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8315038: Capstone disassembler stops when it sees a bad instruction #15763

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theRealAph
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@theRealAph theRealAph commented Sep 15, 2023

At present, the Capstone disassembler stops whenever it encounters an undefined instruction. We really need it not to do that, because we use undefined instructions in JIT-generated code for many things.

The fix is described here:
https://www.capstone-engine.org/skipdata.html


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  • JDK-8315038: Capstone disassembler stops when it sees a bad instruction (Bug - P4)

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👋 Welcome back aph! 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 openjdk bot added the rfr Pull request is ready for review label Sep 15, 2023
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@theRealAph The following labels will be automatically applied to this pull request:

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Some thoughts: that linked page claims this is only available on the next branch, but I see this option is available in the 5.0.1 release from 3 weeks ago (https://github.com/capstone-engine/capstone/releases/tag/5.0.1). That is good.

The page also explains that this option works by skipping bytes until capstone finds something that it can disassemble, which might not be correct. If there is an issue with the approach proposed by this patch in the future, we might want to look at the CS_OPT_SKIPDATA_SETUP, which seems to allow specifying a custom callback that figures out the number of bytes to skip. (Do you have any thoughts about that?). Either way, this patch seems like an improvement over the status quo, so I think we can move forward with it.

@@ -150,6 +150,9 @@ void* decode_instructions_virtual(uintptr_t start_va, uintptr_t end_va,
Options ops = parse_options(options, printf_callback, printf_stream);
cs_option(cs_handle, CS_OPT_SYNTAX, ops.intel_syntax ? CS_OPT_SYNTAX_INTEL : CS_OPT_SYNTAX_ATT);

// Turn on SKIPDATA mode
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The comment is self-evident.

Suggested change
// Turn on SKIPDATA mode
// Turn on SKIPDATA mode to skip broken instructions, which are ubiquitous in JIT-generated code.

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OK now?

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openjdk bot commented Sep 15, 2023

@theRealAph 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:

8315038: Capstone disassembler stops when it sees a bad instruction

Reviewed-by: jvernee, shade

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At the time when this comment was updated there had been 172 new commits pushed to the master branch:

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  • 1203e11: 8294969: Convert jdk.jdeps javap to use the Classfile API
  • fbc766e: 8315988: Parallel: Make TestAggressiveHeap use createTestJvm
  • aa0ebee: 8316341: sun/security/pkcs11/PKCS11Test.java needs adjustment on Linux ppc64le Ubuntu 22
  • ca3fe7b: 8315932: runtime/InvocationTests spend a lot of time on dependency verification
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  • 4b8f5d0: 8316273: JDK-8315818 broke JVMCIPrintProperties on libgraal
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  • c92bdb0: 8316187: Modernize examples in StringTokenizer and {Date,Number}Format
  • 8f46abc: 8315889: Open source several Swing HTMLDocument related tests
  • ... and 162 more: https://git.openjdk.org/jdk/compare/cef9fff0675c85906e134e4c0a8d2036860cd4b3...master

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@openjdk openjdk bot added the ready Pull request is ready to be integrated label Sep 15, 2023
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Some thoughts: that linked page claims this is only available on the next branch, but I see this option is available in the 5.0.1 release from 3 weeks ago (https://github.com/capstone-engine/capstone/releases/tag/5.0.1). That is good.

Yeah. I was wondering about whether to do a #ifdef CS_OPT_SKIPDATA backwards-compatibility hack, but a disassembler that just stops in strange places is so misleading as to be a liability.

The page also explains that this option works by skipping bytes until capstone finds something that it can disassemble, which might not be correct. If there is an issue with the approach proposed by this patch in the future, we might want to look at the CS_OPT_SKIPDATA_SETUP, which seems to allow specifying a custom callback that figures out the number of bytes to skip. (Do you have any thoughts about that?).

The default seems pretty smart in the way that it works, and I doubt we can do any better.

The main use of undefined "instructions" in compiled methods is RISC processors, which tend to disallow concurrent code patching. To solve that problem we've used embedded data that's accessed by PC-relative loads. Because RISCs tend to have fixed-size instructions, SKIPDATA always gets it right.

Either way, this patch seems like an improvement over the status quo, so I think we can move forward with it.

Thanks.

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Looks okay to me.

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/integrate

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openjdk bot commented Sep 19, 2023

Going to push as commit 670b456.
Since your change was applied there have been 191 commits pushed to the master branch:

  • fab372d: 8316428: G1: Nmethod count statistics only count last code root set iterated
  • 283c360: 8314877: Make fields final in 'java.net' package
  • 86115c2: 8316420: Serial: Remove unused GenCollectedHeap::oop_iterate
  • d038571: 8030815: Code roots are not accounted for in region prediction
  • 138542d: 8316061: Open source several Swing RootPane and Slider related tests
  • f52e500: 8316104: Open source several Swing SplitPane and RadioButton related tests
  • 3760a04: 8314021: HeapDump: Optimize segmented heap file merging phase
  • f226ceb: 8316440: LambdasInTwoArchives.java failed to find WhiteBox.class
  • 24c3d86: 8315804: Open source several Swing JTabbedPane JTextArea JTextField tests
  • 373e37b: 8313813: Field sun.util.calendar.CalendarDate#forceStandardTime is never set
  • ... and 181 more: https://git.openjdk.org/jdk/compare/cef9fff0675c85906e134e4c0a8d2036860cd4b3...master

Your commit was automatically rebased without conflicts.

@openjdk openjdk bot added the integrated Pull request has been integrated label Sep 19, 2023
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@openjdk openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Sep 19, 2023
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@theRealAph Pushed as commit 670b456.

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