RE: [PATCH] arm64: Add config to limit user space to 47bits

Zheng Xu
2016-07-14T07:04:08Z

LuaJIT also fix the 48VA issue by allocating heap memory below 47 bits.

For mozjs issue, if there are pointers to .rodata, it can be a problem. Does it happen on master and do we have any case to reproduce the issue so that I can take a look?

Thanks,
Zheng

-----Original Message-----
From: Alexander Graf [mailto:agraf@suse.de]
Sent: 14 July 2016 14:39
To: Steve Capper
Cc: Ard Biesheuvel; Mark Rutland; mbrugger@suse.com; Catalin Marinas; Will Deacon; linux-kernel@vger.kernel.org; linux-arm-kernel@lists.infradead.org; Stuart Monteith; Zheng Xu
Subject: Re: [PATCH] arm64: Add config to limit user space to 47bits

> On 14 Jul 2016, at 03:08, Steve Capper <steve.capper@arm.com> wrote:
>
> Hi Alex,
>
> Thanks for posting this.
>
> On Wed, Jul 13, 2016 at 06:14:11PM +0200, Alexander Graf wrote:
>> On 07/13/2016 05:59 PM, Ard Biesheuvel wrote:
>>> On 13 July 2016 at 17:42, Alexander Graf <agraf@suse.de> wrote:
>>>> Some user space applications are known to break with 48 bits
>>>> virtual
>>> known by whom? At least I wasn't aware of it, so could you please
>>> share some examples?
>>
>> Sure! Known to me so far are:
>>
>> * mozjs17
>> * mozjs24
>> * mozjs38
>> * js-1.8.5
>> * java-1.7 (older JITs, fixed in newer ones)
>>
>> I'm not sure if there are more, but the fact that I've run into this
>> problem more than once doesn't make me incredibly happy :).
>>
>
> I came across this too: on bootup via polkitd (which pulled in mozJS) :-(.

Yup, that’s where I stumbled over it first. Gnome uses mozjs too, as do Firefox and Thunderbird obviously.

>
>>>
>>>> address space. As interim step until the world is healed and
>>>> everyone embraces correct code, this patch allows to only expose 47
>>>> bits of virtual address space to user space.
>>>>
>>> Is this a code generation/toolchain issue?
>>
>> mozjs uses a single 64bit value to combine doubles, ints and pointers
>> into a single variable. It is very smart and uses the upper
>> 17 bits for metadata such as "which type of variable is this".
>> Coincidentally those bits happen to overlap the "double is an
>> infinite number" bits, so that you can also express a NaN with it.
>> When using such a value, the upper 17 bits get masked out.
>>
>> That one was fixed upstream by force allocating the javascript heap
>> starting at a fixed location which is below 47 bits.
>>
>> js-1.8.5 has the same as above, but also uses pointers to .rodata as
>> javascript pointers, so it doesn't only use the heap, it also uses
>> pointers to the library itself, which gets mapped high up the address
>> space. I don't have a solution for that one yet.
>
> Is this Spidermonkey 1.8.5? I wasn't aware of this issue.

Exactly. If you’re interested in fixing it, be my guest :).

>
>>
>> IcedTea for java-1.7 had a bug where it incorrectly caused an
>> overflow when trying to calculating a relative adrp offset from
>> to , so that the resulting
>> pointer had the upper bits set as 1s. That one is long fixed
>> upstream, we only ran into it because we used an ancient IcedTea
>> snapshot.
>
> I would recommend updating the sources used for OpenJDK anyway as
> there have been a few other stability and performance fixes put in
> over the last year to my knowledge.

Sure, that’s what we’ve done of course. I mostly mentioned it to answer the question where I had seen problems with 48 bits VA.

>
>>
>> My main concern however is with code that I do not know is broken today.
>>
>
> I think if we set the 47-bit VA we are just ignoring the fundamental
> problem and even allowing the problem to get worse (as future code may
> adopt unsafe pointer tagging); thus I agree with Mark Rutland's NAK.

Yeah, I’m torn on that one. I agree that we do allow broken code to work. However, going above 47 bits means we’re different from x86_64. And that *may* be a compatibility problem. Unfortunately we won’t know until at least a few hundred ISVs started to port their crufty user space code to ARM and things fell apart from time to time ;).

> Personally, I would only ever tag bits in the VA space that I control
> (i.e. at the bottom of the pointer if I enforce alignment).

Don’t blame the messenger :). For code that I write I tend to agree, but we’re talking about software that is a core dependency of other code (CouchDB uses Spidermonkey, polkit uses mozjs, etc) and that has been unmaintained for almost a decade by now, written in times when 47 bits of VA was more than the contents of the whole internet ;).

Alex

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