RFR: 8342449: reimplement: JDK-8327114 Attach in Linux may have wrong behavior when pid == ns_pid [v2]

[Kevin Walls]

On Mon, 28 Oct 2024 20:27:27 GMT, Larry Cable <duke at openjdk.org> wrote:

>> the implementation I originally provided does not in fact solve the issue!
>> 
>> the attach protocol initiation "handshake" requires that the "attacher" (the caller of this code) and the "attachee"(the target JVM to be "attached" to) *must* share a "/tmp" (or access to the attachee's `cwd`)  in common in order to rendezvous on the "attach" socket (created in the /tmp or attachee `cwd` filesystem).
>> 
>> "attacher" and "attachee" JVM processes are guaranteed to share a common directory/filesystem when thy occupy the same "mount namespace", this is the environment in which "peers" exist, and the attach
>> handshake (initiated by the attacher) can use "/tmp" to establish the socket connection with the attachee.
>> 
>> with the advent of "containers" (implemented in Linux via various namespaces, e.g.: pid & mount) another "attacher" and "attachee" relationship exists, that of "attacher" (namespace ancestor) and "attachee" (namespace descendant).
>> 
>> in this environment it is possible (and highly probable) that the "attacher" and the "attachee" do not share a directory in common.
>> 
>> In this scenario the "attacher" must resort to handshaking with the attachee via the /proc filesystem in order to access the "attachee's" directory from the "attacher".
>> 
>> In order to achieve this rendezvous, the "attachee" must occupy a descendant, or same, (pid) namespace of, or as, the "attacher".
>> 
>> since (pid) namespaces are hierarchical, a descendant process (in its own descendent pid namespace) will also occupy all its ancestor (pid) namespaces (between it and the 'root' or 'host' pid namespace) with a unique pid in each of those "interstitial" (pid) namespace(s).
>> 
>> thus the "attachee" directory is accessible, via an "ancestor's" (or peer's) /proc filesystem using the pid of the "attachee" that is associated with it in that (pid) namespace.
>> 
>> thus an "ancestor" "attacher" can handshake with a descendant "attachee" in this fashion.
>> 
>> therefore an "attacher" has two choices when attempting to attach:
>> 
>> - use the /proc/<pid>/root/tmp path to the "attachee's" /tmp (or its cwd)
>>   - this works with both peers and descendants
>> 
>> - use /tmp
>>   - this only works if the "attacher" and "attachee" share a /tmp in common
>> 
>> the obvious choice is to default to /proc/<pid>/root/tmp (or cwd) however there is an issue with this; should the attachee have elevated privileges, the attacher may not have r/w permission on the attachee's /proc/<pid>/root (or cwd) path.
>> 
>> I...
>
> Larry Cable has updated the pull request incrementally with one additional commit since the last revision:
> 
>   JDK-8342449: changed logic of attach loop to throw if target still not ready when timed out and consolidated comments

src/jdk.attach/linux/classes/sun/tools/attach/VirtualMachineImpl.java line 114:

> 112:                         String.format("Unable to open socket file %s: " +
> 113:                           "target process %d doesn't respond within %dms " +
> 114:                           "or HotSpot VM not loaded", socket_path, time_spend));

Do we still need a pid argument after this format string?
time_spent would read more easily than "spend" 8-) but we had have that for years.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21688#discussion_r1821191477