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

From: Vincenzo Frascino <vincenzo.frascino@arm.com>
To: Szabolcs Nagy <Szabolcs.Nagy@arm.com>,
	Catalin Marinas <Catalin.Marinas@arm.com>
Cc: "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
	"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
	Andrey Konovalov <andreyknvl@google.com>,
	Will Deacon <Will.Deacon@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	"linux-kselftest@vger.kernel.org"
	<linux-kselftest@vger.kernel.org>, nd <nd@arm.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v4 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt
Date: Thu, 13 Jun 2019 15:03:12 +0100	[thread overview]
Message-ID: <5963d144-be9b-78d8-9130-ef92bc66b1fd@arm.com> (raw)
In-Reply-To: <8e3c9537-de10-0d0d-f5bb-c33bde92443f@arm.com>

On 13/06/2019 13:28, Szabolcs Nagy wrote:
> On 13/06/2019 12:16, Vincenzo Frascino wrote:
>> Hi Szabolcs,
>>
>> thank you for your review.
>>
>> On 13/06/2019 11:14, Szabolcs Nagy wrote:
>>> On 13/06/2019 10:20, Catalin Marinas wrote:
>>>> Hi Szabolcs,
>>>>
>>>> On Wed, Jun 12, 2019 at 05:30:34PM +0100, Szabolcs Nagy wrote:
>>>>> On 12/06/2019 15:21, Vincenzo Frascino wrote:
>>>>>> +2. ARM64 Tagged Address ABI
>>>>>> +---------------------------
>>>>>> +
>>>>>> +From the kernel syscall interface prospective, we define, for the purposes
>>>>>                                      ^^^^^^^^^^^
>>>>> perspective
>>>>>
>>>>>> +of this document, a "valid tagged pointer" as a pointer that either it has
>>>>>> +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"
>>>>>
>>>>> this does not make it clear if MAP_FIXED or other flags are valid
>>>>> (there are many map flags i don't know, but at least fixed should work
>>>>> and stack/growsdown. i'd expect anything that's not incompatible with
>>>>> private|anon to work).
>>>>
>>>> Just to clarify, this document tries to define the memory ranges from
>>>> where tagged addresses can be passed into the kernel in the context
>>>> of TBI only (not MTE); that is for hwasan support. FIXED or GROWSDOWN
>>>> should not affect this.
>>>
>>> yes, so either the text should list MAP_* flags that don't affect
>>> the pointer tagging semantics or specify private|anon mapping
>>> with different wording.
>>>
>>
>> Good point. Could you please propose a wording that would be suitable for this case?
> 
> i don't know all the MAP_ magic, but i think it's enough to change
> the "flags =" to
> 
> * flags have MAP_PRIVATE and MAP_ANONYMOUS set or
> * flags have MAP_PRIVATE set and the file descriptor refers to...
> 
> 

Fine by me.  I will add it the next iterations.

>>>>>> +  - a mapping below sbrk(0) done by the process itself
>>>>>
>>>>> doesn't the mmap rule cover this?
>>>>
>>>> IIUC it doesn't cover it as that's memory mapped by the kernel
>>>> automatically on access vs a pointer returned by mmap(). The statement
>>>> above talks about how the address is obtained by the user.
>>>
>>> ok i read 'mapping below sbrk' as an mmap (possibly MAP_FIXED)
>>> that happens to be below the heap area.
>>>
>>> i think "below sbrk(0)" is not the best term to use: there
>>> may be address range below the heap area that can be mmapped
>>> and thus below sbrk(0) and sbrk is a posix api not a linux
>>> syscall, the libc can implement it with mmap or whatever.
>>>
>>> i'm not sure what the right term for 'heap area' is
>>> (the address range between syscall(__NR_brk,0) at
>>> program startup and its current value?)
>>>
>>
>> I used sbrk(0) with the meaning of "end of the process's data segment" not
>> implying that this is a syscall, but just as a useful way to identify the mapping.
>> I agree that it is a posix function implemented by libc but when it is used with
>> 0 finds the current location of the program break, which can be changed by brk()
>> and depending on the new address passed to this syscall can have the effect of
>> allocating or deallocating memory.
>>
>> Will changing sbrk(0) with "end of the process's data segment" make it more clear?
> 
> i don't understand what's the relevance of the *end*
> of the data segment.
> 
> i'd expect the text to say something about the address
> range of the data segment.
> 
> i can do
> 
> mmap((void*)65536, 65536, PROT_READ|PROT_WRITE, MAP_FIXED|MAP_SHARED|MAP_ANON, -1, 0);
> 
> and it will be below the end of the data segment.
>

As far as I understand the data segment "lives" below the program break, hence
it is a way of describing the range from which the user can obtain a valid
tagged pointer.

Said that, I am not really sure on how do you want me to document this (my aim
is for this to be clear to the userspace developers). Could you please propose
something?

>>
>> I will add what you are suggesting about the heap area.
>>

-- 
Regards,
Vincenzo

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