Re: [PATCH v4 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt

Vincenzo Frascino
2019-06-13T10:15:34Z

Hi Catalin,

On 12/06/2019 16:35, Catalin Marinas wrote:
> Hi Vincenzo,
>
> Some minor comments below but it looks fine to me overall. Cc'ing
> Szabolcs as well since I'd like a view from the libc people.
>

Thanks for this, I saw Szabolcs comments.

> On Wed, Jun 12, 2019 at 03:21:10PM +0100, Vincenzo Frascino wrote:
>> diff --git a/Documentation/arm64/tagged-address-abi.txt b/Documentation/arm64/tagged-address-abi.txt
>> new file mode 100644
>> index 000000000000..96e149e2c55c
>> --- /dev/null
>> +++ b/Documentation/arm64/tagged-address-abi.txt
>> @@ -0,0 +1,111 @@
>> +ARM64 TAGGED ADDRESS ABI
>> +========================
>> +
>> +This document describes the usage and semantics of the Tagged Address
>> +ABI on arm64.
>> +
>> +1. Introduction
>> +---------------
>> +
>> +On arm64 the TCR_EL1.TBI0 bit has been always enabled on the arm64 kernel,
>> +hence the userspace (EL0) is allowed to set a non-zero value in the top
>
> I'd be clearer here: "userspace (EL0) is allowed to perform a user
> memory access through a 64-bit pointer with a non-zero top byte" (or
> something along the lines). Otherwise setting a non-zero top byte is
> allowed on any architecture, dereferencing it is a problem.
>

Ok.

>> +byte but the resulting pointers are not allowed at the user-kernel syscall
>> +ABI boundary.
>> +
>> +This document describes a relaxation of the ABI with which it is possible
>
> "relaxation of the ABI that makes it possible to..."
>
>> +to pass tagged tagged pointers to the syscalls, when these pointers are in
>> +memory ranges obtained as described in paragraph 2.
>
> "section 2" is better. There are a lot more paragraphs.
>

Agree.

>> +
>> +Since it is not desirable to relax the ABI to allow tagged user addresses
>> +into the kernel indiscriminately, arm64 provides a new sysctl interface
>> +(/proc/sys/abi/tagged_addr) that is used to prevent the applications from
>> +enabling the relaxed ABI and a new prctl() interface that can be used to
>> +enable or disable the relaxed ABI.
>> +
>> +The sysctl is meant also for testing purposes in order to provide a simple
>> +way for the userspace to verify the return error checking of the prctl()
>> +command without having to reconfigure the kernel.
>> +
>> +The ABI properties are inherited by threads of the same application and
>> +fork()'ed children but cleared when a new process is spawn (execve()).
>
> "spawned".
>
> I guess you could drop these three paragraphs here and mention the
> inheritance properties when introducing the prctl() below. You can also
> mention the global sysctl switch after the prctl() was introduced.
>

I will move the last two (rewording them) to the _section_ 2, but I would still
prefer the Introduction to give an overview of the solution as well.

>> +
>> +2. ARM64 Tagged Address ABI
>> +---------------------------
>> +
>> +From the kernel syscall interface prospective, we define, for the purposes
>> +of this document, a "valid tagged pointer" as a pointer that either it has
>
> "either has" (no 'it') sounds slightly better but I'm not a native
> English speaker either.
>
>> +a zero value set in the top byte or it has a non-zero value, it is in memory
>> +ranges privately owned by a userspace process and it is obtained in one of
>> +the following ways:
>> + - mmap() done by the process itself, where either:
>> + * flags = MAP_PRIVATE | MAP_ANONYMOUS
>> + * flags = MAP_PRIVATE and the file descriptor refers to a regular
>> + file or "/dev/zero"
>> + - a mapping below sbrk(0) done by the process itself
>> + - any memory mapped by the kernel in the process's address space during
>> + creation and following the restrictions presented above (i.e. data, bss,
>> + stack).
>> +
>> +The ARM64 Tagged Address ABI is an opt-in feature, and an application can
>> +control it using the following prctl()s:
>> + - PR_SET_TAGGED_ADDR_CTRL: can be used to enable the Tagged Address ABI.
>
> enable or disable (not sure we need the latter but it doesn't heart).
>
> I'd add the arg2 description here as well.
>

Good point I missed this.

>> + - PR_GET_TAGGED_ADDR_CTRL: can be used to check the status of the Tagged
>> + Address ABI.
>> +
>> +As a consequence of invoking PR_SET_TAGGED_ADDR_CTRL prctl() by an applications,
>> +the ABI guarantees the following behaviours:
>> +
>> + - Every current or newly introduced syscall can accept any valid tagged
>> + pointers.
>> +
>> + - If a non valid tagged pointer is passed to a syscall then the behaviour
>> + is undefined.
>> +
>> + - Every valid tagged pointer is expected to work as an untagged one.
>> +
>> + - The kernel preserves any valid tagged pointers and returns them to the
>> + userspace unchanged in all the cases except the ones documented in the
>> + "Preserving tags" paragraph of tagged-pointers.txt.
>
> I'd think we need to qualify the context here in which the kernel
> preserves the tagged pointers. Did you mean on the syscall return?
>

What this means is that on syscall return the tags are preserved, but if for
example you have tagged pointers inside siginfo_t, they will not because
according to tagged-pointers.txt non-zero tags are not preserved when delivering
signals.

>> +
>> +A definition of the meaning of tagged pointers on arm64 can be found in:
>> +Documentation/arm64/tagged-pointers.txt.
>> +
>> +3. ARM64 Tagged Address ABI Exceptions
>> +--------------------------------------
>> +
>> +The behaviours described in paragraph 2, with particular reference to the
>
> "section 2"
>
>> +acceptance by the syscalls of any valid tagged pointer are not applicable
>> +to the following cases:
>> + - mmap() addr parameter.
>> + - mremap() new_address parameter.
>> + - prctl_set_mm() struct prctl_map fields.
>> + - prctl_set_mm_map() struct prctl_map fields.
>> +
>> +4. Example of correct usage
>> +---------------------------
>> +
>> +void main(void)
>> +{
>> + static int tbi_enabled = 0;
>> + unsigned long tag = 0;
>> +
>> + char *ptr = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE,
>> + MAP_ANONYMOUS, -1, 0);
>> +
>> + if (prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE,
>> + 0, 0, 0) == 0)
>> + tbi_enabled = 1;
>> +
>> + if (!ptr)
>> + return -1;
>> +
>> + if (tbi_enabled)
>> + tag = rand() & 0xff;
>> +
>> + ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT));
>> +
>> + *ptr = 'a';
>> +
>> + ...
>> +}
>> +
>> --
>> 2.21.0
>

--
Regards,
Vincenzo