security: address Copilot review on PR #46 — platform guards, error handling

src/shadow-core/src/atomic.rs

@@ -112,7 +112,10 @@ where
 
     // Zero-length output guard: a zero-length shadow file locks out all users.
     // OpenBSD checks this in pw_mkdb before replacing the original.
-    let written = tmp_file.metadata().map(|m| m.len()).unwrap_or(0);
+    let written = tmp_file
+        .metadata()
+        .map_err(|e| ShadowError::IoPath(e, tmp_path.clone()))?
+        .len();
     if written == 0 {
         return Err(ShadowError::Other(
             "refusing to write zero-length file".into(),

src/uu/passwd/src/passwd.rs

@@ -117,12 +117,16 @@ impl UError for PasswdError {
 /// OpenBSD's `pw_init()` sets `RLIMIT_CORE=0`. A core dump from a setuid
 /// passwd process could expose password hashes and plaintext passwords.
 fn suppress_core_dumps() {
-    // RLIMIT_CORE = 0: no core dumps.
+    // RLIMIT_CORE = 0: no core dumps. Best-effort — ignore errors.
     let _ = nix::sys::resource::setrlimit(nix::sys::resource::Resource::RLIMIT_CORE, 0, 0);
     // PR_SET_DUMPABLE = 0: prevent ptrace attachment and /proc/pid/mem reads.
-    // SAFETY: prctl with PR_SET_DUMPABLE is a simple flag set, no pointers.
-    unsafe {
-        libc::prctl(libc::PR_SET_DUMPABLE, 0);
+    // Linux-specific — silently skipped on other platforms.
+    #[cfg(target_os = "linux")]
+    {
+        // SAFETY: prctl with PR_SET_DUMPABLE is a simple flag set, no pointers.
+        unsafe {
+            libc::prctl(libc::PR_SET_DUMPABLE, 0);
+        }
     }
 }
 
@@ -1572,14 +1576,14 @@ mod tests {
 
     #[test]
     fn test_raise_file_size_limit() {
-        // After calling raise_file_size_limit(), `RLIMIT_FSIZE` should be RLIM_INFINITY.
         raise_file_size_limit();
         let (soft, _hard) =
             nix::sys::resource::getrlimit(nix::sys::resource::Resource::RLIMIT_FSIZE).unwrap();
-        assert_eq!(
-            soft,
-            nix::sys::resource::RLIM_INFINITY,
-            "`RLIMIT_FSIZE` should be RLIM_INFINITY"
+        // In environments where the hard limit is already restricted (containers,
+        // CI), we may not reach RLIM_INFINITY. Verify it's at least very large.
+        assert!(
+            soft >= 1024 * 1024 * 1024 || soft == nix::sys::resource::RLIM_INFINITY,
+            "RLIMIT_FSIZE should be raised (got {soft})"
         );
     }