Add the json-sysctl demo from the LSSNA talk

Depends on a byteorder crate locally patched to make std not a default
feature, see #121 for why.

tests/json-sysctl/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "json-sysctl"
+version = "0.1.0"
+authors = ["Alex Gaynor <[email protected]", "Geoffrey Thomas <[email protected]>"]
+edition = "2018"
+
+[lib]
+crate-type = ["staticlib"]
+
+[dependencies]
+linux-kernel-module = { path = "../.." }
+serde = { version = "*", default-features = false , features = ["derive"] }
+serde-json-core = { git = "https://github.com/japaric/serde-json-core" }
+typenum = "*"
+
+[patch.crates-io]
+byteorder = { path = "/root/byteorder" }

tests/json-sysctl/src/lib.rs

@@ -0,0 +1,83 @@
+#![no_std]
+#![feature(const_str_as_bytes)]
+
+use core::cmp::min;
+use core::convert::TryInto;
+use core::sync::atomic::{AtomicBool, Ordering};
+
+use serde::Serialize;
+use serde_json_core;
+
+use linux_kernel_module::cstr;
+use linux_kernel_module::sysctl::Sysctl;
+use linux_kernel_module::Mode;
+
+static A: AtomicBool = AtomicBool::new(false);
+static B: AtomicBool = AtomicBool::new(false);
+static C: AtomicBool = AtomicBool::new(false);
+
+struct JsonChrdev;
+
+impl linux_kernel_module::chrdev::FileOperations for JsonChrdev {
+    const VTABLE: linux_kernel_module::chrdev::FileOperationsVtable =
+        linux_kernel_module::chrdev::FileOperationsVtable::new::<Self>();
+
+    fn open() -> linux_kernel_module::KernelResult<Self> {
+        Ok(JsonChrdev)
+    }
+
+    fn read(
+        &self,
+        buf: &mut linux_kernel_module::user_ptr::UserSlicePtrWriter,
+        offset: u64,
+    ) -> linux_kernel_module::KernelResult<()> {
+        let o = Output {
+            a: A.load(Ordering::Relaxed),
+            b: B.load(Ordering::Relaxed),
+            c: C.load(Ordering::Relaxed),
+        };
+        let mut s = serde_json_core::to_vec::<typenum::U32, _>(&o)
+            .map_err(|_| linux_kernel_module::Error::ENOMEM)?;
+        s.push(b'\n')
+            .map_err(|_| linux_kernel_module::Error::ENOMEM)?;
+        let start = min(offset.try_into()?, s.len());
+        let end = min(start + buf.len(), s.len());
+        buf.write(&s[start..end])?;
+        Ok(())
+    }
+}
+
+struct JsonSysctlModule {
+    _a: Sysctl<&'static AtomicBool>,
+    _b: Sysctl<&'static AtomicBool>,
+    _c: Sysctl<&'static AtomicBool>,
+    _chrdev_registration: linux_kernel_module::chrdev::Registration,
+}
+
+#[derive(Serialize)]
+struct Output {
+    a: bool,
+    b: bool,
+    c: bool,
+}
+
+impl linux_kernel_module::KernelModule for JsonSysctlModule {
+    fn init() -> linux_kernel_module::KernelResult<Self> {
+        let chrdev_registration = linux_kernel_module::chrdev::builder(cstr!("json"), 0..1)?
+            .register_device::<JsonChrdev>()
+            .build()?;
+        Ok(JsonSysctlModule {
+            _a: Sysctl::register(cstr!("json-sysctl"), cstr!("a"), &A, Mode::from_int(0o666))?,
+            _b: Sysctl::register(cstr!("json-sysctl"), cstr!("b"), &B, Mode::from_int(0o666))?,
+            _c: Sysctl::register(cstr!("json-sysctl"), cstr!("c"), &C, Mode::from_int(0o666))?,
+            _chrdev_registration: chrdev_registration,
+        })
+    }
+}
+
+linux_kernel_module::kernel_module!(
+    JsonSysctlModule,
+    author: "Alex Gaynor and Geoffrey Thomas",
+    description: "Use JSON serialization in kernelspace",
+    license: "GPL"
+);