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

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Message-ID: <ba822b33-a822-02ef-9b85-725f4353596a@arm.com>
Date:   Thu, 13 Jun 2019 15:32:42 +0000
From:   Szabolcs Nagy <Szabolcs.Nagy@....com>
To:     Vincenzo Frascino <Vincenzo.Frascino@....com>,
        Catalin Marinas <Catalin.Marinas@....com>
CC:     nd <nd@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Will Deacon <Will.Deacon@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v4 1/2] arm64: Define
 Documentation/arm64/tagged-address-abi.txt

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.