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Subject: Re: [PATCH v4 1/2] arm64: Define
 Documentation/arm64/tagged-address-abi.txt
To:     Szabolcs Nagy <Szabolcs.Nagy@arm.com>,
        Catalin Marinas <Catalin.Marinas@arm.com>
Cc:     nd <nd@arm.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
        "linux-kselftest@vger.kernel.org" <linux-kselftest@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Will Deacon <Will.Deacon@arm.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>
References: <cover.1560339705.git.andreyknvl@google.com>
 <20190612142111.28161-1-vincenzo.frascino@arm.com>
 <20190612142111.28161-2-vincenzo.frascino@arm.com>
 <a90da586-8ff6-4bed-d940-9306d517a18c@arm.com>
 <20190613092054.GO28951@C02TF0J2HF1T.local>
 <dee7f192-d0f0-558e-3007-eba805c6f2da@arm.com>
 <6ebbda37-5dd9-d0d5-d9cb-286c7a5b7f8e@arm.com>
 <8e3c9537-de10-0d0d-f5bb-c33bde92443f@arm.com>
 <5963d144-be9b-78d8-9130-ef92bc66b1fd@arm.com>
 <ba822b33-a822-02ef-9b85-725f4353596a@arm.com>
From:   Vincenzo Frascino <vincenzo.frascino@arm.com>
Message-ID: <a05a2dfb-3398-455d-8586-b79dfb7a772f@arm.com>
Date:   Thu, 13 Jun 2019 16:35:35 +0100
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On 13/06/2019 16:32, Szabolcs Nagy wrote:
> On 13/06/2019 15:03, Vincenzo Frascino wrote:
>> On 13/06/2019 13:28, Szabolcs Nagy wrote:
>>> On 13/06/2019 12:16, Vincenzo Frascino wrote:
>>>> On 13/06/2019 11:14, Szabolcs Nagy wrote:
>>>>> On 13/06/2019 10:20, Catalin Marinas wrote:
>>>>>> On Wed, Jun 12, 2019 at 05:30:34PM +0100, Szabolcs Nagy wrote:
>>>>>>> On 12/06/2019 15:21, Vincenzo Frascino wrote:
>>>>>>>> +  - 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?
> 
> [...], it is in the memory ranges privately owned by a
> userspace process and it is obtained in one of the
> following ways:
> 
> - mmap done by the process itself, [...]
> 
> - brk syscall done by the process itself.
>   (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 [...]
> 
> the data segment that's part of the process image is
> already covered by the last point.
> 

Thanks Szabolcs, I will update the document accordingly.

-- 
Regards,
Vincenzo