``, ``arg4``, and ``arg5`` must be 0.
>> +
>> + The ABI properties described above are thread-scoped, inherited on
>> + clone() and fork() and cleared on exec().
>> +
>> + Calling ``prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0)``
>> + returns ``-EINVAL`` if the AArch64 Tagged Address ABI is globally
>> + disabled by ``sysctl abi.tagged_addr_disabled=1``. The default
>> + ``sysctl abi.tagged_addr_disabled`` configuration is 0.
>> +
>> +When the AArch64 Tagged Address ABI is enabled for a thread, the
>> +following behaviours are guaranteed:
>> +
>> +- All syscalls except the cases mentioned in section 3 can accept any
>> + valid tagged pointer.
>> +
>> +- The syscall behaviour is undefined for invalid tagged pointers: it may
>> + result in an error code being returned, a (fatal) signal being raised,
>> + or other modes of failure.
>> +
>> +- The syscall behaviour for a valid tagged pointer is the same as for
>> + the corresponding untagged pointer.
>> +
>> +
>> +A definition of the meaning of tagged pointers on AArch64 can be found
>> +in Documentation/arm64/tagged-pointers.rst.
>> +
>> +3. AArch64 Tagged Address ABI Exceptions
>> +-----------------------------------------
>> +
>> +The following system call parameters must be untagged regardless of the
>> +ABI relaxation:
>> +
>> +- ``prctl()`` other than pointers to user data either passed directly or
>> + indirectly as arguments to be accessed by the kernel.
>> +
>> +- ``ioctl()`` other than pointers to user data either passed directly or
>> + indirectly as arguments to be accessed by the kernel.
>> +
>> +- ``shmat()`` and ``shmdt()``.
>> +
>> +Any attempt to use non-zero tagged pointers may result in an error code
>> +being returned, a (fatal) signal being raised, or other modes of
>> +failure.
>> +
>> +4. Example of correct usage
>> +---------------------------
>> +.. code-block:: c
>> +
>> + #include <stdlib.h>
>> + #include <string.h>
>> + #include <unistd.h>
>> + #include <sys/mman.h>
>> + #include <sys/prctl.h>
>> +
>> + #define PR_SET_TAGGED_ADDR_CTRL 55
>> + #define PR_TAGGED_ADDR_ENABLE (1UL << 0)
>> +
>> + #define TAG_SHIFT 56
>> +
>> + int main(void)
>> + {
>> + int tbi_enabled = 0;
>> + unsigned long tag = 0;
>> + char *ptr;
>> +
>> + /* check/enable the tagged address ABI */
>> + if (!prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0))
>> + tbi_enabled = 1;
>> +
>> + /* memory allocation */
>> + ptr = mmap(NULL, sysconf(_SC_PAGE_SIZE), PROT_READ | PROT_WRITE,
>> + MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>> + if (ptr == MAP_FAILED)
>> + return 1;
>> +
>> + /* set a non-zero tag if the ABI is available */
>> + if (tbi_enabled)
>> + tag = rand() & 0xff;
>> + ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT));
>> +
>> + /* memory access to a tagged address */
>> + strcpy(ptr, "tagged pointer\n");
>> +
>> + /* syscall with a tagged pointer */
>> + write(1, ptr, strlen(ptr));
>> +
>> + return 0;
>> + }