On Fri, 19 May 2023 15:43:30 GMT, Roger Riggs <rri...@openjdk.org> wrote:
>>> Sorry, but I don't understand this argument. If we do a short read we will
>>> work with corrupted ChildStuff and SpawnInfo
>>> structures. This can in the extreme case execute arbitrary code (e.g. if
>>> ChildStuff.argv is not fully read from the parent). You are
>>> basically saying it is better to work on corrupted data rather than
>>> reporting an error.
>>
>> No I am simply pointing out that this has changed more than just the issue
>> with close. And maybe a short-read does indicate data "corruption" and maybe
>> it should be a fatal error. But I don't know exactly how this might manifest
>> so perhaps there are benign short-reads that actually do happen. Regardless
>> it might be better to split this part out and focus on the close issue here.
>
>> > Sorry, but I don't understand this argument. If we do a short read we will
>> > work with corrupted ChildStuff and SpawnInfo
>> > structures. This can in the extreme case execute arbitrary code (e.g. if
>> > ChildStuff.argv is not fully read from the parent). You are
>> > basically saying it is better to work on corrupted data rather than
>> > reporting an error.
>>
>> No I am simply pointing out that this has changed more than just the issue
>> with close. And maybe a short-read does indicate data "corruption" and maybe
>> it should be a fatal error. But I don't know exactly how this might manifest
>> so perhaps there are benign short-reads that actually do happen. Regardless
>> it might be better to split this part out and focus on the close issue here.
>
> Given the purpose and implementation of the `readFully` function, I don't see
> how it can return anything other than an error or the full requested read
> length.
@RogerRiggs , @tstuefe sorry for bothering you one more time, but I think
@mlichtblau brought up an interesting case yesterday which isn't fully resolved
by the current fix. In @mlichtblau example,
`Java_java_lang_ProcessImpl_forkAndExec()` got stuck waiting on the ping from
`jspawnhelper` which itself blocks in `readFully()` because of a truncated
`write()` from the parent.
The current fix already replaces `write()` with `restartableWrite()` which
handles the case where a `write()` call was interrupted **before** writing a
single byte. But there's a second case, namely when `write()` was interrupted
after it already wrote some (but not all) of the requested bytes. The `write()`
man page states:
> Note that a successful `write()` may transfer fewer than `count` bytes. Such
> partial writes can occur for various reasons; for example, because .., or
> because a blocked `write()` to a socket, pipe, or similar was interrupted
> by a signal handler after it had transferred some, but before it had
> transferred all of the requested bytes. In the event of a partial write, the
> caller can make another `write()` call to transfer the remaining bytes.
So in order to safely handle the second case as well, we have to replace
`restartableWrite()` with something like `writeFully()` and that's exactly what
this new commit does. I've also added a new test case which reproduces the
issue by simulating a truncated `write()`.
For your convenience I've tried to explain all the additional changes (except
for trivial cleanups and renamings in the test) below:
Thanks in advance,
Volker
##### `jspawnhelper.c`:
+ // The file descriptor for reporting errors back to our parent we got on
the command
+ // line should be the same like the one in the ChildStuff struct we've
just read.
+ assert(c.fail[1] == fdout);
Just trying to be overly cautious.
##### childproc.{c,h}
-ssize_t restartableWrite(int fd, const void *buf, size_t count);
+ssize_t writeFully(int fd, const void *buf, size_t count);
-ssize_t
-restartableWrite(int fd, const void *buf, size_t count)
...
-}
+ssize_t
+writeFully(int fd, const void *buf, size_t nbyte)
+#ifdef DEBUG
...
+#endif
...
+}
Replace `restartableWrite()` with `writeFully()` which is basically a copy of
`readFully()` (with additional testing code for jtreg). This handles both
cases, when `write()` is interrupted before having written a single byte and
returns EINTR, as well as the case where `write()` already wrote some (but not
all) bytes before it was interrupted.
- restartableWrite(fail_pipe_fd, &code, sizeof(code));
+ if (writeFully(fail_pipe_fd, &code, sizeof(code)) != sizeof(code)) {
+ goto WhyCantJohnnyExec;
+ }
Treat an incomplete write as error and bail out.
- restartableWrite(fail_pipe_fd, &errnum, sizeof(errnum));
+ writeFully(fail_pipe_fd, &errnum, sizeof(errnum));
We're already in the error handling code here (i.e. `WhyCantJohnnyExec`) so not
much we can do. We'll close the pipe and exit with an error anyway.
##### `ProcessImpl_md.c`
- restartableWrite(c->childenv[1], (char *)&magic, sizeof(magic)); // magic
number first
+ if (writeFully(c->childenv[1], (char *)&magic, sizeof(magic)) !=
sizeof(magic)) { // magic number first
+ return -1;
+ }
...
- restartableWrite(c->childenv[1], (char *)c, sizeof(*c));
- restartableWrite(c->childenv[1], (char *)&sp, sizeof(sp));
- restartableWrite(c->childenv[1], buf, bufsize);
+ if (writeFully(c->childenv[1], (char *)c, sizeof(*c)) != sizeof(*c) ||
+ writeFully(c->childenv[1], (char *)&sp, sizeof(sp)) != sizeof(sp) ||
+ writeFully(c->childenv[1], buf, bufsize) != bufsize) {
+ return -1;
+ }
Make sure we've written all the required data to jspawnhelper or return an
error otherwise. The `-1` return value will be handled in
`Java_java_lang_ProcessImpl_forkAndExec()` by throwing an
`IOException("posix_spawn failed")`.
+ /* We're done. Let jspwanhelper know he can't expect any more data from
us. */
+ close(c->childenv[1]);
+ c->childenv[1] = -1;
Close the write end of the pipe to jspawnhelper after we've written all data.
This will prevent `jspawnhelper` to block on the reading side if somthing goes
wrong (e.g. we've not written enough data).
+ // Reset errno to protect against bogus error messages
+ errno = 0;
In `Java_java_lang_ProcessImpl_forkAndExec()` we might call
`throwIOException(env, errno, ..)` in error situations not caused by a failing
system call (e.g. if `startChild()` returned `-1` because of a bad response
from `jspawnhelper`). It is therefor better to reset `errno` at the beginning
of the method to avoid bogus error messages in the generated `IOExceptions`s.
- assert(errnum == CHILD_IS_ALIVE);
if (errnum != CHILD_IS_ALIVE) {
- /* Should never happen since the first thing the spawn
- * helper should do is to send an alive ping to the parent,
- * before doing any subsequent work. */
+ /* This can happen if the spawn helper encounters an error
+ * before or during the handshake with the parent. */
throwIOException(env, 0, "Bad code from spawn helper "
This assertion isn't useful. We'd rather want to see the generated exception if
`errnum != CHILD_IS_ALIVE` instead of crashing the process. Also, the comment
is wrong, because `jspawnhelper` does quite some stuff (e.g. allocating,
reading from parent) before sending the alive ping and if these actions fail,
`jspawnhelper` will send an error code (e.g. `ERR_MALLOC`, `ERR_PIPE`) which is
not equal to `CHILD_IS_ALIVE`.
##### JspawnhelperProtocol.java
+ private static void simulateTruncatedWriteInParent(int stage) throws
Exception {
...
+ }
Added a new test to simualte a truncated write in the parent process to
jspawnhelper communication.
- Optional<String> cmd = ph.info().command();
- if (cmd.isPresent() && cmd.get().endsWith("jspawnhelper")) {
- throw new Exception("jspawnhelper still alive after parent
Java process terminated");
+ try {
+ // Give jspawnhelper a chance to exit gracefully
+ ph.onExit().get(TIMEOUT, TimeUnit.SECONDS);
+ } catch (TimeoutException te) {
+ Optional<String> cmd = ph.info().command();
+ if (cmd.isPresent() && cmd.get().endsWith("jspawnhelper")) {
+ throw new Exception("jspawnhelper still alive after parent
Java process terminated");
+ }
Make the `simulateCrashInParent()` sub-test more robust by allowing
`jspawnhelper` up to `TIMEOUT` seconds to terminate.
-------------
PR Comment: https://git.openjdk.org/jdk/pull/13956#issuecomment-1561271712
