8326692: JVMCI Local.endBci is off-by-one #18087

gilles-duboscq · 2024-03-01T17:48:51Z

In class files, in the local variable table, local variables have a start BCI and a length. The local variable has a value from BCI (inclusive) until BCI + length (exclusive).
On the other end, JVMCI stores that information in Local objects with a start BCI and an end BCI (inclusive).
Currently the parser just uses BCI+length to compute the end BCI, leading to an off-by-one error.

A simple test checking that the start and end BCIs are within the method's bytecode is added. It fails without the fix.

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Issue

JDK-8326692: JVMCI Local.endBci is off-by-one (Bug - P5)

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Doug Simon (@dougxc - Reviewer) ⚠️ Review applies to 90e96b4e
Tom Rodriguez (@tkrodriguez - Reviewer) ⚠️ Review applies to 90e96b4e
Guoxiong Li (@lgxbslgx - Committer)

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bridgekeeper · 2024-03-01T17:49:23Z

👋 Welcome back gdub! 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 · 2024-03-01T17:50:39Z

@gilles-duboscq The following labels will be automatically applied to this pull request:

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mlbridge · 2024-03-01T17:54:00Z

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01: Full - Incremental (fe1ee476)
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dougxc

Thanks for fixing this Gilles.

openjdk · 2024-03-01T18:52:50Z

@gilles-duboscq 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:

8326692: JVMCI Local.endBci is off-by-one

Reviewed-by: dnsimon, never, gli

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 79 new commits pushed to the master branch:

37e01ef: 8326586: Improve Speed of System.map
a3d51d2: 8326915: NPE when a validating parser is restricted
f62f2ad: 8325671: Shenandoah: Introduce a ShenandoahGenerationType and templatize certain marking closures with it
f68a4b9: 8327105: compiler.compilercontrol.share.scenario.Executor should listen on loopback address only
a9c17a2: 8327108: compiler.lib.ir_framework.shared.TestFrameworkSocket should listen on loopback address only
7f02f07: 8316708: Augment WorstCaseTests with more cases
8f0fb27: 8327136: javax/management/remote/mandatory/notif/NotifReconnectDeadlockTest.java fails on libgraal
b38a6c5: 8327049: Only export debug.cpp functions on Windows
acc0a74: 8327126: Parallel: Remove unused ParMarkBitMapClosure::_initial_words_remaining
012411a: 8327046: (fs) Files.walk should be clear that depth-first traversal is pre-order
... and 69 more: https://git.openjdk.org/jdk/compare/f62b5789add23adda2634a1cfb80f48b4387be74...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.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

lgxbslgx · 2024-03-02T11:28:43Z

src/jdk.internal.vm.ci/share/classes/jdk/vm/ci/hotspot/HotSpotResolvedJavaMethodImpl.java

-            final int endBci = startBci + UNSAFE.getChar(localVariableTableElement + config.localVariableTableElementLengthOffset);
+            final int endBci = startBci + UNSAFE.getChar(localVariableTableElement + config.localVariableTableElementLengthOffset) - 1;


Just a question: Can the length of a local variable be 0?

If the code length is 0, the endBci here may be less than startBci.

I don't see anything in JVMS 4.7.13 that says it cannot be 0. It basically means the LVT entry is useless (denotes a local that is never alive) but is otherwise harmless.
Maybe add this to the javadoc for getEndBci() to make the API user aware of this corner case:

If the value returned is less than {@link #getStartBCI}, this object denotes a local that is never live.

The reason, which causes this problem, is that the Local::endBci includes itself instead of excluding it. But now, we can only fix the javadoc just as you suggested.

If the value returned is less than {@link #getStartBCI}, this object denotes a local that is never live.

a local variable may be better to a local above.

I had checked the specs on that and came to the same conclusion.
I also think the current state is fine in that regards in terms of code since it just means that there is no bci where this local would be valid when checking both start and end bci.
Adding a note about that to the javadoc is a good idea. I'll do that.

lgxbslgx

Looks good.

gilles-duboscq · 2024-03-03T11:05:07Z

/integrate

openjdk · 2024-03-03T11:05:35Z

Going to push as commit 31ac871.
Since your change was applied there have been 79 commits pushed to the master branch:

37e01ef: 8326586: Improve Speed of System.map
a3d51d2: 8326915: NPE when a validating parser is restricted
f62f2ad: 8325671: Shenandoah: Introduce a ShenandoahGenerationType and templatize certain marking closures with it
f68a4b9: 8327105: compiler.compilercontrol.share.scenario.Executor should listen on loopback address only
a9c17a2: 8327108: compiler.lib.ir_framework.shared.TestFrameworkSocket should listen on loopback address only
7f02f07: 8316708: Augment WorstCaseTests with more cases
8f0fb27: 8327136: javax/management/remote/mandatory/notif/NotifReconnectDeadlockTest.java fails on libgraal
b38a6c5: 8327049: Only export debug.cpp functions on Windows
acc0a74: 8327126: Parallel: Remove unused ParMarkBitMapClosure::_initial_words_remaining
012411a: 8327046: (fs) Files.walk should be clear that depth-first traversal is pre-order
... and 69 more: https://git.openjdk.org/jdk/compare/f62b5789add23adda2634a1cfb80f48b4387be74...master

Your commit was automatically rebased without conflicts.

openjdk · 2024-03-03T11:05:40Z

@gilles-duboscq Pushed as commit 31ac871.

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

gilles-duboscq added 2 commits March 1, 2024 18:44

Add javadoc and minimal test for Local.getStart/EndBCI

94c15b8

Fix JVMCI Local endBCI off-by-one error

90e96b4

openjdk bot added the rfr Pull request is ready for review label Mar 1, 2024

openjdk bot added graal graal-dev@openjdk.org hotspot-compiler hotspot-compiler-dev@openjdk.org labels Mar 1, 2024

dougxc approved these changes Mar 1, 2024

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openjdk bot added the ready Pull request is ready to be integrated label Mar 1, 2024

tkrodriguez approved these changes Mar 1, 2024

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lgxbslgx reviewed Mar 2, 2024

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Add note about zero-length locals

fe1ee47

lgxbslgx approved these changes Mar 2, 2024

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8326692: JVMCI Local.endBci is off-by-one #18087

8326692: JVMCI Local.endBci is off-by-one #18087

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openjdk bot commented Mar 3, 2024

		final int endBci = startBci + UNSAFE.getChar(localVariableTableElement + config.localVariableTableElementLengthOffset);
		final int endBci = startBci + UNSAFE.getChar(localVariableTableElement + config.localVariableTableElementLengthOffset) - 1;

8326692: JVMCI Local.endBci is off-by-one #18087

8326692: JVMCI Local.endBci is off-by-one #18087

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