Fix the mount tests

As originally written by @kamalmarhubi in nix-rust#231, these tests made
clever use of Linux namespaces in order to run as unprivileged users.
However, a subsequent kernel bug broke this functionality, and it hasn't
been fixed even 6 years later.  The tests have been skipped ever since.

Get the tests to run again by removing the namespace stuff and
requiring privileges instead.  Also, remove the custom test harness.
Now Nix will be compatible with tools like cargo-nextest.

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1659087

Cargo.toml

@@ -102,7 +102,6 @@ path = "test/test_clearenv.rs"
 [[test]]
 name = "test-mount"
 path = "test/test_mount.rs"
-harness = false
 
 [[test]]
 name = "test-prctl"

test/test_mount.rs

@@ -1,34 +1,32 @@
+#[macro_use]
 mod common;
 
-// Implementation note: to allow unprivileged users to run it, this test makes
-// use of user and mount namespaces. On systems that allow unprivileged user
-// namespaces (Linux >= 3.8 compiled with CONFIG_USER_NS), the test should run
-// without root.
-
 #[cfg(target_os = "linux")]
 mod test_mount {
     use std::fs::{self, File};
-    use std::io::{self, Read, Write};
+    use std::io::{Read, Write};
     use std::os::unix::fs::OpenOptionsExt;
     use std::os::unix::fs::PermissionsExt;
-    use std::process::{self, Command};
+    use std::process::Command;
 
     use libc::{EACCES, EROFS};
 
-    use nix::errno::Errno;
     use nix::mount::{mount, umount, MsFlags};
-    use nix::sched::{unshare, CloneFlags};
     use nix::sys::stat::{self, Mode};
-    use nix::unistd::getuid;
 
     static SCRIPT_CONTENTS: &[u8] = b"#!/bin/sh
 exit 23";
 
     const EXPECTED_STATUS: i32 = 23;
 
     const NONE: Option<&'static [u8]> = None;
-    #[allow(clippy::bind_instead_of_map)] // False positive
-    pub fn test_mount_tmpfs_without_flags_allows_rwx() {
+
+    #[test]
+    fn test_mount_tmpfs_without_flags_allows_rwx() {
+        require_capability!(
+            "test_mount_tmpfs_without_flags_allows_rwx",
+            CAP_SYS_ADMIN
+        );
         let tempdir = tempfile::tempdir().unwrap();
 
         mount(
@@ -48,28 +46,6 @@ exit 23";
             .write(true)
             .mode((Mode::S_IRWXU | Mode::S_IRWXG | Mode::S_IRWXO).bits())
             .open(&test_path)
-            .or_else(|e| {
-                if Errno::from_raw(e.raw_os_error().unwrap())
-                    == Errno::EOVERFLOW
-                {
-                    // Skip tests on certain Linux kernels which have a bug
-                    // regarding tmpfs in namespaces.
-                    // Ubuntu 14.04 and 16.04 are known to be affected; 16.10 is
-                    // not.  There is no legitimate reason for open(2) to return
-                    // EOVERFLOW here.
-                    // https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1659087
-                    let stderr = io::stderr();
-                    let mut handle = stderr.lock();
-                    writeln!(
-                        handle,
-                        "Buggy Linux kernel detected.  Skipping test."
-                    )
-                    .unwrap();
-                    process::exit(0);
-                } else {
-                    panic!("open failed: {e}");
-                }
-            })
             .and_then(|mut f| f.write(SCRIPT_CONTENTS))
             .unwrap_or_else(|e| panic!("write failed: {e}"));
 
@@ -93,7 +69,9 @@ exit 23";
         umount(tempdir.path()).unwrap_or_else(|e| panic!("umount failed: {e}"));
     }
 
-    pub fn test_mount_rdonly_disallows_write() {
+    #[test]
+    fn test_mount_rdonly_disallows_write() {
+        require_capability!("test_mount_rdonly_disallows_write", CAP_SYS_ADMIN);
         let tempdir = tempfile::tempdir().unwrap();
 
         mount(
@@ -117,7 +95,9 @@ exit 23";
         umount(tempdir.path()).unwrap_or_else(|e| panic!("umount failed: {e}"));
     }
 
-    pub fn test_mount_noexec_disallows_exec() {
+    #[test]
+    fn test_mount_noexec_disallows_exec() {
+        require_capability!("test_mount_noexec_disallows_exec", CAP_SYS_ADMIN);
         let tempdir = tempfile::tempdir().unwrap();
 
         mount(
@@ -165,7 +145,9 @@ exit 23";
         umount(tempdir.path()).unwrap_or_else(|e| panic!("umount failed: {e}"));
     }
 
-    pub fn test_mount_bind() {
+    #[test]
+    fn test_mount_bind() {
+        require_capability!("test_mount_bind", CAP_SYS_ADMIN);
         let tempdir = tempfile::tempdir().unwrap();
         let file_name = "test";
 
@@ -202,66 +184,4 @@ exit 23";
             .unwrap_or_else(|e| panic!("read failed: {e}"));
         assert_eq!(buf, SCRIPT_CONTENTS);
     }
-
-    pub fn setup_namespaces() {
-        // Hold on to the uid in the parent namespace.
-        let uid = getuid();
-
-        unshare(CloneFlags::CLONE_NEWNS | CloneFlags::CLONE_NEWUSER).unwrap_or_else(|e| {
-            let stderr = io::stderr();
-            let mut handle = stderr.lock();
-            writeln!(handle,
-                     "unshare failed: {e}. Are unprivileged user namespaces available?").unwrap();
-            writeln!(handle, "mount is not being tested").unwrap();
-            // Exit with success because not all systems support unprivileged user namespaces, and
-            // that's not what we're testing for.
-            process::exit(0);
-        });
-
-        // Map user as uid 1000.
-        fs::OpenOptions::new()
-            .write(true)
-            .open("/proc/self/uid_map")
-            .and_then(|mut f| f.write(format!("1000 {uid} 1\n").as_bytes()))
-            .unwrap_or_else(|e| panic!("could not write uid map: {e}"));
-    }
 }
-
-// Test runner
-
-/// Mimic normal test output (hackishly).
-#[cfg(target_os = "linux")]
-macro_rules! run_tests {
-    ( $($test_fn:ident),* ) => {{
-        println!();
-
-        $(
-            print!("test test_mount::{} ... ", stringify!($test_fn));
-            $test_fn();
-            println!("ok");
-        )*
-
-        println!();
-    }}
-}
-
-#[cfg(target_os = "linux")]
-fn main() {
-    use test_mount::{
-        setup_namespaces, test_mount_bind, test_mount_noexec_disallows_exec,
-        test_mount_rdonly_disallows_write,
-        test_mount_tmpfs_without_flags_allows_rwx,
-    };
-    skip_if_cirrus!("Fails for an unknown reason Cirrus CI.  Bug #1351");
-    setup_namespaces();
-
-    run_tests!(
-        test_mount_tmpfs_without_flags_allows_rwx,
-        test_mount_rdonly_disallows_write,
-        test_mount_noexec_disallows_exec,
-        test_mount_bind
-    );
-}
-
-#[cfg(not(target_os = "linux"))]
-fn main() {}