Interest in Java based attach provider implementation?

[Philippe Marschall]

Hello

Since the audience has been expanded let me summarize what would be 
needed to make jdk.attach free of JNI on Unix (AIX, Linux, macOS). 
jdk.attach currently uses JNI for four different things.

1. reading and writing unix domain sockets
2. accessing uid, guid and permissions of a file
3. sending SIGQUIT to an arbitrary PID, including self in the case of 
self attach
4. sysctl({CTL_KERN, KERN_PROC, KERN_PROC_PID, pid}) on macOS

For 1. and 2. there are already supported Java based APIs and I have 
patches in case somebody is willing to sponsor this work / open a JIRA.

3. kill(SIGQUIT)

This can be done quite easily using FFM as the signature of kill() is 
quite simple. The signature and values are the same an Linux and macOS 
and I assume AIX as well, but I don't have access to a machine to 
verify. Overlap with existing code is with ProcessHandleImpl.destroy0 
which sends SIGKILL or SIGTERM to a PID but performs additional checks. 
So an API to send a signal to a PID would be needed. It is my 
understanding that jdk.internal.misc.Signal only supports sending signal 
to self.

4. sysctl({CTL_KERN, KERN_PROC, KERN_PROC_PID, pid})

This is more complicated as the kinfo_proc and extern_proc structs are 
quite large. jextract is the easiest way to come up with the 
definitions. However the amount of generated code is quite large even 
after filtering. The generated code can be further cut down using manual 
editing. I have not yet looked into replacing not needed struct fields 
with padding. To the best of my knowledge jextract hasn't the been 
included into the JDK build.
Overlap here is in ProcessHandleImpl_macosx.c with the functions 
os_getParentPidAndTimings and getUID but they access different fields of 
extern_proc.
Honestly here I'm not sure why this check is needed at all. The 
reasoning given for the check in VirtualMachineImpl.c would in my option 
apply to Linux and AIX as well but this check is not performed there. 
Removing this check from the macOS implementation would obviously be the 
easiest.

I can't speak for Windows as I'm not familiar enough with Windows APIs.

Cheers
Philippe


On 19.06.25 14:40, Magnus Ihse Bursie wrote:
> Hi Philippe,
> 
> There is an ongoing effort about "Panamization" (that is, adapting it to 
> use FFM instead of JNI) of native code in the JDK in general. This is 
> discussed on the core-libs-dev mailing list. I've cc:ed them. I think it 
> would be beneficial if you coordinate your efforts with the Panamization 
> effort.
> 
> /Magnus
> 
> 
> On 2025-06-15 17:43, Philippe Marschall wrote:
>> Hello
>>
>> I further pursued the approach
>>
>> - Rebased the Linux implementation [1], implemented the permission 
>> check using JSR 203 and the Unix domain sockets using JEP 380.
>> - Applied the same changes to the AIX implementation [2].
>> - Switched Linux to an FFM based kill implementation [3], completely 
>> getting rid of JNI.
>> -  Updated the macOS implementation [4], implemented the permission 
>> check using JSR 203 and the Unix domain sockets using JEP 380.
>> - Switched macOS to an FFM based kill and sysctl implementation [5], 
>> completely getting rid of JNI.
>>
>> I ran the serviceability test suite on Linux and macOS and it passes. 
>> I manually verified that I can attach to JVMs using local builds.
>>
>> I could not test on AIX.
>>
>>  [1] https://github.com/marschall/jdk/ 
>> commit/3a7796daadad7c9d2d85e9e4623f170baecc0e41
>>  [2] https://github.com/marschall/jdk/ 
>> commit/962729e0bfb6b7d86af303f25c6670d407d1d2d9
>>  [3] https://github.com/marschall/jdk/ 
>> commit/7b5f1bf6f55458a7f69f50b8fdf4986e22202559
>>  [4] https://github.com/marschall/jdk/ 
>> commit/93372a124eca6078fde5597c2498b381a4ef5dfa
>>  [5] https://github.com/marschall/jdk/commit/ 
>> c5faf9655bbb85cc3ed9b2a7ef15b08ab83d1d8b
>>
>> Cheers
>> Philippe
>>
>> On 20.04.22 22:13, Philippe Marschall wrote:
>>> Hello
>>>
>>> I hope this is the right mailing list. I recently had a look at the
>>> Linux attach provider implementation and could not help but noticing
>>> that a large part, if not all of it, could be replaced with Java.
>>> Besides getting rid of the C code this should allow us to unify the AIX,
>>> Linux and macOS implementations under a single Unix implementation.
>>>
>>> The permission check can be implemented using JSR 203 [1] to access uid,
>>> gid and file mode and using jdk.internal.misc.VM to get the euid and 
>>> egid.
>>>
>>> Reading and writing to Unix domain sockets can be done through JEP 
>>> 380 [2].
>>>
>>> Sending SIGQUIT to a process could in theory done through JEP 102 [3]
>>> however sending SIGQUIT to self is currently blocked. This is required
>>> for the self attach mechanism. There a very small C function is still
>>> needed for now, this is hopefully portable.
>>>
>>> I did a small prototype [4]. The tier1 suite runs and I can attach to a
>>> local JVM.
>>>
>>> The overhead will likely be a bit higher as we go through more JDK
>>> abstractions.
>>>
>>>   [1] https://jcp.org/en/jsr/detail?id=203
>>>   [2] https://openjdk.java.net/jeps/380
>>>   [3] https://openjdk.java.net/jeps/102
>>>   [4]
>>> https://github.com/marschall/jdk/ 
>>> commit/207dac7e4d1bd65450bbd2c9e14d33fc34b7cebc
>>>
>>> Cheers
>>> Philippe
>>