mkfifo: fix race condition by using UmaskGuard for atomic permissions

Author: sylvestre

src/uu/mkfifo/src/mkfifo.rs

@@ -6,8 +6,6 @@
 use clap::{Arg, ArgAction, Command, value_parser};
 use nix::sys::stat::Mode;
 use nix::unistd::mkfifo;
-use std::fs;
-use std::os::unix::fs::PermissionsExt;
 use uucore::display::Quotable;
 use uucore::error::{UResult, USimpleError};
 use uucore::translate;
@@ -39,21 +37,21 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
     };
 
     for f in fifos {
-        if mkfifo(f.as_str(), Mode::from_bits_truncate(0o666)).is_err() {
+        // Create FIFO with exact mode by temporarily setting umask to 0.
+        // This avoids a race condition where the FIFO briefly exists with
+        // umask-based permissions before chmod is called.
+        let result = {
+            let _guard = uucore::mode::UmaskGuard::set(0);
+            mkfifo(f.as_str(), Mode::from_bits_truncate(mode))
+        };
+
+        if result.is_err() {
             show!(USimpleError::new(
                 1,
                 translate!("mkfifo-error-cannot-create-fifo", "path" => f.quote()),
             ));
         }
 
-        // Explicitly set the permissions to ignore umask
-        if let Err(e) = fs::set_permissions(&f, fs::Permissions::from_mode(mode)) {
-            return Err(USimpleError::new(
-                1,
-                translate!("mkfifo-error-cannot-set-permissions", "path" => f.quote(), "error" => e),
-            ));
-        }
-
         // Apply SELinux context if requested
         #[cfg(all(feature = "selinux", target_os = "linux"))]
         {
@@ -66,7 +64,7 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
                 if let Err(e) =
                     uucore::selinux::set_selinux_security_context(Path::new(&f), context)
                 {
-                    let _ = fs::remove_file(f);
+                    let _ = std::fs::remove_file(f);
                     return Err(USimpleError::new(1, e.to_string()));
                 }
             }

tests/by-util/test_mkfifo.rs

@@ -137,19 +137,16 @@ fn test_create_fifo_permission_denied() {
     at.mkdir(no_exec_dir);
     at.set_mode(no_exec_dir, 0o644);
 
-    let err_msg = format!(
-        "mkfifo: cannot create fifo '{named_pipe}': File exists
-mkfifo: cannot set permissions on '{named_pipe}': Permission denied (os error 13)
-"
-    );
-
+    // With atomic permission setting, mkfifo fails with a single error.
+    // The exact error depends on whether the FIFO already exists or the
+    // directory lacks execute permission.
     scene
         .ucmd()
         .arg(named_pipe)
         .arg("-m")
         .arg("666")
         .fails()
-        .stderr_is(err_msg.as_str());
+        .stderr_contains("mkfifo: cannot create fifo");
 }
 
 #[test]