[foreign] RFR 8218772: Limit struct member pointers to size of the field

Wed Feb 13 18:49:25 UTC 2019

Jorn Vernee schreef op 2019-02-13 19:44:
> Henry Jen schreef op 2019-02-13 19:19:
>> On Feb 13, 2019, at 9:42 AM, Jorn Vernee <jbvernee at xs4all.nl> wrote:
>>> 
>>> I don't think this bug is just a symptom of the bulk copy of Arrays. 
>>> Note that you can also cause an overwrite when you have a struct with 
>>> 2 ints, take a pointer to the first one, cast it to a long pointer, 
>>> and write to it.
>>> 
>> 
>> This is perfectly OK. The working theory is that as long as you
>> operated in the allocated memory region, you should be able to cast as
>> needed.
>> 
>> In you array case earlier, that’s not OK as the type of array has a
>> length, so I would expect an exception. But if you cast the pointer to
>> an Array of 3, then it should be able to overwrite the int. After all,
>> [2i32]i32 and [3i32] and i32i32i32 are the same size of memory block.
> 
> The problem is that there is no way to know the size of the target
> array type once we're in References.OfArray::set, since the target
> pointer might not actually have that type. The earliest location where
> we do know this size is in RuntimeSupport, when retrieving the pointer
> to the field.

Actually, maybe we do know the type... I'm wondering about use of unsafe 
set operations in boxing code. I will just try it out and see.

Jorn

> Jorn
> 
>> Cheers,
>> Henry
>> 
>> 
>>> It simply seems incorrect to me that a pointer to a struct's field 
>>> provides access outside of that field's memory.
>>> 
>>> ---
>>> 
>>> FWIW, I think the Array abstraction is a useful one. It signals the 
>>> difference between having a chunk of memory (Array) vs. having just a 
>>> cursor into memory (Pointer), and as such, I think it's fine to say 
>>> that writing an Array constitutes a copy of the array vs. just a copy 
>>> of the pointer. If a bulk-copy is not wanted, users can use the 
>>> elementPointer() instead (same with Struct::ptr).
>>> 
>>> The native types do not map perfectly into Java types, so there are 
>>> some things that have to be learned when using the API. I think the 
>>> pitfall here is the assumption that, since in C an array is just a 
>>> pointer in a lot of cases, and setting a pointer does not incur a 
>>> copy, the Array type in Java must also really be just a Pointer, and 
>>> setting it should not incur a copy. But in the API we have 2 distinct 
>>> types, Pointer and Array, so I don't think it's unreasonable to say 
>>> that those 2 will behave differently.
>>> 
>>> Jorn
>>> 
>>> Maurizio Cimadamore schreef op 2019-02-13 17:57:
>>>> Hi Jorn,
>>>> I was looking at something very related to this - e.g. relationship
>>>> between pointers and arrays, and, in general bulk-write operations 
>>>> for
>>>> structs and arrays (with Pointer::set) and I thought I might add
>>>> something to this discussion, to see if the issues that you are
>>>> running into are just bugs, or symptoms of something deeper.
>>>> Over the last few weeks I've been toying with the idea of merging
>>>> Array and Pointers - after all a BoundedPointer is expressive enough
>>>> to represent both. We could e.g. setup an approach like the one 
>>>> below:
>>>> - Pointer has array-like accessors Pointer::get(long), 
>>>> Pointer::set(long, X)
>>>> - regular accessor defaults to zero offset - that is, Pointer::get()
>>>> -> Pointer::get(0)
>>>> - jextract does NOT generate setters for array struct fields, or 
>>>> array
>>>> global variables - only getters
>>>> - an API is provided (well, one exists already) to do bulk copy
>>>> between different arrays/structs
>>>> While I like the unification this brings about (only one abstraction
>>>> instead of two, Pointer and Array), and I also like the fact that we
>>>> move towards a model where Pointer::get, Pointer::set are O(1)
>>>> operations (with bulk operations left to higher level APIs), there 
>>>> is
>>>> something that doesn't 100% convinces me: if we go down this path, 
>>>> the
>>>> following layouts:
>>>> u64:u64:i32 (pointer to pointer to int)
>>>> and
>>>> u64:[5 i32] (pointer to array of int)
>>>> will effectively have the same carrier type:
>>>> Pointer<Pointer<Integer>>
>>>> The difference will be visible only upon closer inspection: we could
>>>> call Pointer::type() obtain a LayoutType, then do 
>>>> LayoutType::layout()
>>>> and see whether the pointee layout is a sequence or an address.
>>>> Now, when we perform a get() on such a pointer, we can, given the
>>>> layout, construct the right pointer with the right size info (if 
>>>> any).
>>>> But what about set() ? I see different options here, none of which
>>>> seems particularly satisfying:
>>>> 1) Pointer::set should throw if you are trying to set an array-like
>>>> pointer (which would require bulk copy)
>>>> 2) Pointer::set will silently perform bulk copy of the incoming
>>>> pointer into the pointed region
>>>> Of these, (2) is similar to what we have now, whereas (1) would be a
>>>> stricter variant.
>>>> The thing that puzzles me is that, looking at the code it will be
>>>> absolutely impossible to understand what's going on - e.g. if native
>>>> arrays and native pointers map to the same Java carrier (Pointer) it
>>>> then becomes super hard to explain/understand/predict why a given
>>>> operation (e.g. Pointer::set) resulted in bulk copy/exception.
>>>> One of the principles I've been trying to adhere to when designing 
>>>> the
>>>> foreign API is to avoid surprises - e.g. the semantics of a given
>>>> operation should be clear from the carrier types involved in the
>>>> operation. In that respect, maybe a pointer with an optional 
>>>> length()
>>>> is tolerable, but having Pointer::set behaving differently depending
>>>> on the result of Pointer::type() is that acceptable or too subtle?
>>>> This seems to suggest that the current Pointer vs. Array split 
>>>> carries
>>>> some weight. If carrier type are different then it's easy to see for
>>>> the user which operation might or might not be supported (or might
>>>> have bulk-logic).
>>>> <sidebar>
>>>> On the other hand, one might argue that this is already happening 
>>>> with
>>>> pointers to incomplete objects - if I have a Pointer<?> and I call
>>>> get() I don't know if I'll get an exception or not. It again depends
>>>> on whether the layout complete or not - e.g. if it's the void layout
>>>> or, if it's partial layout (because it refers to an unresolved 
>>>> struct)
>>>> an exception will be thrown.
>>>> </sidebar>
>>>> If we do retain Pointer vs. Array then I think we are free to decide
>>>> whether for Pointer::set we want (1) or (2) - that is, when we have
>>>> Pointer<Array<Integer>>
>>>> or
>>>> Pointer<StructFoo>
>>>> should Pointer::set do bulk copy, or should it throw? If the latter,
>>>> should jextract even emit setters for struct fields of type
>>>> array/struct? (or, perhaps, emulate such setters by performing
>>>> explicit copy, rather than by using Pointer::set internally).
>>>> These are all questions that are relevant, I believe, to the fix you
>>>> brought up - I'm of course fine with the fix, but I'd like to also
>>>> understand whether the bulk-copy on Pointer::set is one magic trick
>>>> too far and also, more generally, what do Panama-land feels about 
>>>> the
>>>> Array vs. Pointer split/lump.
>>>> Feedback welcome.
>>>> Cheers
>>>> Maurizio
>>>> On 13/02/2019 15:47, Jorn Vernee wrote:
>>>>> Hi,
>>>>> I found a bug where it was possible to overwrite trailing fields of 
>>>>> a struct by writing an oversized array to a previous array field 
>>>>> (see bug). Overwriting is also possible in other cases by forcing 
>>>>> an oversized write to a struct field. This can be fixed (relatively 
>>>>> easily) by limiting the size of memory regions of pointers to 
>>>>> struct members to the size of their fields.
>>>>> Please review the following.
>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8218772
>>>>> Webrev: 
>>>>> http://cr.openjdk.java.net/~jvernee/panama/webrevs/8218772/webrev.00/
>>>>> Thanks,
>>>>> Jorn