8361712: Improve ShenandoahAsserts printing #26237

tstuefe · 2025-07-10T08:02:05Z

Small changes to ShenandoahAsserts and friends to improve fault-tolerance and usefulness:

In assert_correct, we now check the forwarding pointer first before extracting the narrowKlass from it. The forwarding pointer may be invalid. To do this, the forwarding-pointer-checking-block was moved up to before Klass* checks. We also use SafeFetch-based checks for some added safety.
In assert_correct, we test the narrowKlass before decoding it. Otherwise, if it's 0, we'd assert, if garbage, the resulting Klass* would be garbage.
In print_failure, we now check pointers for readability before handing them to print_obj
In print_obj, we now print the extracted narrowKlass, too (useful on Lilliput with its terse ids)
In print_obj, we print a small hex dump of the header (32bytes). With little cost, we get a glance at the raw header and the first members.
In print_obj, we print to a single stringStream now and use auto indentation. Avoids having to deal with indentation and newlines manually and having to pay for four stringStreams.
In assert_correct, I raised the print level for some of the klass-related assertions to safe_oop since the oop, since the hardened version of print_obj now avoids tripping over invalid Klass*

Tests: I manually tested the patch with several manually broken oops of various flavours.

Example output, invalid forwardee:

Referenced from:
  no interior location recorded (probably a plain heap scan, or detached oop)

Object:
  0x00000007cd800000 - nk 1798104 klass 0x00000000921b6fd8 [Ljava.lang.Object;

        allocated after mark start
        after update watermark
        marked strong
        marked weak
    not in collection set
    mark:  marked(0xdeaddeaddeaddeaf)
    region: | 3812|R  |Y|BTE    7cd800000,    7cd920bf8,    7cdc00000|TAMS    7cd800000|UWM    7cd800000|U  1154K|T     0B|G     0B|S  1154K|L     0B|CP   0

  0x00000007cd800000:   deaddeaddeaddeaf 00000a96001b6fd8 f9b005cff9b0054d f9b006d3f9b00651   .........o......M.......Q.......
  0x00000007cd800020:   f9b00797f9b00755 f9b00992f9b0087d f9b009a0f9b00999 f9b009a7f9b0098e   U.......}.......................

Forwardee:
  0xdeaddeaddeaddeac - safe print, no details

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Commit message must refer to an issue

Issue

JDK-8361712: Improve ShenandoahAsserts printing (Enhancement - P4)

Reviewers

Roman Kennke (@rkennke - Reviewer)
Ashutosh Mehra (@ashu-mehra - Committer)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/26237/head:pull/26237
$ git checkout pull/26237

Update a local copy of the PR:
$ git checkout pull/26237
$ git pull https://git.openjdk.org/jdk.git pull/26237/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 26237

View PR using the GUI difftool:
$ git pr show -t 26237

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/26237.diff

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bridgekeeper · 2025-07-10T08:03:05Z

👋 Welcome back stuefe! 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 · 2025-07-10T08:04:06Z

@tstuefe 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:

8361712: Improve ShenandoahAsserts printing

Reviewed-by: rkennke, asmehra

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 101 new commits pushed to the master branch:

06fdb61: 8361964: Remove outdated algorithms from requirements and add PBES2 algorithms
41c94ee: 8363910: Avoid tuning for Power10 CPUs on Linux ppc64le when gcc < 10 is used
518d5f4: 8361871: [GCC static analyzer] complains about use of uninitialized value ckpObject in p11_util.c
... and 98 more: https://git.openjdk.org/jdk/compare/70c1ff7e1505eee11b2a9acd9e94a39cd2c9a932...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.

openjdk · 2025-07-10T08:04:44Z

@tstuefe The following labels will be automatically applied to this pull request:

hotspot
shenandoah

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

mlbridge · 2025-07-12T06:43:33Z

Webrevs

01: Full - Incremental (c7a9c26e)
00: Full (94fe071e)

rkennke

This looks really useful! I have only two comments/questions.

src/hotspot/share/gc/shenandoah/shenandoahAsserts.cpp

src/hotspot/share/utilities/ostream.hpp

tstuefe · 2025-07-23T04:35:08Z

@rkennke Did I address all your concerns? Anyone else willing to look at this, @earthling-amzn maybe?

rkennke · 2025-07-23T06:32:09Z

@rkennke Did I address all your concerns? Anyone else willing to look at this, @earthling-amzn maybe?

Yes, looks all good to me.

ashu-mehra · 2025-07-24T20:27:48Z

src/hotspot/share/gc/shenandoah/shenandoahAsserts.cpp

  ResourceMark rm;

+  if (!os::is_readable_pointer(obj)) {
+    level = _safe_unknown;


Correct me if I am wrong - shouldn't the caller be responsible for passing correct level to print_failure? If the caller passes a potentially invalid obj with, say level = _safe_oop, then the caller should be updated.

The caller may have erred. Seeing how many callers there are, there are many places that can bitrot. This is just an added safety feature.

ashu-mehra · 2025-07-24T20:27:52Z

src/hotspot/share/gc/shenandoah/shenandoahAsserts.cpp

-
-  stringStream mw_ss;
-  obj->mark().print_on(&mw_ss);
+  StreamIndentor si(&ss);


This seems redundant.

Why?

StreamIndentor indents the future output of the associated stream. We place them wherever we need indentation. We want indentation here - that avoids having to write out manual indentation like the code before did.

right, I missed the print statement at line 74.

ashu-mehra

lgtm, thanks for addressing the concerns.

tstuefe · 2025-07-25T13:33:27Z

Thanks @ashu-mehra and @rkennke !

/integrate

openjdk · 2025-07-25T13:34:33Z

Going to push as commit 75ff7e1.
Since your change was applied there have been 101 commits pushed to the master branch:

06fdb61: 8361964: Remove outdated algorithms from requirements and add PBES2 algorithms
41c94ee: 8363910: Avoid tuning for Power10 CPUs on Linux ppc64le when gcc < 10 is used
518d5f4: 8361871: [GCC static analyzer] complains about use of uninitialized value ckpObject in p11_util.c
... and 98 more: https://git.openjdk.org/jdk/compare/70c1ff7e1505eee11b2a9acd9e94a39cd2c9a932...master

Your commit was automatically rebased without conflicts.

openjdk · 2025-07-25T13:34:41Z

@tstuefe Pushed as commit 75ff7e1.

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

openjdk bot added hotspot hotspot-dev@openjdk.org shenandoah shenandoah-dev@openjdk.org labels Jul 10, 2025

tstuefe changed the title ~~assert_correct~~ JDK-8361712: Improve ShenandoahAsserts printing Jul 10, 2025

openjdk bot changed the title ~~JDK-8361712: Improve ShenandoahAsserts printing~~ 8361712: Improve ShenandoahAsserts printing Jul 10, 2025

tstuefe closed this Jul 11, 2025

tstuefe reopened this Jul 11, 2025

tstuefe added 5 commits July 11, 2025 15:58

assert_correct

27ec3db

wip

18de09e

wip

2a5c9c0

wip

c7aa454

wip

aab9d9c

tstuefe force-pushed the harden-shenandoah-asserts branch from 63b1f4c to aab9d9c Compare July 11, 2025 14:42

tstuefe added 2 commits July 11, 2025 16:46

cosmetics

e4e4404

small revert

94fe071

tstuefe marked this pull request as ready for review July 12, 2025 06:39

openjdk bot added the rfr Pull request is ready for review label Jul 12, 2025

Merge branch 'openjdk:master' into harden-shenandoah-asserts

c7a9c26

rkennke reviewed Jul 17, 2025

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rkennke approved these changes Jul 23, 2025

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ashu-mehra reviewed Jul 24, 2025

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openjdk bot closed this Jul 25, 2025

openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Jul 25, 2025

8361712: Improve ShenandoahAsserts printing #26237

8361712: Improve ShenandoahAsserts printing #26237

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