8364235: Fix for JDK-8361447 breaks the alignment requirements for GuardedMemory

[dholmes-ora]

The fix for JDK-8361447 added a new field to the GuardHeader, not realizing that the size of the GuardHeader must be such that the address of the user-data has the strictest necessary alignment (16-byte).

We need to add a padding field to restore the alignment.

A static assert is added to check the alignment.

Testing:

• tiers 1-3 (in progress)

Thanks

---

Progress

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Issue

• JDK-8364235: Fix for JDK-8361447 breaks the alignment requirements for GuardedMemory (Bug - P2)

Reviewers

• Johan Sjölen (@jdksjolen - Reviewer) Review applies to a7502d8c
• Axel Boldt-Christmas (@xmas92 - Reviewer) Review applies to ab19a75d
• Daniel D. Daugherty (@dcubed-ojdk - Reviewer)

Contributors

• Johan Sjölen <[email protected]>

Reviewing

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@dholmes-ora This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

8364235: Fix for JDK-8361447 breaks the alignment requirements for GuardedMemory

Co-authored-by: Johan Sjölen <[email protected]>
Reviewed-by: dcubed, jsjolen, aboldtch

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At the time when this comment was updated there had been 111 new commits pushed to the master branch:

• f95af74: 8364312: debug agent should set FD_CLOEXEC flag rather than explicitly closing every open file
• 72d1066: 8364722: Parallel: Move CLDG mark clearing to the end of full GC
• 0ceb366: 8356645: Javac should utilize new ZIP file system read-only access mode
• ... and 108 more: https://git.openjdk.org/jdk/compare/4669005123420d8dbe86740dd9bcbee04735bc4f...master

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Webrevs

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[jdksjolen]

LGTM, but I'd prefer to do it the way I suggest.

[dholmes-ora]

/contributor add jdksjolen

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@dholmes-ora jdksjolen was not found in the census.

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[dholmes-ora]

/contributor add @jdksjolen

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@dholmes-ora
Contributor Johan Sjölen <[email protected]> successfully added.

[dholmes-ora]

Thanks for the reviews @xmas92 and @jdksjolen !

[dholmes-ora]

@kimbarrett and I discussed this further at some length after I flagged the max_align_t issue. On further analysis, considering in particular 32-bit systems, there is no guarantee that we actually get a properly aligned based address with alignas(16) when we are malloc'ing ourselves, as the "new alignment" may only be 8-bytes not 16-bytes. To deal with that Kim suggested revising the the GuardedMemory calculations so that we use align_up(size, 16) ensure the user-data will actually placed on a 16-byte boundary, regardless of the initial placement of the GuardHeader or its size. I am working on that revision.

I also want to investigate a potential regression test.

[dholmes-ora]

After even more discussions it seems there is no way to guarantee the required base-address alignment, we just have to assume it is correct (as we implicitly assume today), so the alignas(16) fix will stay. Just need someone to re-approve after the comment edit. Thanks.

[dcubed-ojdk]

Thumbs up.

[dholmes-ora]

Thanks Dan!

/integrate

[zhaosongzs]

/touch

[openjdk]

Going to push as commit 078d0d4.
Since your change was applied there have been 112 commits pushed to the master branch:

• 7e484e2: 8334238: Enhance AddLShortcutTest jpackage test
• f95af74: 8364312: debug agent should set FD_CLOEXEC flag rather than explicitly closing every open file
• 72d1066: 8364722: Parallel: Move CLDG mark clearing to the end of full GC
• ... and 109 more: https://git.openjdk.org/jdk/compare/4669005123420d8dbe86740dd9bcbee04735bc4f...master

Your commit was automatically rebased without conflicts.

[openjdk]

@dholmes-ora Pushed as commit 078d0d4.

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@zhaosongzs The command touch can only be used in open pull requests.

[dholmes-ora]

/backport jdk25u

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@dholmes-ora the backport was successfully created on the branch backport-dholmes-ora-078d0d49-master in my personal fork of openjdk/jdk25u. To create a pull request with this backport targeting openjdk/jdk25u:master, just click the following link:

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Hi all,

This pull request contains a backport of commit 078d0d49 from the openjdk/jdk repository.

The commit being backported was authored by David Holmes on 7 Aug 2025 and was reviewed by Daniel D. Daugherty, Johan Sjölen and Axel Boldt-Christmas.

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