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doc: Add initial USDT documentation

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Both added files are extended in the following commits.

doc/usdt.md is based on earlier work by laanwj.

Co-authored-by: W. J. van der Laan <laanwj@protonmail.com>
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Example scripts for User-space, Statically Defined Tracing (USDT)
=================================================================
This directory contains scripts showcasing User-space, Statically Defined
Tracing (USDT) support for Bitcoin Core on Linux using. For more information on
USDT support in Bitcoin Core see the [USDT documentation].
[USDT documentation]: ../../doc/tracing.md
Examples for the two main eBPF front-ends, [bpftrace] and
[BPF Compiler Collection (BCC)], with support for USDT, are listed. BCC is used
for complex tools and daemons and `bpftrace` is preferred for one-liners and
shorter scripts.
[bpftrace]: https://github.com/iovisor/bpftrace
[BPF Compiler Collection (BCC)]: https://github.com/iovisor/bcc
To develop and run bpftrace and BCC scripts you need to install the
corresponding packages. See [installing bpftrace] and [installing BCC] for more
information. For development there exist a [bpftrace Reference Guide], a
[BCC Reference Guide], and a [bcc Python Developer Tutorial].
[installing bpftrace]: https://github.com/iovisor/bpftrace/blob/master/INSTALL.md
[installing BCC]: https://github.com/iovisor/bcc/blob/master/INSTALL.md
[bpftrace Reference Guide]: https://github.com/iovisor/bpftrace/blob/master/docs/reference_guide.md
[BCC Reference Guide]: https://github.com/iovisor/bcc/blob/master/docs/reference_guide.md
[bcc Python Developer Tutorial]: https://github.com/iovisor/bcc/blob/master/docs/tutorial_bcc_python_developer.md
## Examples
The bpftrace examples contain a relative path to the `bitcoind` binary. By
default, the scripts should be run from the repository-root and assume a
self-compiled `bitcoind` binary. The paths in the examples can be changed, for
example, to point to release builds if needed. See the
[Bitcoin Core USDT documentation] on how to list available tracepoints in your
`bitcoind` binary.
[Bitcoin Core USDT documentation]: ../../doc/tracing.md#listing-available-tracepoints
**WARNING: eBPF programs require root privileges to be loaded into a Linux
kernel VM. This means the bpftrace and BCC examples must be executed with root
privileges. Make sure to carefully review any scripts that you run with root
privileges first!**

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# 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.
## 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->GetAddrName().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)
…
```
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