During the SA attach process on macOS, SA installs an exception handler and
expects to get an EXC_SOFTWARE exception for the expected SIGSTOP signal. On
aarch64, sometimes it instead gets an EXC_BAD_INSTRUCTION exception. I found if
I changed the code to just ignore this, it eventually gets the EXC_SOFTWARE
exception and everything is fine.
The EXC_BAD_INSTRUCTION is from the following code in the JIT and is expected
behavior:
bool NativeInstruction::is_sigill_zombie_not_entrant() {
return uint_at(0) == 0xd4bbd5a1; // dcps1 #0xdead
}
void NativeIllegalInstruction::insert(address code_pos) {
*(juint*)code_pos = 0xd4bbd5a1; // dcps1 #0xdead
}
void NativeJump::patch_verified_entry(address entry, address verified_entry,
address dest) {
...
if (Assembler::reachable_from_branch_at(verified_entry, dest)) {
...
} else {
// We use an illegal instruction for marking a method as
// not_entrant or zombie.
NativeIllegalInstruction::insert(verified_entry);
}
I fixed SA to retry each time it gets EXC_BAD_INSTRUCTION. I also did the same
for EXC_BAD_ACCESS. We saw this once on x64, and it looks like it is for a
trap-based NPE, given that the bad address that was provided as part of the
exception data is for address 0x8.
Although the fix is somewhat straight forward, the context in which it is run
is not. SA does a `ptrace_attach()`. This seems to result in a SIGSTOP being
generated. SA provides an exception handler called
`catch_mach_exception_raise()`. It gets called for any pending exception when
SA calls `mach_msg(&exc_msg.header, MACH_RCV_MSG, ...)` from
`wait_for_exception()`. So `catch_mach_exception_raise()` should see the
SIGSTOP, return KERN_SUCCESS (which means to ignore the exception), and then
`mach_msg()` returns to `wait_for_exception()`, which checks for success of
having received the SIGSTOP, and then allows SA to continue with its attach
process (like suspending all threads).
The details of attaching and exception handling are actually much uglier than
that, and I don't pretend to even understand half of it.
https://www.spaceflint.com/?p=150 is an interesting read if you are looking for
more details.
-------------
Commit messages:
- Fix issues with unexcpected exceptions during attach.
Changes: https://git.openjdk.java.net/jdk/pull/8255/files
Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8255&range=00
Issue: https://bugs.openjdk.java.net/browse/JDK-8283179
Stats: 40 lines in 1 file changed: 25 ins; 0 del; 15 mod
Patch: https://git.openjdk.java.net/jdk/pull/8255.diff
Fetch: git fetch https://git.openjdk.java.net/jdk pull/8255/head:pull/8255
PR: https://git.openjdk.java.net/jdk/pull/8255
