RFR: 8303002: Reject packed structs from linker [v5]

[Maurizio Cimadamore]

On Tue, 18 Apr 2023 22:25:09 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Jorn Vernee has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:
>> 
>>  - fix TestIllegalLink
>>  - Merge branch 'PR_21' into RejectPacked
>>  - Merge branch 'PR_21' into RejectPacked
>>  - add javadoc + fixes for trailing padding
>>  - fix check for padding. Add check for trailing padding too
>>  - Reject packed structs and structs with extra padding
>>  - Check byte order of layouts passed to linker
>
> src/java.base/share/classes/java/lang/foreign/Linker.java line 202:
> 
>> 200:  * Due to limited ABI specification coverage, all the native linker implementations limit the function
>> 201:  * descriptors that they support to those that contain only so-called <em>canonical</em> layouts. These layouts
>> 202:  * have the following restrictions:
> 
> I think it's better to phrase this more or less as:
> 
> A canonical layout has the following characteristics:
> * Its alignment constraint is equal to its natural alignment
> * If the layout is a value layout (linkplain), its byte order matches that of the platform in which the JVM is executing (link to nativeOrder())
> *  If the layout is a group layout (linkplain), it must not contain more padding layout (linkplain) elements than those strictly necessary to satisfy the alignment constraints of the non-padding elements of the group layout.

Also, isn't it the case that, for structs, we want the size of the layout to be a multiple of its alignment constraint?

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PR Review Comment: https://git.openjdk.org/jdk/pull/13164#discussion_r1170624776