std: Synchronize access to global env during `exec`

This commit, after reverting #55359, applies a different fix for #46775
while also fixing #55775. The basic idea was to go back to pre-#55359
libstd, and then fix #46775 in a way that doesn't expose #55775.

The issue described in #46775 boils down to two problems:

* First, the global environment is reset during `exec` but, but if the
  `exec` call fails then the global environment was a dangling pointer
  into free'd memory as the block of memory was deallocated when
  `Command` is dropped. This is fixed in this commit by installing a
  `Drop` stack object which ensures that the `environ` pointer is
  preserved on a failing `exec`.

* Second, the global environment was accessed in an unsynchronized
  fashion during `exec`. This was fixed by ensuring that the
  Rust-specific environment lock is acquired for these system-level
  operations.

Thanks to Alex Gaynor for pioneering the solution here!

Closes #55775
Co-authored-by: NAlex Gaynor <alex.gaynor@gmail.com>

src/libstd/sys/unix/os.rs

@@ -27,15 +27,12 @@
 use ptr;
 use slice;
 use str;
-use sys_common::mutex::Mutex;
+use sys_common::mutex::{Mutex, MutexGuard};
 use sys::cvt;
 use sys::fd;
 use vec;
 
 const TMPBUF_SZ: usize = 128;
-// We never call `ENV_LOCK.init()`, so it is UB to attempt to
-// acquire this mutex reentrantly!
-static ENV_LOCK: Mutex = Mutex::new();
 
 
 extern {
@@ -408,11 +405,18 @@ pub unsafe fn environ() -> *mut *const *const c_char {
     &mut environ
 }
 
+pub unsafe fn env_lock() -> MutexGuard<'static> {
+    // We never call `ENV_LOCK.init()`, so it is UB to attempt to
+    // acquire this mutex reentrantly!
+    static ENV_LOCK: Mutex = Mutex::new();
+    ENV_LOCK.lock()
+}
+
 /// Returns a vector of (variable, value) byte-vector pairs for all the
 /// environment variables of the current process.
 pub fn env() -> Env {
     unsafe {
-        let _guard = ENV_LOCK.lock();
+        let _guard = env_lock();
         let mut environ = *environ();
         let mut result = Vec::new();
         while environ != ptr::null() && *environ != ptr::null() {
@@ -448,7 +452,7 @@ pub fn getenv(k: &OsStr) -> io::Result<Option<OsString>> {
     // always None as well
     let k = CString::new(k.as_bytes())?;
     unsafe {
-        let _guard = ENV_LOCK.lock();
+        let _guard = env_lock();
         let s = libc::getenv(k.as_ptr()) as *const libc::c_char;
         let ret = if s.is_null() {
             None
@@ -464,7 +468,7 @@ pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
     let v = CString::new(v.as_bytes())?;
 
     unsafe {
-        let _guard = ENV_LOCK.lock();
+        let _guard = env_lock();
         cvt(libc::setenv(k.as_ptr(), v.as_ptr(), 1)).map(|_| ())
     }
 }
@@ -473,7 +477,7 @@ pub fn unsetenv(n: &OsStr) -> io::Result<()> {
     let nbuf = CString::new(n.as_bytes())?;
 
     unsafe {
-        let _guard = ENV_LOCK.lock();
+        let _guard = env_lock();
         cvt(libc::unsetenv(nbuf.as_ptr())).map(|_| ())
     }
 }

src/libstd/sys/unix/process/process_unix.rs

@@ -11,9 +11,9 @@
 use io::{self, Error, ErrorKind};
 use libc::{self, c_int, gid_t, pid_t, uid_t};
 use ptr;
-
 use sys::cvt;
 use sys::process::process_common::*;
+use sys;
 
 ////////////////////////////////////////////////////////////////////////////////
 // Command
@@ -22,8 +22,6 @@
 impl Command {
     pub fn spawn(&mut self, default: Stdio, needs_stdin: bool)
                  -> io::Result<(Process, StdioPipes)> {
-        use sys;
-
         const CLOEXEC_MSG_FOOTER: &'static [u8] = b"NOEX";
 
         let envp = self.capture_env();
@@ -41,8 +39,21 @@ pub fn spawn(&mut self, default: Stdio, needs_stdin: bool)
 
         let (input, output) = sys::pipe::anon_pipe()?;
 
+        // Whatever happens after the fork is almost for sure going to touch or
+        // look at the environment in one way or another (PATH in `execvp` or
+        // accessing the `environ` pointer ourselves). Make sure no other thread
+        // is accessing the environment when we do the fork itself.
+        //
+        // Note that as soon as we're done with the fork there's no need to hold
+        // a lock any more because the parent won't do anything and the child is
+        // in its own process.
+        let result = unsafe {
+            let _env_lock = sys::os::env_lock();
+            cvt(libc::fork())?
+        };
+
         let pid = unsafe {
-            match cvt(libc::fork())? {
+            match result {
                 0 => {
                     drop(input);
                     let err = self.do_exec(theirs, envp.as_ref());
@@ -114,7 +125,16 @@ pub fn exec(&mut self, default: Stdio) -> io::Error {
         }
 
         match self.setup_io(default, true) {
-            Ok((_, theirs)) => unsafe { self.do_exec(theirs, envp.as_ref()) },
+            Ok((_, theirs)) => {
+                unsafe {
+                    // Similar to when forking, we want to ensure that access to
+                    // the environment is synchronized, so make sure to grab the
+                    // environment lock before we try to exec.
+                    let _lock = sys::os::env_lock();
+
+                    self.do_exec(theirs, envp.as_ref())
+                }
+            }
             Err(e) => e,
         }
     }
@@ -193,9 +213,6 @@ unsafe fn do_exec(
         if let Some(ref cwd) = *self.get_cwd() {
             t!(cvt(libc::chdir(cwd.as_ptr())));
         }
-        if let Some(envp) = maybe_envp {
-            *sys::os::environ() = envp.as_ptr();
-        }
 
         // emscripten has no signal support.
         #[cfg(not(any(target_os = "emscripten")))]
@@ -231,6 +248,27 @@ unsafe fn do_exec(
             t!(callback());
         }
 
+        // Although we're performing an exec here we may also return with an
+        // error from this function (without actually exec'ing) in which case we
+        // want to be sure to restore the global environment back to what it
+        // once was, ensuring that our temporary override, when free'd, doesn't
+        // corrupt our process's environment.
+        let mut _reset = None;
+        if let Some(envp) = maybe_envp {
+            struct Reset(*const *const libc::c_char);
+
+            impl Drop for Reset {
+                fn drop(&mut self) {
+                    unsafe {
+                        *sys::os::environ() = self.0;
+                    }
+                }
+            }
+
+            _reset = Some(Reset(*sys::os::environ()));
+            *sys::os::environ() = envp.as_ptr();
+        }
+
         libc::execvp(self.get_argv()[0], self.get_argv().as_ptr());
         io::Error::last_os_error()
     }
@@ -330,6 +368,8 @@ fn drop(&mut self) {
                 libc::POSIX_SPAWN_SETSIGMASK;
             cvt(libc::posix_spawnattr_setflags(&mut attrs.0, flags as _))?;
 
+            // Make sure we synchronize access to the global `environ` resource
+            let _env_lock = sys::os::env_lock();
             let envp = envp.map(|c| c.as_ptr())
                 .unwrap_or_else(|| *sys::os::environ() as *const _);
             let ret = libc::posix_spawnp(

src/test/run-pass/command-exec.rs

@@ -48,6 +48,23 @@ fn main() {
                 println!("passed");
             }
 
+            "exec-test5" => {
+                env::set_var("VARIABLE", "ABC");
+                Command::new("definitely-not-a-real-binary").env("VARIABLE", "XYZ").exec();
+                assert_eq!(env::var("VARIABLE").unwrap(), "ABC");
+                println!("passed");
+            }
+
+            "exec-test6" => {
+                let err = Command::new("echo").arg("passed").env_clear().exec();
+                panic!("failed to spawn: {}", err);
+            }
+
+            "exec-test7" => {
+                let err = Command::new("echo").arg("passed").env_remove("PATH").exec();
+                panic!("failed to spawn: {}", err);
+            }
+
             _ => panic!("unknown argument: {}", arg),
         }
         return
@@ -72,4 +89,23 @@ fn main() {
     assert!(output.status.success());
     assert!(output.stderr.is_empty());
     assert_eq!(output.stdout, b"passed\n");
+
+    let output = Command::new(&me).arg("exec-test5").output().unwrap();
+    assert!(output.status.success());
+    assert!(output.stderr.is_empty());
+    assert_eq!(output.stdout, b"passed\n");
+
+    if cfg!(target_os = "linux") {
+        let output = Command::new(&me).arg("exec-test6").output().unwrap();
+        println!("{:?}", output);
+        assert!(output.status.success());
+        assert!(output.stderr.is_empty());
+        assert_eq!(output.stdout, b"passed\n");
+
+        let output = Command::new(&me).arg("exec-test7").output().unwrap();
+        println!("{:?}", output);
+        assert!(output.status.success());
+        assert!(output.stderr.is_empty());
+        assert_eq!(output.stdout, b"passed\n");
+    }
 }