Add collecting system users from passwd file

For allow/deny list later.

src/main.rs

@@ -22,6 +22,8 @@ use util::*;
 
 mod log;
 
+mod sys;
+
 mod interval;
 mod config;
 

src/sys/user.rs

@@ -1 +1,85 @@
 //! passwd queries
+use super::*;
+use std::sync::Mutex;
+use libc::{
+    getpwent,
+    endpwent
+};
+
+/// Safe wrapper around C's passwd struct. Doesn't implement the
+/// `passwd', `gecos' or `expire' fields because we don't use them.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct Passwd {
+    name: String,
+    uid: u32,
+    gid: u32,
+    home: String,
+    shell: String,
+}
+
+/// Returns a vector of the system's users
+pub fn get_users() -> std::vec::Vec<Passwd> {
+    lazy_static::lazy_static! {
+        static ref MUTEX: Mutex<()> = Mutex::new(());
+    }
+    
+    let mut users = vec!();
+
+    unsafe {
+        // This could maybe be simplified but there's no point
+        macro_rules! from_cstr {
+            ($x:expr, $fmt:literal) => {
+                match std::ffi::CStr::from_ptr($x).to_owned().into_string() {
+                    Ok(v) => v,
+                    Err(v) => {
+                        warn!($fmt, v);
+                        continue;
+                    }
+                }
+            };
+            ($x:expr, $fmt:literal, $usr:expr) => {
+                match std::ffi::CStr::from_ptr($x).to_owned().into_string() {
+                    Ok(v) => v,
+                    Err(v) => {
+                        warn!($fmt, $usr, v);
+                        continue;
+                    }
+                }
+            }
+        }
+        
+        let _lock = MUTEX.lock().expect("poisoned");
+        loop {
+            let passwd = getpwent();
+
+            // FFI returns NULL when no more entries
+            if passwd.is_null() {
+                break;
+            }
+
+            let my_passwd = *passwd;
+            let name = from_cstr!(
+                my_passwd.pw_name,
+                "User {} has invalid UTF-8 characters, skipping"
+            );
+            
+            users.push(Passwd{
+                gid: 1,
+                uid: 2,
+                home: from_cstr!(
+                    my_passwd.pw_dir,
+                    "User {}'s home ({}) has invalid UTF-8 characters, skipping",
+                    name
+                ),
+                shell: from_cstr!(
+                    my_passwd.pw_shell,
+                    "User {}'s shell ({}) has invalid UTF-8 characters, skipping",
+                    name
+                ),
+                name
+            });
+        }
+        endpwent();
+    }
+    return users;
+}