On Thu, 25 May 2023 15:25:40 GMT, Volker Simonis <[email protected]> wrote:
>> Since JDK13, executing commands in a sub-process defaults to the so called
>> `POSIX_SPAWN` launching mechanism (i.e.
>> `-Djdk.lang.Process.launchMechanism=POSIX_SPAWN`) on Linux. This works by
>> using `posix_spawn(3)` to firstly start a tiny helper program called
>> `jspawnhelper` in a subprocess. In a second step, `jspawnhelper` reads the
>> command data from the parent Java process over a Unix pipe and finally
>> executes (i.e. `execvp(3)`) the requested command.
>>
>> In cases where the parent process terminates abnormally before
>> `jspawnhelper` has read all the expected data from the pipe, `jspawnhelper`
>> will block indefinitely on the reading end of the pipe. This is especially
>> harmful if the parent process had open sockets, because in that case, the
>> forked `jspawnhelper` process will inherit them and keep all the
>> corresponding ports open effectively preventing other processes to bind to
>> them. Notice that this is not an abstract scenario. We've observed this
>> regularly in production with services which couldn't be restarted after a
>> crash after migrating to JDK 17.
>>
>> The fix of the issue is rather trivial. `jspawnhelper` has to close its
>> writing end of the pipe which connects it with the parent Java process
>> *before* starting to read from that pipe such that reads from the pipe can
>> immediately return with EOF if the parent process terminates abnormally.
>>
>> Also did some cleanup:
>> - in `jspawnhelper.c` correctly chek the result of `readFully()`
>> - in `ProcessImpl_md.c` use `restartableWrite()` instead of `write()` to
>> write the command data from the parent process to `jspawnhelper`
>> - in `childproc.{c,h}` remove remaining traces of `MODE_CLONE` because
>> that's long gone.
>
> Volker Simonis has updated the pull request incrementally with one additional
> commit since the last revision:
>
> Fix for failing CloseRace test
Fixing the last bug :)
If we close the writing end of the pipe to jspawnhelper early in `spawnChild()`
right after the last write (which I still think is a good idea), we have to use
`c->childenv` rather than `childenv` when closing the pipe descriptors at the
end of `Java_java_lang_ProcessImpl_forkAndExec()` in order to avoid double
closing:
- closeSafely(childenv[0]);
- closeSafely(childenv[1]);
+ /* We use 'c->childenv' here rather than 'childenv' because 'spawnChild()'
might have
+ * already closed 'c->childenv[1]' and signaled this by setting
'c->childenv[1]' to '-1'.
+ * Otherwise 'c->childenv' and 'childenv' are the same because we just
copied 'childenv'
+ * to 'c->childenv' (with 'copyPipe()') before calling 'startChild()'. */
+ closeSafely(c->childenv[0]);
+ closeSafely(c->childenv[1]);
This also fixes `test/jdk/java/lang/ProcessBuilder/CloseRace.java` which could
failed sporadically the previous version of the change.
-------------
PR Comment: https://git.openjdk.org/jdk/pull/13956#issuecomment-1563105428
