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• Click Create
• Click Continue
• Click Done
• Click on the "My Certificates"
• Double click on your new lldbopenjdk_codesign certificate
• Turn down the "Trust" disclosure triangle

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The next steps are necessary on SnowLeopard, but are probably because of a bug in how Keychain Access makes certificates. (Note: These also apply for Lion.)

• Option-drag the new lldbopenjdk_codesign certificate from the login keychain to the System keychain in the Keychains pane of the main Keychain Access window to make a copy of this certificate in the System keychain. You'll have to authorize a few more times, set it to be "Always trusted" when asked.
• Switch to the System keychain, and drag the copy of lldbopenjdk_codesign you just made there onto the desktop.

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sudo security add-trust -d -r trustRoot -p basic -p codeSign -k /Library/Keychains/System.keychain ~/Desktop/openjdk_codesign.cer

• Right click on the "lldbopenjdk_codesign" certificate in the "System" keychain (NOT "login", but the one in "System"), and select "Delete" to delete it from
  the "System" keychain.
• Reboot (I've found this step unnecessary.)
• Clean and rebuild OpenJDK and you should be able to debug. (You can just remove the jsadebug,jinfo,jmap binaries in build/macosx-universal/bin instead of cleaning everything.)