Debugging a Core Dump with MDB

2022-08-29 ~ / tinkering /

Our story begins here.

fn queue_notify(&self, vq: &Arc<VirtQueue>, ctx: &DispCtx) {
    let handle = tokio::runtime::Handle::current();
    let _guard = handle.enter();
    futures::executor::block_on(self.handle_guest_virtio_request(vq, ctx));

We've landed in a callback we do not control. This callback is not async and we need to call some async code. We know that the process we're running in has a tokio runtime. So we get a handle to that runtime, enter it and attempt to block on a future.

When we reach this code, the process we are running in crashes and dumps its core. There are a number of things to be learned from the core file. The following analysis is done with the illumos mdb tool. A good guide on mdb can be found here

The first thing the core file tells us is that we are in fact experiencing a segfault.

> ::status
debugging core file of propolis-server (64-bit) from masaka
file: /home/ry/src/propolis/target/release/propolis-server
initial argv: /home/ry/src/propolis/target/release/propolis-server run .falcon/router.toml [:
threading model: native threads
status: process terminated by SIGSEGV (Segmentation Fault), addr=fffffbffc44dced0

A stack trace shows us that the Future::poll method is the point of explosion, which is a few calls down the chain from our queue_notify call.

> $G
> > $C
fffffbffc67fe870 <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h0a10623d62628f3a+0x19a()
fffffbffc67fe8b0 std::thread::local::LocalKey<T>::with::he924ca0bd4e6d9cf+0x56()
fffffbffc67fefd0 futures_executor::local_pool::block_on::h7b551178601388b4+0x4c()
fffffbffc67ff700 <propolis::hw::virtio::softnpu::PciVirtioSoftNPUPort as propolis::hw::virtio::VirtioDevice>::queue_notify::h2900034107523958+0x66()
fffffbffc67ff780 propolis::hw::virtio::pci::PciVirtioState::legacy_write::h683ad0199a8250fb+0x274()
fffffbffc67ff900 propolis::hw::virtio::pci::<impl propolis::hw::pci::device::Device for D>::bar_rw::{{closure}}::hea30c7837f72dad6+0x1aa()
fffffbffc67ffa80 propolis::util::regmap::RegMap<ID>::process::h3d39f905a5a91b8b+0x171()
fffffbffc67ffb50 propolis::hw::pci::device::<impl propolis::hw::pci::Endpoint for D>::bar_rw::h883683c8494c42ce+0x134()
fffffbffc67ffc50 propolis::pio::PioBus::handle_out::h0f978df375855073+0x2aa()
fffffbffc67ffe20 propolis::vcpu_run_loop::h5a6c9e2b54e47a88+0x286()
fffffbffc67ffe50 core::ops::function::FnOnce::call_once{{vtable.shim}}::h96a1f70aa2cd73b4+0x1e()
fffffbffc67ffee0 std::sys_common::backtrace::__rust_begin_short_backtrace::h0c5e358dfc29343a +0x90()
fffffbffc67fff60 core::ops::function::FnOnce::call_once{{vtable.shim}}::hffcdb87c11bdff96+0x95()
fffffbffc67fffb0 std::sys::unix::thread::Thread::new::thread_start::h5ec8d723f4048251+0x27()

pro-tip: the $G here turns on demangling support in mdb without which your eyes will bleed over the rust compilers mangling of method names.

Let's take a closer look at the point of explosion. In the below output Future is substituted for <core::future::from_generator::GenFuture<T> as core::future::future::Future> to make the output a bit more readable.

Below we're asking mdb to disassemble instructions around the instruction pointer at the time of the crash. mdb highlights the function that segfaulted, which does not go well with markdown code blocks, so I've identified the crashing line here with a sequence of exclamation points.

> <rip::dis -n 0xe
Future::poll::h0a10623d62628f3a+0x151:    movq   0xc0(%r14),%rdi
Future::poll::h0a10623d62628f3a+0x158:    call   *0x18(%rax)
Future::poll::h0a10623d62628f3a+0x15b:    movq   0x28(%r15),%rsi
Future::poll::h0a10623d62628f3a+0x15f:    testq  %rsi,%rsi
Future::poll::h0a10623d62628f3a+0x162:    je     +0x34e   <Future::poll::h0a10623d62628f3a+0x4b6>
Future::poll::h0a10623d62628f3a+0x168:    movq   0x18(%r14),%rbx
Future::poll::h0a10623d62628f3a+0x16c:    movl   (%rbx),%edx
Future::poll::h0a10623d62628f3a+0x16e:    movq   0x98(%r14),%rax
Future::poll::h0a10623d62628f3a+0x175:    movq   %rax,-0xa0(%rbp)
Future::poll::h0a10623d62628f3a+0x17c:    movups 0x88(%r14),%xmm0
Future::poll::h0a10623d62628f3a+0x184:    movaps %xmm0,-0xb0(%rbp)
Future::poll::h0a10623d62628f3a+0x18b:    movq   0x30(%r15),%rax
Future::poll::h0a10623d62628f3a+0x18f:    leaq   -0x58(%rbp),%rdi
Future::poll::h0a10623d62628f3a+0x193:    leaq   -0xb0(%rbp),%rcx
Future::poll::h0a10623d62628f3a+0x19a:    call   *0x18(%rax) !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Future::poll::h0a10623d62628f3a+0x19d:    cmpq   $0x0,-0x58(%rbp)
Future::poll::h0a10623d62628f3a+0x1a2:    je     +0x46    <Future::poll::h0a10623d62628f3a+0x1ea>
Future::poll::h0a10623d62628f3a+0x1a4:    leaq   0x8(%rbx),%rcx
Future::poll::h0a10623d62628f3a+0x1a8:    addq   $0x30,%rbx
Future::poll::h0a10623d62628f3a+0x1ac:    movq   -0x38(%rbp),%rax
Future::poll::h0a10623d62628f3a+0x1b0:    movq   %rax,-0x60(%rbp)
Future::poll::h0a10623d62628f3a+0x1b4:    movups -0x58(%rbp),%xmm0
Future::poll::h0a10623d62628f3a+0x1b8:    movups -0x48(%rbp),%xmm1
Future::poll::h0a10623d62628f3a+0x1bc:    movaps %xmm1,-0x70(%rbp)
Future::poll::h0a10623d62628f3a+0x1c0:    movaps %xmm0,-0x80(%rbp)
Future::poll::h0a10623d62628f3a+0x1c4:    movl   -0x30(%rbp),%edx
Future::poll::h0a10623d62628f3a+0x1c7:    leaq   -0x80(%rbp),%rdi
Future::poll::h0a10623d62628f3a+0x1cb:    movq   %rbx,%rsi
Future::poll::h0a10623d62628f3a+0x1ce:    call   +0x233ed <propolis::hw::virtio::softnpu::PciVirtioSoftNPUPort::handle_packet_to_ext_port::h4103b5a93ebf77ce>

Here we see the crashing instruction is call *0x18(%rax). We can see what the value of rax is by dumping the registers at the time of the crash.

> $r
%rax = 0xfffffbffc44dceb8       %r8  = 0x0000000000000000
%rbx = 0x0000000002c2c3f0       %r9  = 0x0000000016ba6f00
%rcx = 0xfffffbffc67fe7c0       %r10 = 0x0000000000000501
%rdx = 0x0000000000000000       %r11 = 0x0000000000000000
%rsi = 0x0000000002f5b550       %r12 = 0x0000000000000063
%rdi = 0xfffffbffc67fe818       %r13 = 0xfffffbffc67fe818
                                %r14 = 0xfffffbffc67fe8c8
                                %r15 = 0x0000000002c8f5c0

%cs = 0x0053    %fs = 0x0000    %gs = 0x0000
%ds = 0x004b    %es = 0x004b    %ss = 0x004b

%rip = 0x000000000139a03a <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::h0a10623d62628f3a+0x19a
%rbp = 0xfffffbffc67fe870
%rsp = 0xfffffbffc67fe7a0

%rflags = 0x00010206
  id=0 vip=0 vif=0 ac=0 vm=0 rf=1 nt=0 iopl=0x0

%gsbase = 0x0000000000000000
%fsbase = 0xfffffbffee0e1a40
%trapno = 0xe
   %err = 0x4

So here we see the value of rax is 0xfffffbffc44dceb8 and if we add an offset of 0x18 to that which is what the call above is doing, we land at an address of fffffbffc44dced0 which is what our ::status dcmd above reported as the segfault address.

In the disassembly above we can see a call to handle_packet_to_ext_port. The corresponding rust code looks like this.

fn handle_guest_packet<'a>(
    index: usize,
    mut pkt: packet_in<'a>,
    data_handles: &Vec<dlpi::DlpiHandle>,
    pipeline: &mut Box<dyn Pipeline>,
    log: &Logger,
) {
    match pipeline.process_packet(index as u8, &mut pkt) {
        Some((mut out_pkt, port)) => {
                &mut out_pkt,
        None => {}

In the assembly code there is only one control flow instruction between the call to handle_packet_to_ext_port and the site of our crash. This must be the match statement in the code above which means that the crashing call must be the call to process_packet.

So now the question becomes why is the call to the pipeline a segmentation fault. To answer this question we need to look at where the pipeline::process_packet address comes from. This is a dynamically loaded program trait object. The loading code looks like this.

async fn load_program(
    pipeline: Arc<tokio::sync::Mutex<Option<Box<dyn Pipeline>>>>,
    log: Logger,
) {

    let lib = match unsafe { libloading::Library::new("/tmp/") } {
        Ok(l) => l,
        Err(e) => {
            warn!(log, "failed to load p4 program: {}", e);
    let func: libloading::Symbol<
        unsafe extern "C" fn() -> *mut dyn p4rs::Pipeline,
        > = match unsafe { lib.get(b"_main_pipeline_create") } {
            Ok(f) => f,
            Err(e) => {
                    "failed to load _main_pipeline_create func: {}", e

    let mut pl = pipeline.lock().await;
    let _ = pl.insert(unsafe { Box::from_raw(func()) });

Reading this code now with the insight of where the crash is occurring, I have a guess as to what's happening. At the end of this function the lib function gets dropped. Which may unload from this process' memory space, the pipeline implementation we are attempting to process packets with at the site of the crash. A look at the drop implementation for the internal Library data structure used by libloading, we see that dlclose is called on drop.

From the man page of dlclose.

Once an object has been closed by dlclose(), referencing symbols contained in that object can cause undefined behavior.

dlclose removes any objects from a corresponding dlopen from the address space of the calling process. This is consistent with what we are seeing.