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

Fri Feb 15 01:10:00 UTC 2019

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