diff --git a/Cargo.lock b/Cargo.lock
index 3ac07b3..3a031c6 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1111,6 +1111,7 @@ dependencies = [
  "rsjudge-judger",
  "rsjudge-rest",
  "rsjudge-runner",
+ "rsjudge-utils",
  "tokio",
  "toml",
 ]
@@ -1121,13 +1122,13 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "futures",
+ "log",
  "prost",
  "prost-types",
  "rsjudge-utils",
  "tokio",
  "tokio-stream",
  "tonic",
- "tonic-buf-build",
  "tonic-build",
 ]
 
@@ -1156,10 +1157,11 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "caps",
+ "log",
  "nix",
- "once_cell",
  "rsjudge-utils",
  "rustc_version",
+ "thiserror",
  "uzers",
 ]
 
@@ -1210,12 +1212,6 @@ version = "1.0.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e86697c916019a8588c99b5fac3cead74ec0b4b819707a682fd4d23fa0ce1ba1"
 
-[[package]]
-name = "scopeguard"
-version = "1.2.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "94143f37725109f92c262ed2cf5e59bce7498c01bcc1502d7b9afe439a4e9f49"
-
 [[package]]
 name = "semver"
 version = "1.0.22"
@@ -1284,19 +1280,6 @@ dependencies = [
  "serde",
 ]
 
-[[package]]
-name = "serde_yaml"
-version = "0.9.34+deprecated"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6a8b1a1a2ebf674015cc02edccce75287f1a0130d394307b36743c2f5d504b47"
-dependencies = [
- "indexmap 2.2.6",
- "itoa",
- "ryu",
- "serde",
- "unsafe-libyaml",
-]
-
 [[package]]
 name = "sh"
 version = "0.2.1"
@@ -1534,20 +1517,6 @@ dependencies = [
  "tracing",
 ]
 
-[[package]]
-name = "tonic-buf-build"
-version = "0.2.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "89fb2d0fdd379526f80038c2bfbc5be061ff966b84b402b8131dbff4015ba25d"
-dependencies = [
- "prost-build",
- "scopeguard",
- "serde",
- "serde_yaml",
- "tonic-build",
- "uuid",
-]
-
 [[package]]
 name = "tonic-build"
 version = "0.11.0"
@@ -1637,28 +1606,12 @@ version = "1.0.12"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3354b9ac3fae1ff6755cb6db53683adb661634f67557942dea4facebec0fee4b"
 
-[[package]]
-name = "unsafe-libyaml"
-version = "0.2.11"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "673aac59facbab8a9007c7f6108d11f63b603f7cabff99fabf650fea5c32b861"
-
 [[package]]
 name = "utf8parse"
 version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "711b9620af191e0cdc7468a8d14e709c3dcdb115b36f838e601583af800a370a"
 
-[[package]]
-name = "uuid"
-version = "1.8.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a183cf7feeba97b4dd1c0d46788634f6221d87fa961b305bed08c851829efcc0"
-dependencies = [
- "getrandom",
- "rand",
-]
-
 [[package]]
 name = "uzers"
 version = "0.11.3"
diff --git a/Cargo.toml b/Cargo.toml
index 02e86d2..a2a91f6 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -17,6 +17,10 @@ license = "Apache-2.0"
 # clap requires Rust 1.74+ to work
 rust-version = "1.74"
 
+[workspace.dependencies]
+log = "0.4.21"
+rsjudge-utils = { version = "0.1.0", path = "crates/rsjudge-utils" }
+
 [package]
 name = "rsjudge"
 version.workspace = true
@@ -131,6 +135,7 @@ dest = "/usr/lib/sysusers.d/rsjudge.conf"
 # Workspace dependencies
 rsjudge-judger = { version = "0.1.0", path = "crates/rsjudge-judger" }
 rsjudge-runner = { version = "0.1.0", path = "crates/rsjudge-runner" }
+rsjudge-utils.workspace = true
 
 # Optional dependencies
 rsjudge-grpc = { version = "0.1.0", path = "crates/rsjudge-grpc", optional = true }
@@ -140,9 +145,9 @@ anyhow = "1.0.80"
 caps = "0.5.5"
 clap = { version = "4.5.3", features = ["derive"] }
 env_logger = "0.11.3"
-log = "0.4.21"
+log.workspace = true
 mimalloc = "0.1.39"
-tokio = { version = "1.36.0", features = ["fs", "rt-multi-thread", "macros"] }
+tokio = { version = "1.37.0", features = ["fs", "rt-multi-thread", "macros"] }
 toml = "0.8.12"
 
 # Unused for now:
diff --git a/crates/rsjudge-grpc/Cargo.toml b/crates/rsjudge-grpc/Cargo.toml
index b8e4857..cd48f8f 100644
--- a/crates/rsjudge-grpc/Cargo.toml
+++ b/crates/rsjudge-grpc/Cargo.toml
@@ -9,12 +9,12 @@ rust-version.workspace = true
 [dependencies]
 anyhow = "1.0.81"
 futures = "0.3.30"
+log.workspace = true
 prost = "0.12.3"
 prost-types = "0.12.3"
 tokio = { version = "1.36.0", features = ["net"] }
 tokio-stream = "0.1.15"
 tonic = "0.11.0"
-tonic-buf-build = "0.2.0"
 
 [build-dependencies]
 anyhow = "1.0.81"
diff --git a/crates/rsjudge-grpc/src/lib.rs b/crates/rsjudge-grpc/src/lib.rs
index 66d80cc..0259d2d 100644
--- a/crates/rsjudge-grpc/src/lib.rs
+++ b/crates/rsjudge-grpc/src/lib.rs
@@ -1,3 +1,5 @@
+#![cfg_attr(not(test), warn(clippy::print_stdout, clippy::print_stderr))]
+
 use std::net::SocketAddr;
 
 use tonic::transport::{Error, Server};
diff --git a/crates/rsjudge-grpc/src/server.rs b/crates/rsjudge-grpc/src/server.rs
index ef993d6..6274d49 100644
--- a/crates/rsjudge-grpc/src/server.rs
+++ b/crates/rsjudge-grpc/src/server.rs
@@ -1,3 +1,4 @@
+use log::debug;
 use tokio_stream::wrappers::ReceiverStream;
 use tonic::{async_trait, Request, Response, Status};
 
@@ -17,8 +18,8 @@ impl JudgeService for JudgeServerImpl {
         &self,
         request: Request<SelfTestRequest>,
     ) -> Result<Response<Self::SelfTestStream>, Status> {
-        let _ = request;
-        todo!()
+        debug!("Received SelfTestRequest: {:?}", request.into_inner());
+        Err(Status::unimplemented("Not implemented yet"))
     }
 
     type SubmitStream = ReceiverStream<Result<SubmitResponse, Status>>;
@@ -27,7 +28,7 @@ impl JudgeService for JudgeServerImpl {
         &self,
         request: Request<SubmitRequest>,
     ) -> Result<Response<Self::SubmitStream>, Status> {
-        let _ = request;
-        todo!()
+        debug!("Received SubmitRequest: {:?}", request.into_inner());
+        Err(Status::unimplemented("Not implemented yet"))
     }
 }
diff --git a/crates/rsjudge-judger/src/lib.rs b/crates/rsjudge-judger/src/lib.rs
index 19e251e..36d6c36 100644
--- a/crates/rsjudge-judger/src/lib.rs
+++ b/crates/rsjudge-judger/src/lib.rs
@@ -1,3 +1,5 @@
+#![cfg_attr(not(test), warn(clippy::print_stdout, clippy::print_stderr))]
+
 pub mod comparer;
 
 pub use comparer::{default_comparer::DefaultComparer, CompareResult, Comparer};
diff --git a/crates/rsjudge-runner/Cargo.toml b/crates/rsjudge-runner/Cargo.toml
index 0254e55..064c3fa 100644
--- a/crates/rsjudge-runner/Cargo.toml
+++ b/crates/rsjudge-runner/Cargo.toml
@@ -8,14 +8,15 @@ rust-version.workspace = true
 description = "Command runner for rsjudge"
 
 [dependencies]
-anyhow = "1.0.81"
 caps = "0.5.5"
+log.workspace = true
 nix = { version = "0.28.0", features = ["user"] }
-once_cell = "1.19.0"
+rsjudge-utils.workspace = true
+thiserror = "1.0.58"
 uzers = "0.11.3"
 
 [build-dependencies]
 rustc_version = "0.4.0"
 
 [dev-dependencies]
-rsjudge-utils = { path = "../rsjudge-utils" }
+anyhow = "1.0.81"
diff --git a/crates/rsjudge-runner/examples/demo.rs b/crates/rsjudge-runner/examples/demo.rs
index 76e9b05..2811ce6 100644
--- a/crates/rsjudge-runner/examples/demo.rs
+++ b/crates/rsjudge-runner/examples/demo.rs
@@ -8,14 +8,14 @@ use rsjudge_runner::{
 use uzers::{get_current_uid, get_user_by_uid};
 fn main() -> anyhow::Result<()> {
     let self_output = Command::new("id")
-        .run_as(&get_user_by_uid(get_current_uid()).ok_or(anyhow!("invalid user"))?)
+        .run_as(&get_user_by_uid(get_current_uid()).ok_or(anyhow!("invalid user"))?)?
         .output()?;
     println!("{}", String::from_utf8_lossy(&self_output.stdout));
 
-    let builder_output = Command::new("id").run_as(builder()?).output()?;
+    let builder_output = Command::new("id").run_as(builder()?)?.output()?;
     println!("{}", String::from_utf8_lossy(&builder_output.stdout));
 
-    let runner_output = Command::new("id").run_as(runner()?).output()?;
+    let runner_output = Command::new("id").run_as(runner()?)?.output()?;
     println!("{}", String::from_utf8_lossy(&runner_output.stdout));
     Ok(())
 }
diff --git a/crates/rsjudge-runner/examples/exploit.rs b/crates/rsjudge-runner/examples/exploit.rs
index 66e8573..828710b 100644
--- a/crates/rsjudge-runner/examples/exploit.rs
+++ b/crates/rsjudge-runner/examples/exploit.rs
@@ -47,12 +47,12 @@ fn main() -> anyhow::Result<()> {
 
     let exploit_inner = examples.join("exploit_inner");
 
-    let status = check_output(Command::new(dbg!(exploit_inner)).run_as(builder()?))?;
+    let status = check_output(Command::new(dbg!(exploit_inner)).run_as(builder()?)?)?;
     println!("{}", String::from_utf8_lossy(&status.stdout));
     println!("{}", String::from_utf8_lossy(&status.stderr));
 
     let normal = examples.join("normal");
-    let status = check_output(Command::new(normal).run_as(builder()?))?;
+    let status = check_output(Command::new(normal).run_as(builder()?)?)?;
     println!("{}", String::from_utf8_lossy(&status.stdout));
     println!("{}", String::from_utf8_lossy(&status.stderr));
 
diff --git a/crates/rsjudge-runner/src/error.rs b/crates/rsjudge-runner/src/error.rs
new file mode 100644
index 0000000..0d477c3
--- /dev/null
+++ b/crates/rsjudge-runner/src/error.rs
@@ -0,0 +1,17 @@
+use caps::Capability;
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum Error {
+    #[error("{cap} is required to run as another user.")]
+    CapsRequired { cap: Capability },
+    #[error("User '{name}' not found")]
+    UserNotFound { name: String },
+
+    #[error("I/O error: {0}")]
+    Io(#[from] std::io::Error),
+    #[error("{0}")]
+    CapsError(#[from] caps::errors::CapsError),
+}
+
+pub type Result<T> = std::result::Result<T, Error>;
diff --git a/crates/rsjudge-runner/src/lib.rs b/crates/rsjudge-runner/src/lib.rs
index b92c4b9..9092c5d 100644
--- a/crates/rsjudge-runner/src/lib.rs
+++ b/crates/rsjudge-runner/src/lib.rs
@@ -1,15 +1,35 @@
+#![cfg_attr(not(test), warn(clippy::print_stdout, clippy::print_stderr))]
+
 use std::{os::unix::process::CommandExt as _, process::Command};
 
+use caps::{has_cap, CapSet, Capability};
 use nix::unistd::{setgroups, Gid};
 use uzers::User;
 
+use crate::error::{Error, Result};
+
+pub mod error;
 pub mod user;
 pub trait RunAs {
-    fn run_as(&mut self, user: &User) -> &mut Command;
+    type Error;
+    fn run_as(&mut self, user: &User) -> Result<&mut Self>;
 }
 
 impl RunAs for Command {
-    fn run_as(&mut self, user: &User) -> &mut Self {
+    type Error = Error;
+    fn run_as(&mut self, user: &User) -> Result<&mut Self> {
+        if !has_cap(None, CapSet::Effective, Capability::CAP_SETUID)? {
+            Err(Error::CapsRequired {
+                cap: Capability::CAP_SETUID,
+            })?;
+        }
+
+        has_cap(None, CapSet::Effective, Capability::CAP_SETGID).map_err(|_| {
+            Error::CapsRequired {
+                cap: Capability::CAP_SETGID,
+            }
+        })?;
+
         let uid = user.uid();
         let gid = user.primary_group_id();
 
@@ -47,6 +67,6 @@ impl RunAs for Command {
             self.groups(groups);
         }
 
-        self
+        Ok(self)
     }
 }
diff --git a/crates/rsjudge-runner/src/user.rs b/crates/rsjudge-runner/src/user.rs
index 2182f7e..c32c5e3 100644
--- a/crates/rsjudge-runner/src/user.rs
+++ b/crates/rsjudge-runner/src/user.rs
@@ -1,25 +1,36 @@
-use anyhow::anyhow;
-use once_cell::sync::Lazy;
+use std::sync::OnceLock;
+
 use uzers::{get_user_by_name, User};
 
-pub static SUPERVISOR: Lazy<Option<User>> = Lazy::new(|| get_user_by_name("rsjudge-supervisor"));
-pub static BUILDER: Lazy<Option<User>> = Lazy::new(|| get_user_by_name("rsjudge-builder"));
-pub static RUNNER: Lazy<Option<User>> = Lazy::new(|| get_user_by_name("rsjudge-runner"));
+use crate::error::{Error, Result};
+
+pub static SUPERVISOR: OnceLock<Option<User>> = OnceLock::new();
+pub static BUILDER: OnceLock<Option<User>> = OnceLock::new();
+pub static RUNNER: OnceLock<Option<User>> = OnceLock::new();
 
-pub fn supervisor<'a>() -> anyhow::Result<&'a User> {
+pub fn supervisor<'a>() -> Result<&'a User> {
     SUPERVISOR
+        .get_or_init(|| get_user_by_name("rsjudge-supervisor"))
         .as_ref()
-        .ok_or_else(|| anyhow!("User `rsjudge-supervisor` not found"))
+        .ok_or_else(|| Error::UserNotFound {
+            name: "rsjudge-supervisor".to_string(),
+        })
 }
 
-pub fn builder<'a>() -> anyhow::Result<&'a User> {
+pub fn builder<'a>() -> Result<&'a User> {
     BUILDER
+        .get_or_init(|| get_user_by_name("rsjudge-builder"))
         .as_ref()
-        .ok_or_else(|| anyhow!("User `rsjudge-builder` not found"))
+        .ok_or_else(|| Error::UserNotFound {
+            name: "rsjudge-builder".to_string(),
+        })
 }
 
-pub fn runner<'a>() -> anyhow::Result<&'a User> {
+pub fn runner<'a>() -> Result<&'a User> {
     RUNNER
+        .get_or_init(|| get_user_by_name("rsjudge-runner"))
         .as_ref()
-        .ok_or_else(|| anyhow!("User `rsjudge-runner` not found"))
+        .ok_or_else(|| Error::UserNotFound {
+            name: "rsjudge-runner".to_string(),
+        })
 }
diff --git a/crates/rsjudge-utils/src/lib.rs b/crates/rsjudge-utils/src/lib.rs
index a621fce..94351b8 100644
--- a/crates/rsjudge-utils/src/lib.rs
+++ b/crates/rsjudge-utils/src/lib.rs
@@ -1,3 +1,5 @@
+#![warn(clippy::print_stdout, clippy::print_stderr)]
+
 //! A collection of utility functions for the rsjudge project.
 
 #![warn(missing_docs)]
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..1e7f748
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,48 @@
+#![cfg_attr(not(test), warn(clippy::print_stdout, clippy::print_stderr))]
+
+use std::process::Command;
+
+use clap::Parser as _;
+use env_logger::Env;
+use log::{debug, info, warn};
+use rsjudge_runner::{user::builder, RunAs as _};
+use rsjudge_utils::command::display_cmd;
+use tokio::fs::read;
+
+use crate::cli::Args;
+
+mod cli;
+
+pub async fn main_impl() -> anyhow::Result<()> {
+    env_logger::Builder::from_env(
+        Env::new()
+            .filter_or("RSJUDGE_LOG", "info")
+            .write_style("RSJUDGE_LOG_STYLE"),
+    )
+    .format_timestamp_millis()
+    .format_module_path(true)
+    .try_init()?;
+
+    let args = Args::try_parse()?;
+    debug!("{:?}", args);
+
+    let config = read(args.config_dir.join("executors.toml")).await?;
+
+    info!("Executing \"id\" as rsjudge-builder");
+
+    match Command::new("id").run_as(builder()?) {
+        Ok(it) => {
+            debug!("{} exited with {}", display_cmd(it), it.status()?);
+        }
+        Err(err) => {
+            warn!("Failed to run \"id\" as rsjudge-builder: {}", err);
+        }
+    };
+
+    debug!(
+        "Config:\n{:#?}",
+        String::from_utf8_lossy(&config).parse::<toml::Value>()?
+    );
+
+    Ok(())
+}
diff --git a/src/main.rs b/src/main.rs
index 5372427..a08e648 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,14 +1,8 @@
-use std::process::Command;
+#![cfg_attr(not(test), warn(clippy::print_stdout, clippy::print_stderr))]
 
-use caps::{has_cap, CapSet, Capability};
-use clap::Parser;
-use env_logger::Env;
-use log::{debug, info, trace};
+use log::error;
 use mimalloc::MiMalloc;
-use rsjudge_runner::{user::builder, RunAs};
-use tokio::fs::read;
-
-use crate::cli::Args;
+use rsjudge::main_impl;
 
 #[global_allocator]
 static GLOBAL: MiMalloc = MiMalloc;
@@ -17,32 +11,8 @@ mod cli;
 
 #[tokio::main]
 async fn main() -> anyhow::Result<()> {
-    env_logger::Builder::from_env(
-        Env::new()
-            .filter_or("RSJUDGE_LOG", "info")
-            .write_style("RSJUDGE_LOG_STYLE"),
-    )
-    .format_timestamp_millis()
-    .format_module_path(true)
-    .try_init()?;
-
-    let args = Args::try_parse()?;
-    info!("{:?}", args);
-
-    let config = read(args.config_dir.join("executors.toml")).await?;
-    if has_cap(None, CapSet::Permitted, Capability::CAP_SETUID)?
-        && has_cap(None, CapSet::Permitted, Capability::CAP_SETGID)?
-    {
-        debug!("Executing `id` as `rsjudge-builder`");
-        info!("{}", Command::new("id").run_as(builder()?).status()?);
-    } else {
-        info!("CAP_SETUID and CAP_SETGID not set, skipping.");
+    if let Err(err) = main_impl().await {
+        error!("{:?}", err);
     }
-
-    trace!(
-        "Config:\n{:#?}",
-        String::from_utf8_lossy(&config).parse::<toml::Value>()?
-    );
-
     Ok(())
 }