# User-space, Statically Defined Tracing (USDT) for Bitcoin Core

Bitcoin Core includes statically defined tracepoints to allow for more
observability during development, debugging, code review, and production usage.
These tracepoints make it possible to keep track of custom statistics and
enable detailed monitoring of otherwise hidden internals. They have
little to no performance impact when unused.

```
eBPF and USDT Overview
======================

                ┌──────────────────┐            ┌──────────────┐
                │ tracing script   │            │ bitcoind     │
                │==================│      2.    │==============│
                │  eBPF  │ tracing │      hooks │              │
                │  code  │ logic   │      into┌─┤►tracepoint 1─┼───┐ 3.
                └────┬───┴──▲──────┘          ├─┤►tracepoint 2 │   │ pass args
            1.       │      │ 4.              │ │ ...          │   │ to eBPF
    User    compiles │      │ pass data to    │ └──────────────┘   │ program
    Space    & loads │      │ tracing script  │                    │
    ─────────────────┼──────┼─────────────────┼────────────────────┼───
    Kernel           │      │                 │                    │
    Space       ┌──┬─▼──────┴─────────────────┴────────────┐       │
                │  │  eBPF program                         │◄──────┘
                │  └───────────────────────────────────────┤
                │ eBPF kernel Virtual Machine (sandboxed)  │
                └──────────────────────────────────────────┘

1. The tracing script compiles the eBPF code and loads the eBPF program into a kernel VM
2. The eBPF program hooks into one or more tracepoints
3. When the tracepoint is called, the arguments are passed to the eBPF program
4. The eBPF program processes the arguments and returns data to the tracing script
```

The Linux kernel can hook into the tracepoints during runtime and pass data to
sandboxed [eBPF] programs running in the kernel. These eBPF programs can, for
example, collect statistics or pass data back to user-space scripts for further
processing.

[eBPF]: https://ebpf.io/

The two main eBPF front-ends with support for USDT are [bpftrace] and
[BPF Compiler Collection (BCC)]. BCC is used for complex tools and daemons and
`bpftrace` is preferred for one-liners and shorter scripts. Examples for both can
be found in [contrib/tracing].

[bpftrace]: https://github.com/iovisor/bpftrace
[BPF Compiler Collection (BCC)]: https://github.com/iovisor/bcc
[contrib/tracing]: ../contrib/tracing/

## Tracepoint documentation

The currently available tracepoints are listed here.

### Context `net`

#### Tracepoint `net:inbound_message`

Is called when a message is received from a peer over the P2P network. Passes
information about our peer, the connection and the message as arguments.

Arguments passed:
1. Peer ID as `int64`
2. Peer Address and Port (IPv4, IPv6, Tor v3, I2P, ...) as `pointer to C-style String` (max. length 68 characters)
3. Connection Type (inbound, feeler, outbound-full-relay, ...) as `pointer to C-style String` (max. length 20 characters)
4. Message Type (inv, ping, getdata, addrv2, ...) as `pointer to C-style String` (max. length 20 characters)
5. Message Size in bytes as `uint64`
6. Message Bytes as `pointer to unsigned chars` (i.e. bytes)

Note: The message is passed to the tracepoint in full, however, due to space
limitations in the eBPF kernel VM it might not be possible to pass the message
to user-space in full. Messages longer than a 32kb might be cut off. This can
be detected in tracing scripts by comparing the message size to the length of
the passed message.

#### Tracepoint `net:outbound_message`

Is called when a message is sent to a peer over the P2P network. Passes
information about our peer, the connection and the message as arguments.

Arguments passed:
1. Peer ID as `int64`
2. Peer Address and Port (IPv4, IPv6, Tor v3, I2P, ...) as `pointer to C-style String` (max. length 68 characters)
3. Connection Type (inbound, feeler, outbound-full-relay, ...) as `pointer to C-style String` (max. length 20 characters)
4. Message Type (inv, ping, getdata, addrv2, ...) as `pointer to C-style String` (max. length 20 characters)
5. Message Size in bytes as `uint64`
6. Message Bytes as `pointer to unsigned chars` (i.e. bytes)

Note: The message is passed to the tracepoint in full, however, due to space
limitations in the eBPF kernel VM it might not be possible to pass the message
to user-space in full. Messages longer than a 32kb might be cut off. This can
be detected in tracing scripts by comparing the message size to the length of
the passed message.

### Context `validation`

#### Tracepoint `validation:block_connected`

Is called *after* a block is connected to the chain. Can, for example, be used
to benchmark block connections together with `-reindex`.

Arguments passed:
1. Block Header Hash as `pointer to unsigned chars` (i.e. 32 bytes in little-endian)
2. Block Height as `int32`
3. Transactions in the Block as `uint64`
4. Inputs spend in the Block as `int32`
5. SigOps in the Block (excluding coinbase SigOps) `uint64`
6. Time it took to connect the Block in microseconds (µs) as `uint64`

### Context `utxocache`

The following tracepoints cover the in-memory UTXO cache. UTXOs are, for example,
added to and removed (spent) from the cache when we connect a new block.
**Note**: Bitcoin Core uses temporary clones of the _main_ UTXO cache
(`chainstate.CoinsTip()`). For example, the RPCs `generateblock` and
`getblocktemplate` call `TestBlockValidity()`, which applies the UTXO set
changes to a temporary cache. Similarly, mempool consistency checks, which are
frequent on regtest, also apply the UTXO set changes to a temporary cache.
Changes to the _main_ UTXO cache and to temporary caches trigger the tracepoints.
We can't tell if a temporary cache or the _main_ cache was changed.

#### Tracepoint `utxocache:flush`

Is called *after* the in-memory UTXO cache is flushed.

Arguments passed:
1. Time it took to flush the cache microseconds as `int64`
2. Flush state mode as `uint32`. It's an enumerator class with values `0`
   (`NONE`), `1` (`IF_NEEDED`), `2` (`PERIODIC`), `3` (`ALWAYS`)
3. Cache size (number of coins) before the flush as `uint64`
4. Cache memory usage in bytes as `uint64`
5. If pruning caused the flush as `bool`

#### Tracepoint `utxocache:add`

Is called when a coin is added to a UTXO cache. This can be a temporary UTXO cache too.

Arguments passed:
1. Transaction ID (hash) as `pointer to unsigned chars` (i.e. 32 bytes in little-endian)
2. Output index as `uint32`
3. Block height the coin was added to the UTXO-set as  `uint32`
4. Value of the coin as `int64`
5. If the coin is a coinbase as `bool`

#### Tracepoint `utxocache:spent`

Is called when a coin is spent from a UTXO cache. This can be a temporary UTXO cache too.

Arguments passed:
1. Transaction ID (hash) as `pointer to unsigned chars` (i.e. 32 bytes in little-endian)
2. Output index as `uint32`
3. Block height the coin was spent, as `uint32`
4. Value of the coin as `int64`
5. If the coin is a coinbase as `bool`

#### Tracepoint `utxocache:uncache`

Is called when a coin is purposefully unloaded from a UTXO cache. This
happens, for example, when we load an UTXO into a cache when trying to accept
a transaction that turns out to be invalid. The loaded UTXO is uncached to avoid
filling our UTXO cache up with irrelevant UTXOs.

Arguments passed:
1. Transaction ID (hash) as `pointer to unsigned chars` (i.e. 32 bytes in little-endian)
2. Output index as `uint32`
3. Block height the coin was uncached, as `uint32`
4. Value of the coin as `int64`
5. If the coin is a coinbase as `bool`

### Context `coin_selection`

#### Tracepoint `coin_selection:selected_coins`

Is called when `SelectCoins` completes.

Arguments passed:
1. Wallet name as `pointer to C-style string`
2. Coin selection algorithm name as `pointer to C-style string`
3. Selection target value as `int64`
4. Calculated waste metric of the solution as `int64`
5. Total value of the selected inputs as `int64`

#### Tracepoint `coin_selection:normal_create_tx_internal`

Is called when the first `CreateTransactionInternal` completes.

Arguments passed:
1. Wallet name as `pointer to C-style string`
2. Whether `CreateTransactionInternal` succeeded as `bool`
3. The expected transaction fee as an `int64`
4. The position of the change output as an `int32`

#### Tracepoint `coin_selection:attempting_aps_create_tx`

Is called when `CreateTransactionInternal` is called the second time for the optimistic
Avoid Partial Spends selection attempt. This is used to determine whether the next
tracepoints called are for the Avoid Partial Spends solution, or a different transaction.

Arguments passed:
1. Wallet name as `pointer to C-style string`

#### Tracepoint `coin_selection:aps_create_tx_internal`

Is called when the second `CreateTransactionInternal` with Avoid Partial Spends enabled completes.

Arguments passed:
1. Wallet name as `pointer to C-style string`
2. Whether the Avoid Partial Spends solution will be used as `bool`
3. Whether `CreateTransactionInternal` succeeded as` bool`
4. The expected transaction fee as an `int64`
5. The position of the change output as an `int32`

## Adding tracepoints to Bitcoin Core

To add a new tracepoint, `#include <util/trace.h>` in the compilation unit where
the tracepoint is inserted. Use one of the `TRACEx` macros listed below
depending on the number of arguments passed to the tracepoint. Up to 12
arguments can be provided. The `context` and `event` specify the names by which
the tracepoint is referred to. Please use `snake_case` and try to make sure that
the tracepoint names make sense even without detailed knowledge of the
implementation details. Do not forget to update the tracepoint list in this
document.

```c
#define TRACE(context, event)
#define TRACE1(context, event, a)
#define TRACE2(context, event, a, b)
#define TRACE3(context, event, a, b, c)
#define TRACE4(context, event, a, b, c, d)
#define TRACE5(context, event, a, b, c, d, e)
#define TRACE6(context, event, a, b, c, d, e, f)
#define TRACE7(context, event, a, b, c, d, e, f, g)
#define TRACE8(context, event, a, b, c, d, e, f, g, h)
#define TRACE9(context, event, a, b, c, d, e, f, g, h, i)
#define TRACE10(context, event, a, b, c, d, e, f, g, h, i, j)
#define TRACE11(context, event, a, b, c, d, e, f, g, h, i, j, k)
#define TRACE12(context, event, a, b, c, d, e, f, g, h, i, j, k, l)
```

For example:

```C++
TRACE6(net, inbound_message,
    pnode->GetId(),
    pnode->m_addr_name.c_str(),
    pnode->ConnectionTypeAsString().c_str(),
    sanitizedType.c_str(),
    msg.data.size(),
    msg.data.data()
);
```

### Guidelines and best practices

#### Clear motivation and use case
Tracepoints need a clear motivation and use case. The motivation should
outweigh the impact on, for example, code readability. There is no point in
adding tracepoints that don't end up being used.

#### Provide an example
When adding a new tracepoint, provide an example. Examples can show the use case
and help reviewers testing that the tracepoint works as intended. The examples
can be kept simple but should give others a starting point when working with
the tracepoint. See existing examples in [contrib/tracing/].

[contrib/tracing/]: ../contrib/tracing/

#### No expensive computations for tracepoints
Data passed to the tracepoint should be inexpensive to compute. Although the
tracepoint itself only has overhead when enabled, the code to compute arguments
is always run - even if the tracepoint is not used. For example, avoid
serialization and parsing.

#### Semi-stable API
Tracepoints should have a semi-stable API. Users should be able to rely on the
tracepoints for scripting. This means tracepoints need to be documented, and the
argument order ideally should not change. If there is an important reason to
change argument order, make sure to document the change and update the examples
using the tracepoint.

#### eBPF Virtual Machine limits
Keep the eBPF Virtual Machine limits in mind. eBPF programs receiving data from
the tracepoints run in a sandboxed Linux kernel VM. This VM has a limited stack
size of 512 bytes. Check if it makes sense to pass larger amounts of data, for
example, with a tracing script that can handle the passed data.

#### `bpftrace` argument limit
While tracepoints can have up to 12 arguments, bpftrace scripts currently only
support reading from the first six arguments (`arg0` till `arg5`) on `x86_64`.
bpftrace currently lacks real support for handling and printing binary data,
like block header hashes and txids. When a tracepoint passes more than six
arguments, then string and integer arguments should preferably be placed in the
first six argument fields. Binary data can be placed in later arguments. The BCC
supports reading from all 12 arguments.

#### Strings as C-style String
Generally, strings should be passed into the `TRACEx` macros as pointers to
C-style strings (a null-terminated sequence of characters). For C++
`std::strings`, [`c_str()`]  can be used. It's recommended to document the
maximum expected string size if known.


[`c_str()`]: https://www.cplusplus.com/reference/string/string/c_str/


## Listing available tracepoints

Multiple tools can list the available tracepoints in a `bitcoind` binary with
USDT support.

### GDB - GNU Project Debugger

To list probes in Bitcoin Core, use `info probes` in `gdb`:

```
$ gdb ./src/bitcoind
…
(gdb) info probes
Type Provider   Name             Where              Semaphore Object
stap net        inbound_message  0x000000000014419e /src/bitcoind
stap net        outbound_message 0x0000000000107c05 /src/bitcoind
stap validation block_connected  0x00000000002fb10c /src/bitcoind
…
```

### With `readelf`

The `readelf` tool can be used to display the USDT tracepoints in Bitcoin Core.
Look for the notes with the description `NT_STAPSDT`.

```
$ readelf -n ./src/bitcoind | grep NT_STAPSDT -A 4 -B 2
Displaying notes found in: .note.stapsdt
  Owner                 Data size	Description
  stapsdt              0x0000005d	NT_STAPSDT (SystemTap probe descriptors)
    Provider: net
    Name: outbound_message
    Location: 0x0000000000107c05, Base: 0x0000000000579c90, Semaphore: 0x0000000000000000
    Arguments: -8@%r12 8@%rbx 8@%rdi 8@192(%rsp) 8@%rax 8@%rdx
…
```

### With `tplist`

The `tplist` tool is provided by BCC (see [Installing BCC]). It displays kernel
tracepoints or USDT probes and their formats (for more information, see the
[`tplist` usage demonstration]). There are slight binary naming differences
between distributions. For example, on
[Ubuntu the binary is called `tplist-bpfcc`][ubuntu binary].

[Installing BCC]: https://github.com/iovisor/bcc/blob/master/INSTALL.md
[`tplist` usage demonstration]: https://github.com/iovisor/bcc/blob/master/tools/tplist_example.txt
[ubuntu binary]: https://github.com/iovisor/bcc/blob/master/INSTALL.md#ubuntu---binary

```
$ tplist -l ./src/bitcoind -v
b'net':b'outbound_message' [sema 0x0]
  1 location(s)
  6 argument(s)
…
```
