On Tue, 24 May 2022 14:40:56 GMT, Maurizio Cimadamore <[email protected]>
wrote:
> Constructing indexed var handles using the `MemoryLayout` API produces
> `VarHandle` which do not check the input indices for out-of-bounds conditions.
> While this can never result in a VM crash (after all the memory segment will
> protect against "true" OOB access), it is still possible for an access
> expression to refer to parts of a segment that are logically unrelated.
>
> This patch adds a "logical" bound check to all indexed var handles generated
> using the layout API.
> Benchmarks are not affected by the check. Users are still able to create
> custom "unchecked" var handles, using the combinator API in `MethodHandles`.
src/java.base/share/classes/java/lang/foreign/MemoryLayout.java line 537:
> 535: *
> 536: * <ul>
> 537: * <li>if {@code F > 0}, then {@code B = ceilDiv(C - S,
> F)}</li>
These formulas come from the formula for computing the accessed index A:
`A = S + I * F`
And then deriving the value for I, by equating `A = C` (for F > 0) and `A = -1`
(for F < 0) - that is equating the accessed index to the "first" out of bound
index. `ceilDiv` ensures there is "some room" between the max/min index and the
selected one.
src/java.base/share/classes/jdk/internal/foreign/LayoutPath.java line 109:
> 107: SequenceLayout seq = (SequenceLayout)layout;
> 108: checkSequenceBounds(seq, index);
> 109: long elemSize = seq.elementLayout().bitSize();
I've simplified the code here, as it still had traces of attempts to avoid the
call to `bitSize` (this method used to be partial).
-------------
PR: https://git.openjdk.java.net/jdk/pull/8868
