id: The --real flag should only affect -u, -g, and -G

src/uu/id/src/id.rs

@@ -230,10 +230,14 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
             return Ok(());
         }
 
-        let (uid, gid) = possible_pw.as_ref().map(|p| (p.uid, p.gid)).unwrap_or((
-            if state.rflag { getuid() } else { geteuid() },
-            if state.rflag { getgid() } else { getegid() },
-        ));
+        let (uid, gid) = possible_pw.as_ref().map(|p| (p.uid, p.gid)).unwrap_or({
+            let use_effective = !state.rflag && (state.uflag || state.gflag || state.gsflag);
+            if use_effective {
+                (geteuid(), getegid())
+            } else {
+                (getuid(), getgid())
+            }
+        });
         state.ids = Some(Ids {
             uid,
             gid,

tests/by-util/test_id.rs

@@ -8,7 +8,7 @@
 use uutests::new_ucmd;
 use uutests::unwrap_or_return;
 use uutests::util::{TestScenario, check_coreutil_version, expected_result, is_ci, whoami};
-use uutests::util_name;
+use uutests::{at_and_ucmd, util_name};
 
 const VERSION_MIN_MULTIPLE_USERS: &str = "8.31"; // this feature was introduced in GNU's coreutils 8.31
 
@@ -478,3 +478,31 @@ fn test_id_pretty_print_password_record() {
         .fails()
         .stderr_contains("the argument '-p' cannot be used with '-P'");
 }
+
+fn compile_preload_file_with_gcc(
+    c_file: &str,
+    so_file: &str,
+) -> Result<std::process::ExitStatus, String> {
+    Ok(std::process::Command::new("gcc")
+        .args(["-fPIC", "-shared", "-o", &so_file, &c_file])
+        .status()
+        .map_err(|_| "`gcc` is not installed")?)
+}
+
+#[test]
+#[cfg(all(unix, not(target_os = "android")))]
+fn test_id_different_uid_and_euid() {
+    let (at, mut ucmd) = at_and_ucmd!();
+
+    // Compile the preload file required for mocking different UID and EUID.
+    // The UID should be 1000, whereas the EUID should be 0.
+    let c_file = at.as_string() + "/different_uid_and_euid.c";
+    let so_file = at.as_string() + "/different_uid_and_euid.so";
+    let status = unwrap_or_return!(compile_preload_file_with_gcc(&c_file, &so_file));
+    if !status.success() {
+        panic!("Preload file compilation failed")
+    }
+
+    let result = ucmd.env("LD_PRELOAD", so_file).run();
+    result.stdout_contains("uid=1000").stdout_contains("euid=0");
+}

tests/fixtures/id/different_uid_and_euid.c

@@ -0,0 +1,7 @@
+#include <unistd.h>
+
+const uid_t USER_UID = 1000;
+const uid_t ROOT_UID = 0;
+
+uid_t getuid(void) { return USER_UID; }
+uid_t geteuid(void) { return ROOT_UID; }