8356865: C2: Unreasonable values for debug flag FastAllocateSizeLimit can lead to left-shift-overflow, which is UB #25834

benoitmaillard · 2025-06-16T14:50:46Z

This PR adds a range constraint for the -XX:FastAllocateSizeLimit debug flag. This prevents undefined behavior caused by left-shift overflow of the flag value in GraphKit::new_array.

Testing

GitHub Actions
tier1-3, plus some internal testing
Manual testing with values known to previously cause undefined behavior

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Issue

JDK-8356865: C2: Unreasonable values for debug flag FastAllocateSizeLimit can lead to left-shift-overflow, which is UB (Bug - P4)

Reviewers

Manuel Hässig (@mhaessig - Author) Review applies to 486ddcb6
Emanuel Peter (@eme64 - Reviewer)

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Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/25834/head:pull/25834
$ git checkout pull/25834

Update a local copy of the PR:
$ git checkout pull/25834
$ git pull https://git.openjdk.org/jdk.git pull/25834/head

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Checkout this PR locally:
$ git pr checkout 25834

View PR using the GUI difftool:
$ git pr show -t 25834

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https://git.openjdk.org/jdk/pull/25834.diff

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bridgekeeper · 2025-06-16T14:51:29Z

👋 Welcome back bmaillard! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2025-06-16T14:52:05Z

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

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

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

8356865: C2: Unreasonable values for debug flag FastAllocateSizeLimit can lead to left-shift-overflow, which is UB

Reviewed-by: epeter, mhaessig

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 64 new commits pushed to the master branch:

3d35b40: 8357220: Introduce a BSMAttributeEntry struct
de34bb8: 8359687: Use PassFailJFrame for java/awt/print/Dialog/DialogType.java
620df7e: 8359801: RISC-V: Simplify Interpreter::profile_taken_branch
... and 61 more: https://git.openjdk.org/jdk/compare/9d060574e5dbd13e634f00d749d0108ceff1fae8...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

As you do not have Committer status in this project an existing Committer must agree to sponsor your change. Possible candidates are the reviewers of this PR (@eme64) but any other Committer may sponsor as well.

➡️ To flag this PR as ready for integration with the above commit message, type /integrate in a new comment. (Afterwards, your sponsor types /sponsor in a new comment to perform the integration).

openjdk · 2025-06-16T14:52:40Z

@benoitmaillard The following label will be automatically applied to this pull request:

hotspot

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

mlbridge · 2025-06-16T15:03:21Z

Webrevs

benoitmaillard · 2025-06-17T08:06:18Z

/label add hotspot-compiler

openjdk · 2025-06-17T08:07:52Z

@benoitmaillard
The hotspot-compiler label was successfully added.

mhaessig

Thank you for working on this, @benoitmaillard. This looks good to me.

src/hotspot/share/runtime/globals.hpp

eastig · 2025-06-17T16:39:56Z

src/hotspot/share/opto/graphKit.cpp

@@ -3804,6 +3804,7 @@ Node* GraphKit::new_array(Node* klass_node,     // array klass (maybe variable)
    // Increase the size limit if we have exact knowledge of array type.
    int log2_esize = Klass::layout_helper_log2_element_size(layout_con);
    fast_size_limit <<= (LogBytesPerLong - log2_esize);
+    assert (fast_size_limit > 0, "increasing the size limit should not produce negative values");


Prior C++14 left shit producing a negative value is undefined behavior: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2161.pdf

Do we compile c++ source specifying the C++ standard?

Yes we use -std=c++14, but creating a negative value in this way still feels like a kind of overflow to me.

Thanks for the comments!

I added the assert because the issue in the JBS mentioned a specific case where we ended up with negative values.

Should I leave it like this, or rather convert it to a more specific check (ie. making sure that the LogBytesPerLong - log2_esize most significant bits are not used before shifting)?

IMO your assert is obfuscating the overflow problem.
I think the assert should be before doing the shift.
It can be like:

assert((fast_size_limit == 0) || (count_leading_zeros(fast_size_limit) > (LogBytesPerLong - log2_esize), "fast_size_limit (%d) overflow when shifted left by %d", fast_size_limit, (LogBytesPerLong - log2_esize));

Thanks for the tip, I made the requested changes!

test/hotspot/jtreg/compiler/arguments/TestFastAllocateSizeLimit.java

eme64

Thanks for the updates! Nice work :)

benoitmaillard · 2025-06-23T07:36:12Z

/integrate

openjdk · 2025-06-23T07:37:30Z

@benoitmaillard
Your change (at version 8241b21) is now ready to be sponsored by a Committer.

eme64 · 2025-06-23T07:49:32Z

/sponsor

openjdk · 2025-06-23T07:52:03Z

Going to push as commit c220b13.
Since your change was applied there have been 65 commits pushed to the master branch:

9ae39b6: 8355276: Sort C2 includes
3d35b40: 8357220: Introduce a BSMAttributeEntry struct
de34bb8: 8359687: Use PassFailJFrame for java/awt/print/Dialog/DialogType.java
... and 62 more: https://git.openjdk.org/jdk/compare/9d060574e5dbd13e634f00d749d0108ceff1fae8...master

Your commit was automatically rebased without conflicts.

openjdk · 2025-06-23T07:52:20Z

@eme64 @benoitmaillard Pushed as commit c220b13.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

benoitmaillard added 3 commits June 16, 2025 15:36

8356865: Add range for FastAllocateSizeLimit flag

0839c31

8356865: Add assert for sanity check

d9c6b45

8356865: Add test for -XX:FastAllocateSizeLimit

486ddcb

openjdk bot changed the title ~~8356865~~ 8356865: C2: Unreasonable values for debug flag FastAllocateSizeLimit can lead to left-shift-overflow, which is UB Jun 16, 2025

openjdk bot added the hotspot [email protected] label Jun 16, 2025

benoitmaillard marked this pull request as ready for review June 16, 2025 14:58

openjdk bot added the rfr Pull request is ready for review label Jun 16, 2025

openjdk bot added the hotspot-compiler [email protected] label Jun 17, 2025

mhaessig approved these changes Jun 17, 2025

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eastig reviewed Jun 17, 2025

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benoitmaillard added 2 commits June 18, 2025 09:24

8356865: Add comment to clarify the flag range

c8904a2

8356865: Change assert

97f52b4

eme64 reviewed Jun 23, 2025

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test/hotspot/jtreg/compiler/arguments/TestFastAllocateSizeLimit.java Show resolved Hide resolved

8356865: Add comment for range in test

8241b21

eme64 approved these changes Jun 23, 2025

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openjdk bot added the ready Pull request is ready to be integrated label Jun 23, 2025

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openjdk bot closed this Jun 23, 2025

openjdk bot removed the ready Pull request is ready to be integrated label Jun 23, 2025

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8356865: C2: Unreasonable values for debug flag FastAllocateSizeLimit can lead to left-shift-overflow, which is UB #25834

8356865: C2: Unreasonable values for debug flag FastAllocateSizeLimit can lead to left-shift-overflow, which is UB #25834

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