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

Fri Feb 15 12:53:20 UTC 2019

+1

Maurizio

On 15/02/2019 12:28, Jorn Vernee wrote:
> Actually... I realized that checkBounds was only being used by 
> checkRange, so at this point we could just remove it if we wanted.
>
> Webrev: 
> http://cr.openjdk.java.net/~jvernee/panama/webrevs/nocheck/webrev.00/
>
> Jorn
>
> Maurizio Cimadamore schreef op 2019-02-15 02:10:
>> Pretty solid patch - thanks for the explanations (perhaps at some
>> point it could be worth renaming checkBounds/checkRange to something
>> else).
>>
>> Go for it
>>
>> Maurizio
>>
>> On 15/02/2019 00:41, Jorn Vernee wrote:
>>> Maurizio Cimadamore schreef op 2019-02-14 22:52:
>>>> On 14/02/2019 21:23, Jorn Vernee wrote:
>>>>
>>>>> Yes, this is a good idea.
>>>>>
>>>>> Update webrev:
>>>>>
>>>> http://cr.openjdk.java.net/~jvernee/panama/webrevs/8218772/webrev.02/
>>>>>
>>>>>
>>>>> Struct::assign and Array::assign were pretty similar, so I factored
>>>>> out the code into a Pointer::assign method. I also implemented
>>>>> equals/hashCode/toString for LayoutTypeImpl for that, which seem
>>>>> useful to have any ways. (I can imagine there are scenarios where a
>>>>> user would want to have a Map<LayoutType, ...> for instance).
>>>>
>>>> Not uber convinced about Pointer::assign, mostly because we already
>>>> have a Pointer::copy - we should only have the latter, and perhaps
>>>> that's the right name for the new method?
>>>
>>> Sounds good. I've renamed 'assign' to 'copy'.
>>>
>>>> Also, not greatly convinced by the need of the Array::ptr method -
>>>> maybe hide that into the impl class?
>>>
>>> Yeah, I'll hide that in the impl class. I originally added it to 
>>> Array to avoid casting in the implementation, and I thought maybe if 
>>> the method is useful for us, it's useful to users as well. But I 
>>> guess it's a little too confusing to have both ptr() and 
>>> elementPointer().
>>>
>>>>> The new checking also uncovered a bug where allocation of a 0 length
>>>>> array would return Pointer.nullPointer(), but this does not have the
>>>>> right LayoutType. Besides, there's already a special case for 0
>>>>> length regions by returning MemoryBoundInfo.NOTHING. I just removed
>>>>> the nullPointer() return, so we end up with a pointer with the right
>>>>> LayoutType, and perhaps more importantly, the right Scope.
>>>>>
>>>>> However, this again caused a crash in ScopeTest, so I've also
>>>>> updated the bounds checking code to remove the special checkBounds
>>>>> case for accessing NOTHING at offset 0, and added a special case to
>>>>> checkRange w.r.t. empty ranges, which fixes the issue.
>>>>
>>>> Could you expand a bit on exactly who was calling checkRange with
>>>> offset and length set to 0? I guess it was Pointer::addr?
>>>
>>> I was seeing a segmentation fault with the following stack trace:
>>>
>>> j jdk.internal.foreign.memory.BoundedPointer.unsafeGetBits()J+111 
>>> java.base at 13-internal
>>> j  jdk.internal.foreign.memory.BoundedPointer.getBits()J+14 
>>> java.base at 13-internal
>>> j 
>>> jdk.internal.foreign.memory.References$OfByte.getByte(Ljava/foreign/memory/Pointer;)B+4 
>>> java.base at 13-internal
>>> j 
>>> java.lang.invoke.DirectMethodHandle$Holder.invokeStatic(Ljava/lang/Object;Ljava/lang/Object;)I+10 
>>> java.base at 13-internal
>>> j 
>>> java.lang.invoke.LambdaForm$MH.invoke(Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;+15 
>>> java.base at 13-internal
>>> j 
>>> java.lang.invoke.LambdaForm$MH.invoke_MT(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;+18 
>>> java.base at 13-internal
>>> j  java.foreign.memory.Array.get(J)Ljava/lang/Object;+23 
>>> java.base at 13-internal
>>> j 
>>> ScopeTest.assertEmptyArray(Ljava/foreign/memory/Array;Ljava/lang/Object;)V+26
>>> j  ScopeTest.testNullAllocation()V+44
>>>
>>> This is because checkBounds explicitly allows access at offset = 0 
>>> and regionLength = 0 (bad imho). I think this wasn't previously a 
>>> problem since NOTHING was only every used with References.OfGrumpy, 
>>> which explicitly throws an exception when trying to access.
>>>
>>>> The changes in MemoryBoundInfo look odd - for instance:
>>>>
>>>> if (length == 0) {
>>>> +            if (offset < 0 || offset > this.length) {
>>>> +                throw new IllegalStateException("offset: " + offset +
>>>> ", region length: " + this.length);
>>>> +            }
>>>> +        }
>>>>
>>>> This  stuff inside the braces is 99% the same as checkBounds(offset),
>>>> except for a '<' instead of '<='.
>>>>
>>>> Wouldn't it be better to special case checkBounds to deal with zero
>>>> length gracefully, and then just call it from checkRange?
>>>
>>> The problem is that we have 2 requirements, 1.) bounds check for 
>>> accessing a byte in a memory region. 2.) checking if one region is a 
>>> sub-region of another. For 1. the offset can not be equal to the 
>>> length of the region, because then we are reading outside of the 
>>> region, but for 2. this is fine, as long as the sub-region has a 
>>> length of 0.
>>>
>>> checkBounds works like an array index check, so it works well for 
>>> 1., but not for 2. It seemed better to handle 2. in checkRange, 
>>> since we actually know the length of the range being accessed there, 
>>> which could be 0. I really don't think checkBounds is the right 
>>> place to try and fix this problem. Tbh, it seems like a mistake that 
>>> checkRange relies on checkBounds in the first place.
>>>
>>> I agree that the current code looks kind of cryptic. I've tried to 
>>> improve this by removing the dependency on checkBounds from checkRange.
>>>
>>>> Also, and probably even better - if this stuff is there just to handle
>>>> NOTHING - wouldn't we better off overriding the check methods for the
>>>> NOTHING instance (as we do for everything) ? I'm not a fan of these
>>>> hard-coded special cases, which will be nearly impossible to read 2
>>>> months from now.
>>>
>>> Actually, I think I spoke too soon. I think the actual use case was 
>>> to allow passing a null pointer to a native function, since the 
>>> unboxing incurs a bounds check. The re-write of checkRange takes 
>>> care of that as well.
>>>
>>> Update webrev: 
>>> http://cr.openjdk.java.net/~jvernee/panama/webrevs/8218772/webrev.03/
>>>
>>> (FWIW, re-writing the bounds check seems to have improved perf as 
>>> well. My test runs are finishing about a minute faster).
>>>
>>> Jorn
>>>
>>>> Cheers
>>>> Maurizio
>>>>
>>>>> Jorn
>>>>>
>>>>> Maurizio Cimadamore schreef op 2019-02-14 16:19:
>>>>> On 14/02/2019 15:18, Maurizio Cimadamore wrote:
>>>>> Why not using the Array.assign API point, which already performs all
>>>>> these checks?
>>>>>
>>>>> Similarly, I think References.OfStruct::set should use Struct.assign
>>>>> -
>>>>> as there's always a chance of mis-using the setter MH to write a
>>>>> struct that is bigger or smaller than the expected one.
>>>>>
>>>>> Maurizio
>>>>>
>>>>> Maurizio
>>>>>
>>>>> On 14/02/2019 14:31, Jorn Vernee wrote:
>>>>> Okay, I have implemented the check in References.OfArray::set.
>>>>>
>>>>> Updated webrev:
>>>>>
>>>> http://cr.openjdk.java.net/~jvernee/panama/webrevs/8218772/webrev.01/
>>>>>
>>>>>
>>>>> I was also tinkering a long time with simplifying the original code,
>>>>> but kept running into issues with bounds checks in MemoryBoundInfo.
>>>>> The current bounds checking code isn't really suited for dealing
>>>>> with 0 length range checks.
>>>>>
>>>>> Jorn
>>>>>
>>>>> Jorn Vernee schreef op 2019-02-13 19:49:
>>>>> 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