compositor: set scheduler to SCHED_RR before dropping CAP_SYS_NICE

src/cli.rs

@@ -234,7 +234,9 @@ impl ValueEnum for &'static Format {
 
 pub fn main() {
     let cli = Jay::parse();
-    drop_all_pr_caps();
+    if not_matches!(cli.command, Cmd::Run(_)) {
+        drop_all_pr_caps();
+    }
     match cli.command {
         Cmd::Run(a) => start_compositor(cli.global, a),
         Cmd::GenerateCompletion(g) => generate::main(g),

src/compositor.rs

@@ -39,6 +39,7 @@ use {
         logger::Logger,
         output_schedule::OutputSchedule,
         portal::{self, PortalStartup},
+        pr_caps::pr_caps,
         scale::Scale,
         sighand::{self, SighandError},
         state::{ConnectorData, IdleState, ScreenlockState, State, XWaylandState},
@@ -51,9 +52,17 @@ use {
         },
         user_session::import_environment,
         utils::{
-            clone3::ensure_reaper, clonecell::CloneCell, errorfmt::ErrorFmt, fdcloser::FdCloser,
-            numcell::NumCell, oserror::OsError, queue::AsyncQueue, refcounted::RefCounted,
-            run_toplevel::RunToplevel, tri::Try,
+            clone3::ensure_reaper,
+            clonecell::CloneCell,
+            errorfmt::ErrorFmt,
+            fdcloser::FdCloser,
+            nice::{did_elevate_scheduler, elevate_scheduler},
+            numcell::NumCell,
+            oserror::OsError,
+            queue::AsyncQueue,
+            refcounted::RefCounted,
+            run_toplevel::RunToplevel,
+            tri::Try,
         },
         version::VERSION,
         video::drm::wait_for_sync_obj::WaitForSyncObj,
@@ -72,6 +81,11 @@ pub const MAX_EXTENTS: i32 = (1 << 22) - 1;
 pub fn start_compositor(global: GlobalArgs, args: RunArgs) {
     sighand::reset_all();
     let reaper_pid = ensure_reaper();
+    let caps = pr_caps().into_comp();
+    if caps.has_nice() {
+        elevate_scheduler();
+    }
+    drop(caps);
     let forker = create_forker(reaper_pid);
     let portal = portal::run_from_compositor(global.log_level.into());
     enable_profiler();
@@ -146,6 +160,9 @@ fn start_compositor2(
 ) -> Result<(), CompositorError> {
     log::info!("pid = {}", uapi::getpid());
     log::info!("version = {VERSION}");
+    if did_elevate_scheduler() {
+        log::info!("Running with elevated scheduler: SCHED_RR");
+    }
     init_fd_limit();
     leaks::init();
     clientmem::init()?;
@@ -680,7 +697,7 @@ fn create_dummy_output(state: &Rc<State>) {
     state.dummy_output.set(Some(dummy_output));
 }
 
-fn config_dir() -> Option<String> {
+pub fn config_dir() -> Option<String> {
     if let Ok(xdg) = env::var("XDG_CONFIG_HOME") {
         Some(format!("{}/jay", xdg))
     } else if let Ok(home) = env::var("HOME") {

src/config.rs

@@ -10,8 +10,13 @@ use {
         state::State,
         tree::ToplevelData,
         utils::{
-            clonecell::CloneCell, numcell::NumCell, ptr_ext::PtrExt,
-            toplevel_identifier::ToplevelIdentifier, unlink_on_drop::UnlinkOnDrop, xrd::xrd,
+            clonecell::CloneCell,
+            nice::{JAY_NO_REALTIME, dont_allow_config_so},
+            numcell::NumCell,
+            ptr_ext::PtrExt,
+            toplevel_identifier::ToplevelIdentifier,
+            unlink_on_drop::UnlinkOnDrop,
+            xrd::xrd,
         },
     },
     bincode::Options,
@@ -26,7 +31,7 @@ use {
         window::{self, TileState},
     },
     libloading::Library,
-    std::{cell::Cell, io, mem, ptr, rc::Rc},
+    std::{cell::Cell, io, mem, path::Path, ptr, rc::Rc},
     thiserror::Error,
 };
 
@@ -42,6 +47,8 @@ pub enum ConfigError {
     CopyConfigFile(#[source] io::Error),
     #[error("XDG_RUNTIME_DIR is not set")]
     XrdNotSet,
+    #[error("Custom config.so is not permitted")]
+    NotPermitted,
 }
 
 pub struct ConfigProxy {
@@ -280,11 +287,24 @@ impl ConfigProxy {
     }
 
     pub fn from_config_dir(state: &Rc<State>) -> Result<Self, ConfigError> {
+        if dont_allow_config_so() {
+            if have_config_so(state.config_dir.as_deref()) {
+                log::warn!("Not loading config.so because");
+                log::warn!("  1. Jay was started with CAP_SYS_NICE");
+                log::warn!("  2. Jay was not started with {}=1", JAY_NO_REALTIME);
+                log::warn!("  3. The scheduler was elevated to SCHED_RR");
+                log::warn!(
+                    "  4. Jay was not compiled with {}=1",
+                    jay_allow_realtime_config_so!(),
+                );
+            }
+            return Err(ConfigError::NotPermitted);
+        }
         let dir = match state.config_dir.as_deref() {
             Some(d) => d,
             _ => return Err(ConfigError::ConfigDirNotSet),
         };
-        let file = format!("{}/config.so", dir);
+        let file = format!("{}/{CONFIG_SO}", dir);
         unsafe { Self::from_file(&file, state) }
     }
 
@@ -355,3 +375,15 @@ pub struct InvokedShortcut {
     pub effective_mods: Modifiers,
     pub sym: KeySym,
 }
+
+const CONFIG_SO: &str = "config.so";
+
+pub fn have_config_so(config_dir: Option<&str>) -> bool {
+    let Some(dir) = config_dir else {
+        return false;
+    };
+    let mut dir = dir.to_owned();
+    dir.push_str("/");
+    dir.push_str(CONFIG_SO);
+    Path::new(&dir).exists()
+}

src/macros.rs

@@ -760,3 +760,15 @@ macro_rules! client_wire_scale_to_logical {
         }
     };
 }
+
+macro_rules! not_matches {
+    ($($tt:tt)*) => {
+        !matches!($($tt)*)
+    };
+}
+
+macro_rules! jay_allow_realtime_config_so {
+    () => {
+        "JAY_ALLOW_REALTIME_CONFIG_SO"
+    };
+}

src/pr_caps.rs

@@ -1,17 +1,52 @@
 use {
     crate::{
         pr_caps::sys::{
-            _LINUX_CAPABILITY_U32S_3, _LINUX_CAPABILITY_VERSION_3, cap_user_data_t,
+            _LINUX_CAPABILITY_U32S_3, _LINUX_CAPABILITY_VERSION_3, CAP_SYS_NICE, cap_user_data_t,
             cap_user_header_t,
         },
-        utils::{errorfmt::ErrorFmt, oserror::OsError},
+        utils::{bitflags::BitflagsExt, errorfmt::ErrorFmt, oserror::OsError},
     },
     uapi::{
-        c::{SYS_capset, syscall},
+        c::{SYS_capget, SYS_capset, syscall},
         map_err,
     },
 };
 
+pub struct PrCaps {
+    effective: u64,
+    permitted: u64,
+    inheritable: u64,
+}
+
+pub struct PrCompCaps {
+    caps: PrCaps,
+}
+
+pub fn pr_caps() -> PrCaps {
+    let mut hdr = cap_user_header_t {
+        version: _LINUX_CAPABILITY_VERSION_3,
+        pid: 0,
+    };
+    let mut caps = [cap_user_data_t::default(); _LINUX_CAPABILITY_U32S_3];
+    let ret = unsafe { syscall(SYS_capget, &mut hdr, &mut caps) };
+    if let Err(e) = map_err!(ret) {
+        eprintln!(
+            "Could not get process capabilities: {}",
+            ErrorFmt(OsError(e.0))
+        );
+        return PrCaps {
+            effective: 0,
+            permitted: 0,
+            inheritable: 0,
+        };
+    }
+    PrCaps {
+        effective: caps[0].effective as u64 | ((caps[1].effective as u64) << 32),
+        permitted: caps[0].permitted as u64 | ((caps[1].permitted as u64) << 32),
+        inheritable: caps[0].inheritable as u64 | ((caps[1].inheritable as u64) << 32),
+    }
+}
+
 pub fn drop_all_pr_caps() {
     let mut hdr = cap_user_header_t {
         version: _LINUX_CAPABILITY_VERSION_3,
@@ -27,6 +62,55 @@ pub fn drop_all_pr_caps() {
     }
 }
 
+impl PrCaps {
+    pub fn into_comp(mut self) -> PrCompCaps {
+        let mut caps = 0;
+        macro_rules! add_cap {
+            ($name:ident) => {
+                if self.permitted.contains(1 << $name) {
+                    caps |= 1 << $name;
+                }
+            };
+        }
+        add_cap!(CAP_SYS_NICE);
+        let mut hdr = cap_user_header_t {
+            version: _LINUX_CAPABILITY_VERSION_3,
+            pid: 0,
+        };
+        let caps_hi = (caps >> 32) as u32;
+        let caps_lo = caps as u32;
+        let mut data = [cap_user_data_t::default(); _LINUX_CAPABILITY_U32S_3];
+        data[0].effective = caps_lo;
+        data[1].effective = caps_hi;
+        data[0].permitted = caps_lo;
+        data[1].permitted = caps_hi;
+        let ret = unsafe { syscall(SYS_capset, &mut hdr, &data) };
+        if let Err(e) = map_err!(ret) {
+            eprintln!(
+                "Could not get set compositor capabilities: {}",
+                ErrorFmt(OsError(e.0))
+            );
+            return PrCompCaps { caps: self };
+        }
+        self.effective = caps;
+        self.permitted = caps;
+        self.inheritable = 0;
+        PrCompCaps { caps: self }
+    }
+}
+
+impl PrCompCaps {
+    pub fn has_nice(&self) -> bool {
+        self.caps.effective.contains(1 << CAP_SYS_NICE)
+    }
+}
+
+impl Drop for PrCaps {
+    fn drop(&mut self) {
+        drop_all_pr_caps();
+    }
+}
+
 mod sys {
     #![allow(dead_code)]
 

src/utils.rs

@@ -28,6 +28,7 @@ pub mod line_logger;
 pub mod linkedlist;
 pub mod log_on_drop;
 pub mod mmap;
+pub mod nice;
 pub mod nonblock;
 pub mod num_cpus;
 pub mod numcell;

src/utils/clone3.rs

@@ -1,6 +1,7 @@
 use {
     crate::{
         forker::ForkerError,
+        pr_caps::drop_all_pr_caps,
         utils::{errorfmt::ErrorFmt, on_drop::OnDrop, process_name::set_process_name},
     },
     std::{env, mem::MaybeUninit, process, slice, str::FromStr},
@@ -161,6 +162,7 @@ pub fn ensure_reaper() -> c::pid_t {
         set_deathsig();
         return reaper_pid;
     };
+    drop_all_pr_caps();
     set_process_name("jay reaper");
     while let Ok((pid, status)) = uapi::wait() {
         if pid == main_process_id {

src/utils/nice.rs

@@ -0,0 +1,40 @@
+use {
+    crate::{compositor::config_dir, config::have_config_so},
+    c::sched_setscheduler,
+    std::{
+        env, mem,
+        sync::atomic::{AtomicBool, Ordering::Relaxed},
+    },
+    uapi::c::{self, SCHED_RESET_ON_FORK, SCHED_RR, sched_param},
+};
+
+static DID_ELEVATE_SCHEDULER: AtomicBool = AtomicBool::new(false);
+
+pub const JAY_NO_REALTIME: &str = "JAY_NO_REALTIME";
+
+pub fn elevate_scheduler() {
+    if env::var(JAY_NO_REALTIME).as_deref().unwrap_or_default() == "1" {
+        return;
+    }
+    if have_config_so(config_dir().as_deref()) && dont_allow_realtime_config_so() {
+        return;
+    }
+    let mut param = unsafe { mem::zeroed::<sched_param>() };
+    param.sched_priority = 1;
+    let res = unsafe { sched_setscheduler(0, SCHED_RR | SCHED_RESET_ON_FORK, &param) };
+    if res == 0 {
+        DID_ELEVATE_SCHEDULER.store(true, Relaxed);
+    }
+}
+
+pub fn did_elevate_scheduler() -> bool {
+    DID_ELEVATE_SCHEDULER.load(Relaxed)
+}
+
+fn dont_allow_realtime_config_so() -> bool {
+    option_env!(jay_allow_realtime_config_so!()).unwrap_or_default() != "1"
+}
+
+pub fn dont_allow_config_so() -> bool {
+    did_elevate_scheduler() && dont_allow_realtime_config_so()
+}