Add a chapter about logging custom structs with PerfEventArray

Signed-off-by: Michal Rostecki <vadorovsky@gmail.com>

Author: vadorovsky

docs/book/start/getting-data-to-user-space.md

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+# Getting data to user-space
+
+In previous chapters we were logging packets with aya-log. However, what if we
+need to send additional data about the packet or any other type of information
+for the user-space program to utilize? In this chapter, we will explore how to
+leverage perf buffers and define data structures in the *-common* crate to
+transfer data from eBPF program to user-space applications. By doing so,
+user-space programs can access and utilize the transferred data effectively.
+
+!!! example "Source code"
+
+    Full code for the example in this chapter is available
+    [here](https://github.com/aya-rs/book/tree/main/examples/xdp-perfbuf-custom-data)
+
+## Sharing data
+
+In this chapter, we'll be sending data from the kernel-space to the user-space
+by writing it into a struct and outputting it to user-space. We can achieve this
+by using an eBPF map. There are different types of maps available, but in this
+case, we'll use `PerfEventArray`.
+
+`PerfEventArray` is a collection of per-CPU circular buffers that enable the
+kernel to emit events (defined as custom structs) to user-space. Each CPU has
+its own buffer, and the eBPF program emits an event to the buffer of the CPU
+it's currently running on. The events are out of order, meaning that they arrive
+in the user-space in a different order than they were created and sent from the
+eBPF program.
+
+To gather events from all CPUs, a user-space program needs to spawn a thread for
+each CPU to poll for the events and then iterate over them.
+
+The data structure we'll be using needs to hold an IPv4 address and a port.
+
+```rust linenums="1" title="xdp-perfbuf-custom-data-common/src/lib.rs"
+--8<-- "examples/xdp-perfbuf-custom-data/xdp-perfbuf-custom-data-common/src/lib.rs"
+```
+
+1. We implement the `aya::Pod` trait for our struct since it is Plain Old Data
+as can be safely converted to a byte-slice and back.
+
+!!! tip "Alignment, padding and verifier errors"
+
+    At program load time, the eBPF verifier checks that all the memory used is
+    properly initialized. This can be a problem if - to ensure alignment - the
+    compiler inserts padding bytes between fields in your types.
+
+    **Example:**
+
+    ```rust
+    #[repr(C)]
+    struct SourceInfo {
+        source_port: u16,
+        source_ip: u32,
+    }
+
+    let source_port = ...;
+    let source_ip = ...;
+    let si = SourceInfo { source_port, source_ip };
+    ```
+
+    In the example above, the compiler will insert two extra bytes between the
+    struct fields `source_port` and `source_ip` to make sure that `source_ip` is
+    correctly aligned to a 4 bytes address (assuming `mem::align_of::<u32>() ==
+    4`).  Since padding bytes are typically not initialized by the compiler,
+    this will result in the infamous `invalid indirect read from stack` verifier
+    error.
+
+    To avoid the error, you can either manually ensure that all the fields in
+    your types are correctly aligned (e.g. by explicitly adding padding or by
+    making field types larger to enforce alignment) or use `#[repr(packed)]`.
+    Since the latter comes with its own foot-guns and can perform less
+    efficiently, explicitly adding padding or tweaking alignment is recommended.
+
+    **Solution ensuring alignment using larger types:**
+
+    ```rust
+    #[repr(C)]
+    struct SourceInfo {
+        source_port: u32,
+        source_ip: u32,
+    }
+
+    let source_port = ...;
+    let source_ip = ...;
+    let si = SourceInfo { source_port, source_ip };
+    ```
+
+    **Solution with explicit padding:**
+
+    ```rust
+    #[repr(C)]
+    struct SourceInfo {
+        source_port: u16,
+        padding: u16,
+        source_ip: u32,
+    }
+
+    let source_port = ...;
+    let source_ip = ...;
+    let si = SourceInfo { source_port, padding: 0, source_ip };
+    ```
+
+## Getting packet data from the context and into the map
+
+Our eBPF program code is going to be similar to the one in the previous chapters.
+It's going to get the information about source IP address and port from packet
+headers.
+
+The difference is that once we get the data from headers, instead of logging
+them, we create a `PacketLog` struct and output it to our `PerfEventArray`
+
+The resulting code looks like this:
+
+```rust linenums="1" title="xdp-perfbuf-custom-data-ebpf/src/main.rs"
+--8<-- "examples/xdp-perfbuf-custom-data/xdp-perfbuf-custom-data-ebpf/src/main.rs"
+```
+
+1. Create our map.
+2. Output the event to the map.
+
+Don't forget to rebuild your eBPF program!
+
+## Reading data
+
+In order to read from the `AsyncPerfEventArray`, we have to call
+`AsyncPerfEventArray::open()` for each online CPU, then we have to poll the file
+descriptor for events. While this is do-able using `PerfEventArray` and `mio` or
+`epoll`, the code is much less easy to follow. Instead, we'll use `tokio`, which
+was added to our template for us.
+
+We'll need to add a dependency on `bytes` to `xdp-log/Cargo.toml` since
+this will make it easier to deal with the chunks of bytes yielded by the
+`AsyncPerfEventArray`.
+
+Here's the code:
+
+```rust linenums="1" title="xdp-perfbuf-custom-data/src/main.rs"
+--8<-- "examples/xdp-perfbuf-custom-data/xdp-perfbuf-custom-data/src/main.rs"
+```
+
+1. Define our map.
+2. Call `open()` for each online CPU.
+3. Spawn a `tokio::task`.
+4. Create buffers.
+5. Read events in to buffers.
+6. Use `read_unaligned` to read the event data into a `PacketLog`.
+7. Log the packet data to the console.
+
+## Running the program
+
+As before, the interface can be overwritten by providing the interface name as a
+parameter, for example, `RUST_LOG=info cargo xtask run -- --iface wlp2s0`.
+
+```console
+$ RUST_LOG=info cargo xtask run
+[2023-01-25T08:57:41Z INFO  xdp_perfbuf_custom_data] SRC IP: 60.235.240.157, SRC_PORT: 443
+[2023-01-25T08:57:41Z INFO  xdp_perfbuf_custom_data] SRC IP: 98.21.76.76, SRC_PORT: 443
+[2023-01-25T08:57:41Z INFO  xdp_perfbuf_custom_data] SRC IP: 95.194.217.172, SRC_PORT: 443
+[2023-01-25T08:57:41Z INFO  xdp_perfbuf_custom_data] SRC IP: 95.194.217.172, SRC_PORT: 443
+[2023-01-25T08:57:41Z INFO  xdp_perfbuf_custom_data] SRC IP: 95.10.251.142, SRC_PORT: 443
+```

examples/xdp-perfbuf-custom-data/.cargo/config.toml

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+[alias]
+xtask = "run --package xtask --"

examples/xdp-perfbuf-custom-data/.gitignore

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+### https://raw.github.com/github/gitignore/master/Rust.gitignore
+
+# Generated by Cargo
+# will have compiled files and executables
+debug/
+target/
+
+# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
+# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
+Cargo.lock
+
+# These are backup files generated by rustfmt
+**/*.rs.bk

examples/xdp-perfbuf-custom-data/.vim/coc-settings.json

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+{
+  "rust-analyzer.linkedProjects": ["Cargo.toml", "xdp-perfbuf-custom-data-ebpf/Cargo.toml"]
+}

examples/xdp-perfbuf-custom-data/.vscode/settings.json

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+{
+  "rust-analyzer.linkedProjects": ["Cargo.toml", "xdp-perfbuf-custom-data-ebpf/Cargo.toml"]
+}

examples/xdp-perfbuf-custom-data/Cargo.toml

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+[workspace]
+members = ["xdp-perfbuf-custom-data", "xdp-perfbuf-custom-data-common", "xtask"]

examples/xdp-perfbuf-custom-data/README.md

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+# xdp-perfbuf-custom-data
+
+## Prerequisites
+
+1. Install a rust stable toolchain: `rustup install stable`
+1. Install a rust nightly toolchain: `rustup install nightly`
+1. Install bpf-linker: `cargo install bpf-linker`
+
+## Build eBPF
+
+```bash
+cargo xtask build-ebpf
+```
+
+To perform a release build you can use the `--release` flag.
+You may also change the target architecture with the `--target` flag
+
+## Build Userspace
+
+```bash
+cargo build
+```
+
+## Run
+
+```bash
+RUST_LOG=info cargo xtask run
+```

examples/xdp-perfbuf-custom-data/xdp-perfbuf-custom-data-common/Cargo.toml

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+[package]
+name = "xdp-perfbuf-custom-data-common"
+version = "0.1.0"
+edition = "2021"
+
+[features]
+default = []
+user = [ "aya" ]
+
+[dependencies]
+aya = { version = ">=0.11", optional=true }
+
+[lib]
+path = "src/lib.rs"

examples/xdp-perfbuf-custom-data/xdp-perfbuf-custom-data-common/src/lib.rs

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+#![no_std]
+
+#[repr(C)]
+#[derive(Clone, Copy)]
+pub struct PacketLog {
+    pub ipv4_address: u32,
+    pub port: u32,
+}
+
+#[cfg(feature = "user")]
+unsafe impl aya::Pod for PacketLog {} // (1)

examples/xdp-perfbuf-custom-data/xdp-perfbuf-custom-data-ebpf/.cargo/config.toml

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+[build]
+target-dir = "../target"
+target = "bpfel-unknown-none"
+
+[unstable]
+build-std = ["core"]