From 64e5327b6e7ad79f4a3ca7de17ac105c8c59277e Mon Sep 17 00:00:00 2001
From: Vytautas Astrauskas <astrauv@amazon.com>
Date: Mon, 30 Mar 2020 20:55:17 -0700
Subject: [PATCH 1/9] Fix double-free and undefined behaviour in
 libstd::syn::unix::Thread::new.

---
 src/libstd/sys/cloudabi/thread.rs | 13 ++++++++++---
 src/libstd/sys/hermit/thread.rs   | 14 ++++++++------
 src/libstd/sys/unix/thread.rs     | 13 ++++++++++---
 src/libstd/sys/vxworks/thread.rs  | 13 ++++++++++---
 src/libstd/sys/windows/thread.rs  |  8 +++++---
 5 files changed, 43 insertions(+), 18 deletions(-)

diff --git a/src/libstd/sys/cloudabi/thread.rs b/src/libstd/sys/cloudabi/thread.rs
index 3afcae7ae7516..a3595debaf591 100644
--- a/src/libstd/sys/cloudabi/thread.rs
+++ b/src/libstd/sys/cloudabi/thread.rs
@@ -22,7 +22,7 @@ unsafe impl Sync for Thread {}
 impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements
     pub unsafe fn new(stack: usize, p: Box<dyn FnOnce()>) -> io::Result<Thread> {
-        let p = box p;
+        let mut p = mem::ManuallyDrop::new(box p);
         let mut native: libc::pthread_t = mem::zeroed();
         let mut attr: libc::pthread_attr_t = mem::zeroed();
         assert_eq!(libc::pthread_attr_init(&mut attr), 0);
@@ -30,13 +30,20 @@ impl Thread {
         let stack_size = cmp::max(stack, min_stack_size(&attr));
         assert_eq!(libc::pthread_attr_setstacksize(&mut attr, stack_size), 0);
 
-        let ret = libc::pthread_create(&mut native, &attr, thread_start, &*p as *const _ as *mut _);
+        let ret = libc::pthread_create(
+            &mut native,
+            &attr,
+            thread_start,
+            &mut *p as &mut Box<dyn FnOnce()> as *mut _ as *mut _,
+        );
         assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
 
         return if ret != 0 {
+            // The thread failed to start and as a result p was not consumed. Therefore, it is
+            // safe to manually drop it.
+            mem::ManuallyDrop::drop(&mut p);
             Err(io::Error::from_raw_os_error(ret))
         } else {
-            mem::forget(p); // ownership passed to pthread_create
             Ok(Thread { id: native })
         };
 
diff --git a/src/libstd/sys/hermit/thread.rs b/src/libstd/sys/hermit/thread.rs
index c3c29c93826de..8e15208abc246 100644
--- a/src/libstd/sys/hermit/thread.rs
+++ b/src/libstd/sys/hermit/thread.rs
@@ -49,21 +49,23 @@ impl Thread {
         p: Box<dyn FnOnce()>,
         core_id: isize,
     ) -> io::Result<Thread> {
-        let p = box p;
+        let mut p = mem::ManuallyDrop::new(box p);
         let mut tid: Tid = u32::MAX;
         let ret = abi::spawn(
             &mut tid as *mut Tid,
             thread_start,
-            &*p as *const _ as *const u8 as usize,
+            &mut *p as &mut Box<dyn FnOnce()> as *mut _ as *mut u8 as usize,
             Priority::into(NORMAL_PRIO),
             core_id,
         );
 
-        return if ret == 0 {
-            mem::forget(p); // ownership passed to pthread_create
-            Ok(Thread { tid: tid })
-        } else {
+        return if ret != 0 {
+            // The thread failed to start and as a result p was not consumed. Therefore, it is
+            // safe to manually drop it.
+            mem::ManuallyDrop::drop(&mut p);
             Err(io::Error::new(io::ErrorKind::Other, "Unable to create thread!"))
+        } else {
+            Ok(Thread { tid: tid })
         };
 
         extern "C" fn thread_start(main: usize) {
diff --git a/src/libstd/sys/unix/thread.rs b/src/libstd/sys/unix/thread.rs
index 674d4c7113801..efcd614202468 100644
--- a/src/libstd/sys/unix/thread.rs
+++ b/src/libstd/sys/unix/thread.rs
@@ -43,7 +43,7 @@ unsafe fn pthread_attr_setstacksize(
 impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements
     pub unsafe fn new(stack: usize, p: Box<dyn FnOnce()>) -> io::Result<Thread> {
-        let p = box p;
+        let mut p = mem::ManuallyDrop::new(box p);
         let mut native: libc::pthread_t = mem::zeroed();
         let mut attr: libc::pthread_attr_t = mem::zeroed();
         assert_eq!(libc::pthread_attr_init(&mut attr), 0);
@@ -65,13 +65,20 @@ impl Thread {
             }
         };
 
-        let ret = libc::pthread_create(&mut native, &attr, thread_start, &*p as *const _ as *mut _);
+        let ret = libc::pthread_create(
+            &mut native,
+            &attr,
+            thread_start,
+            &mut *p as &mut Box<dyn FnOnce()> as *mut _ as *mut _,
+        );
         assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
 
         return if ret != 0 {
+            // The thread failed to start and as a result p was not consumed. Therefore, it is
+            // safe to manually drop it.
+            mem::ManuallyDrop::drop(&mut p);
             Err(io::Error::from_raw_os_error(ret))
         } else {
-            mem::forget(p); // ownership passed to pthread_create
             Ok(Thread { id: native })
         };
 
diff --git a/src/libstd/sys/vxworks/thread.rs b/src/libstd/sys/vxworks/thread.rs
index e0d104b5f3ec9..81233c1975c79 100644
--- a/src/libstd/sys/vxworks/thread.rs
+++ b/src/libstd/sys/vxworks/thread.rs
@@ -31,7 +31,7 @@ unsafe fn pthread_attr_setstacksize(
 impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements
     pub unsafe fn new(stack: usize, p: Box<dyn FnOnce()>) -> io::Result<Thread> {
-        let p = box p;
+        let mut p = mem::ManuallyDrop::new(box p);
         let mut native: libc::pthread_t = mem::zeroed();
         let mut attr: libc::pthread_attr_t = mem::zeroed();
         assert_eq!(libc::pthread_attr_init(&mut attr), 0);
@@ -53,13 +53,20 @@ impl Thread {
             }
         };
 
-        let ret = libc::pthread_create(&mut native, &attr, thread_start, &*p as *const _ as *mut _);
+        let ret = libc::pthread_create(
+            &mut native,
+            &attr,
+            thread_start,
+            &mut *p as &mut Box<dyn FnOnce()> as *mut _ as *mut _,
+        );
         assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
 
         return if ret != 0 {
+            // The thread failed to start and as a result p was not consumed. Therefore, it is
+            // safe to manually drop it.
+            mem::ManuallyDrop::drop(&mut p);
             Err(io::Error::from_raw_os_error(ret))
         } else {
-            mem::forget(p); // ownership passed to pthread_create
             Ok(Thread { id: native })
         };
 
diff --git a/src/libstd/sys/windows/thread.rs b/src/libstd/sys/windows/thread.rs
index c828243a59b11..052f51a33ceeb 100644
--- a/src/libstd/sys/windows/thread.rs
+++ b/src/libstd/sys/windows/thread.rs
@@ -20,7 +20,7 @@ pub struct Thread {
 impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements
     pub unsafe fn new(stack: usize, p: Box<dyn FnOnce()>) -> io::Result<Thread> {
-        let p = box p;
+        let mut p = mem::ManuallyDrop::new(box p);
 
         // FIXME On UNIX, we guard against stack sizes that are too small but
         // that's because pthreads enforces that stacks are at least
@@ -34,15 +34,17 @@ impl Thread {
             ptr::null_mut(),
             stack_size,
             thread_start,
-            &*p as *const _ as *mut _,
+            &mut *p as &mut Box<dyn FnOnce()> as *mut _ as *mut _,
             c::STACK_SIZE_PARAM_IS_A_RESERVATION,
             ptr::null_mut(),
         );
 
         return if ret as usize == 0 {
+            // The thread failed to start and as a result p was not consumed. Therefore, it is
+            // safe to manually drop it.
+            mem::ManuallyDrop::drop(&mut p);
             Err(io::Error::last_os_error())
         } else {
-            mem::forget(p); // ownership passed to CreateThread
             Ok(Thread { handle: Handle::new(ret) })
         };
 

From 753bc7ddf8a0f00acf039731947a12d06ad30884 Mon Sep 17 00:00:00 2001
From: Vytautas Astrauskas <astrauv@amazon.com>
Date: Tue, 31 Mar 2020 12:35:07 -0700
Subject: [PATCH 2/9] Inline start_thread into its callers.

---
 src/libstd/sys/cloudabi/thread.rs |  8 ++++++--
 src/libstd/sys/hermit/thread.rs   |  9 ++++++---
 src/libstd/sys/unix/thread.rs     | 10 ++++++----
 src/libstd/sys/vxworks/thread.rs  | 10 ++++++----
 src/libstd/sys/windows/thread.rs  |  8 ++++++--
 src/libstd/sys_common/thread.rs   | 11 -----------
 6 files changed, 30 insertions(+), 26 deletions(-)

diff --git a/src/libstd/sys/cloudabi/thread.rs b/src/libstd/sys/cloudabi/thread.rs
index a3595debaf591..112bb1ce3af0a 100644
--- a/src/libstd/sys/cloudabi/thread.rs
+++ b/src/libstd/sys/cloudabi/thread.rs
@@ -4,8 +4,8 @@ use crate::io;
 use crate::mem;
 use crate::ptr;
 use crate::sys::cloudabi::abi;
+use crate::sys::stack_overflow;
 use crate::sys::time::checked_dur2intervals;
-use crate::sys_common::thread::*;
 use crate::time::Duration;
 
 pub const DEFAULT_MIN_STACK_SIZE: usize = 2 * 1024 * 1024;
@@ -49,7 +49,11 @@ impl Thread {
 
         extern "C" fn thread_start(main: *mut libc::c_void) -> *mut libc::c_void {
             unsafe {
-                start_thread(main as *mut u8);
+                // Next, set up our stack overflow handler which may get triggered if we run
+                // out of stack.
+                let _handler = stack_overflow::Handler::new();
+                // Finally, let's run some code.
+                Box::from_raw(main as *mut Box<dyn FnOnce()>)();
             }
             ptr::null_mut()
         }
diff --git a/src/libstd/sys/hermit/thread.rs b/src/libstd/sys/hermit/thread.rs
index 8e15208abc246..f92c18a3a4521 100644
--- a/src/libstd/sys/hermit/thread.rs
+++ b/src/libstd/sys/hermit/thread.rs
@@ -5,11 +5,10 @@ use crate::fmt;
 use crate::io;
 use crate::mem;
 use crate::sys::hermit::abi;
+use crate::sys::stack_overflow;
 use crate::time::Duration;
 use core::u32;
 
-use crate::sys_common::thread::*;
-
 pub type Tid = abi::Tid;
 
 /// Priority of a task
@@ -70,7 +69,11 @@ impl Thread {
 
         extern "C" fn thread_start(main: usize) {
             unsafe {
-                start_thread(main as *mut u8);
+                // Next, set up our stack overflow handler which may get triggered if we run
+                // out of stack.
+                let _handler = stack_overflow::Handler::new();
+                // Finally, let's run some code.
+                Box::from_raw(main as *mut Box<dyn FnOnce()>)();
             }
         }
     }
diff --git a/src/libstd/sys/unix/thread.rs b/src/libstd/sys/unix/thread.rs
index efcd614202468..513a1fde331b7 100644
--- a/src/libstd/sys/unix/thread.rs
+++ b/src/libstd/sys/unix/thread.rs
@@ -3,11 +3,9 @@ use crate::ffi::CStr;
 use crate::io;
 use crate::mem;
 use crate::ptr;
-use crate::sys::os;
+use crate::sys::{os, stack_overflow};
 use crate::time::Duration;
 
-use crate::sys_common::thread::*;
-
 #[cfg(not(target_os = "l4re"))]
 pub const DEFAULT_MIN_STACK_SIZE: usize = 2 * 1024 * 1024;
 #[cfg(target_os = "l4re")]
@@ -84,7 +82,11 @@ impl Thread {
 
         extern "C" fn thread_start(main: *mut libc::c_void) -> *mut libc::c_void {
             unsafe {
-                start_thread(main as *mut u8);
+                // Next, set up our stack overflow handler which may get triggered if we run
+                // out of stack.
+                let _handler = stack_overflow::Handler::new();
+                // Finally, let's run some code.
+                Box::from_raw(main as *mut Box<dyn FnOnce()>)();
             }
             ptr::null_mut()
         }
diff --git a/src/libstd/sys/vxworks/thread.rs b/src/libstd/sys/vxworks/thread.rs
index 81233c1975c79..92163865b799a 100644
--- a/src/libstd/sys/vxworks/thread.rs
+++ b/src/libstd/sys/vxworks/thread.rs
@@ -3,11 +3,9 @@ use crate::ffi::CStr;
 use crate::io;
 use crate::mem;
 use crate::ptr;
-use crate::sys::os;
+use crate::sys::{os, stack_overflow};
 use crate::time::Duration;
 
-use crate::sys_common::thread::*;
-
 pub const DEFAULT_MIN_STACK_SIZE: usize = 0x40000; // 256K
 
 pub struct Thread {
@@ -72,7 +70,11 @@ impl Thread {
 
         extern "C" fn thread_start(main: *mut libc::c_void) -> *mut libc::c_void {
             unsafe {
-                start_thread(main as *mut u8);
+                // Next, set up our stack overflow handler which may get triggered if we run
+                // out of stack.
+                let _handler = stack_overflow::Handler::new();
+                // Finally, let's run some code.
+                Box::from_raw(main as *mut Box<dyn FnOnce()>)();
             }
             ptr::null_mut()
         }
diff --git a/src/libstd/sys/windows/thread.rs b/src/libstd/sys/windows/thread.rs
index 052f51a33ceeb..a1cad19d0f57e 100644
--- a/src/libstd/sys/windows/thread.rs
+++ b/src/libstd/sys/windows/thread.rs
@@ -4,7 +4,7 @@ use crate::mem;
 use crate::ptr;
 use crate::sys::c;
 use crate::sys::handle::Handle;
-use crate::sys_common::thread::*;
+use crate::sys::stack_overflow;
 use crate::time::Duration;
 
 use libc::c_void;
@@ -50,7 +50,11 @@ impl Thread {
 
         extern "system" fn thread_start(main: *mut c_void) -> c::DWORD {
             unsafe {
-                start_thread(main as *mut u8);
+                // Next, set up our stack overflow handler which may get triggered if we run
+                // out of stack.
+                let _handler = stack_overflow::Handler::new();
+                // Finally, let's run some code.
+                Box::from_raw(main as *mut Box<dyn FnOnce()>)();
             }
             0
         }
diff --git a/src/libstd/sys_common/thread.rs b/src/libstd/sys_common/thread.rs
index 6ab0d5cbe9c96..f3a8bef8f718f 100644
--- a/src/libstd/sys_common/thread.rs
+++ b/src/libstd/sys_common/thread.rs
@@ -1,18 +1,7 @@
 use crate::env;
 use crate::sync::atomic::{self, Ordering};
-use crate::sys::stack_overflow;
 use crate::sys::thread as imp;
 
-#[allow(dead_code)]
-pub unsafe fn start_thread(main: *mut u8) {
-    // Next, set up our stack overflow handler which may get triggered if we run
-    // out of stack.
-    let _handler = stack_overflow::Handler::new();
-
-    // Finally, let's run some code.
-    Box::from_raw(main as *mut Box<dyn FnOnce()>)()
-}
-
 pub fn min_stack() -> usize {
     static MIN: atomic::AtomicUsize = atomic::AtomicUsize::new(0);
     match MIN.load(Ordering::SeqCst) {

From 5382347064ac47a2a5ac56b57cec0d91b9b40edc Mon Sep 17 00:00:00 2001
From: Vytautas Astrauskas <astrauv@amazon.com>
Date: Tue, 31 Mar 2020 17:51:51 -0700
Subject: [PATCH 3/9] Use Box::into_raw instead of ManuallyDrop in Thread::new.

---
 src/libstd/sys/cloudabi/thread.rs | 13 ++++---------
 src/libstd/sys/hermit/thread.rs   |  8 ++++----
 src/libstd/sys/unix/thread.rs     | 13 ++++---------
 src/libstd/sys/vxworks/thread.rs  | 13 ++++---------
 src/libstd/sys/windows/thread.rs  |  9 ++++-----
 5 files changed, 20 insertions(+), 36 deletions(-)

diff --git a/src/libstd/sys/cloudabi/thread.rs b/src/libstd/sys/cloudabi/thread.rs
index 112bb1ce3af0a..9d95a61c3154d 100644
--- a/src/libstd/sys/cloudabi/thread.rs
+++ b/src/libstd/sys/cloudabi/thread.rs
@@ -22,7 +22,7 @@ unsafe impl Sync for Thread {}
 impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements
     pub unsafe fn new(stack: usize, p: Box<dyn FnOnce()>) -> io::Result<Thread> {
-        let mut p = mem::ManuallyDrop::new(box p);
+        let p = Box::into_raw(box p);
         let mut native: libc::pthread_t = mem::zeroed();
         let mut attr: libc::pthread_attr_t = mem::zeroed();
         assert_eq!(libc::pthread_attr_init(&mut attr), 0);
@@ -30,18 +30,13 @@ impl Thread {
         let stack_size = cmp::max(stack, min_stack_size(&attr));
         assert_eq!(libc::pthread_attr_setstacksize(&mut attr, stack_size), 0);
 
-        let ret = libc::pthread_create(
-            &mut native,
-            &attr,
-            thread_start,
-            &mut *p as &mut Box<dyn FnOnce()> as *mut _ as *mut _,
-        );
+        let ret = libc::pthread_create(&mut native, &attr, thread_start, p as *mut _);
         assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
 
         return if ret != 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
-            // safe to manually drop it.
-            mem::ManuallyDrop::drop(&mut p);
+            // safe to reconstruct the box so that it gets deallocated.
+            let _ = Box::from_raw(p);
             Err(io::Error::from_raw_os_error(ret))
         } else {
             Ok(Thread { id: native })
diff --git a/src/libstd/sys/hermit/thread.rs b/src/libstd/sys/hermit/thread.rs
index f92c18a3a4521..6b00903780541 100644
--- a/src/libstd/sys/hermit/thread.rs
+++ b/src/libstd/sys/hermit/thread.rs
@@ -48,20 +48,20 @@ impl Thread {
         p: Box<dyn FnOnce()>,
         core_id: isize,
     ) -> io::Result<Thread> {
-        let mut p = mem::ManuallyDrop::new(box p);
+        let p = Box::into_raw(box p);
         let mut tid: Tid = u32::MAX;
         let ret = abi::spawn(
             &mut tid as *mut Tid,
             thread_start,
-            &mut *p as &mut Box<dyn FnOnce()> as *mut _ as *mut u8 as usize,
+            p as *mut u8 as usize,
             Priority::into(NORMAL_PRIO),
             core_id,
         );
 
         return if ret != 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
-            // safe to manually drop it.
-            mem::ManuallyDrop::drop(&mut p);
+            // safe to reconstruct the box so that it gets deallocated.
+            let _ = Box::from_raw(p);
             Err(io::Error::new(io::ErrorKind::Other, "Unable to create thread!"))
         } else {
             Ok(Thread { tid: tid })
diff --git a/src/libstd/sys/unix/thread.rs b/src/libstd/sys/unix/thread.rs
index 513a1fde331b7..1cad474e33e63 100644
--- a/src/libstd/sys/unix/thread.rs
+++ b/src/libstd/sys/unix/thread.rs
@@ -41,7 +41,7 @@ unsafe fn pthread_attr_setstacksize(
 impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements
     pub unsafe fn new(stack: usize, p: Box<dyn FnOnce()>) -> io::Result<Thread> {
-        let mut p = mem::ManuallyDrop::new(box p);
+        let p = Box::into_raw(box p);
         let mut native: libc::pthread_t = mem::zeroed();
         let mut attr: libc::pthread_attr_t = mem::zeroed();
         assert_eq!(libc::pthread_attr_init(&mut attr), 0);
@@ -63,18 +63,13 @@ impl Thread {
             }
         };
 
-        let ret = libc::pthread_create(
-            &mut native,
-            &attr,
-            thread_start,
-            &mut *p as &mut Box<dyn FnOnce()> as *mut _ as *mut _,
-        );
+        let ret = libc::pthread_create(&mut native, &attr, thread_start, p as *mut _);
         assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
 
         return if ret != 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
-            // safe to manually drop it.
-            mem::ManuallyDrop::drop(&mut p);
+            // safe to reconstruct the box so that it gets deallocated.
+            let _ = Box::from_raw(p);
             Err(io::Error::from_raw_os_error(ret))
         } else {
             Ok(Thread { id: native })
diff --git a/src/libstd/sys/vxworks/thread.rs b/src/libstd/sys/vxworks/thread.rs
index 92163865b799a..3c9557db94ade 100644
--- a/src/libstd/sys/vxworks/thread.rs
+++ b/src/libstd/sys/vxworks/thread.rs
@@ -29,7 +29,7 @@ unsafe fn pthread_attr_setstacksize(
 impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements
     pub unsafe fn new(stack: usize, p: Box<dyn FnOnce()>) -> io::Result<Thread> {
-        let mut p = mem::ManuallyDrop::new(box p);
+        let p = Box::into_raw(box p);
         let mut native: libc::pthread_t = mem::zeroed();
         let mut attr: libc::pthread_attr_t = mem::zeroed();
         assert_eq!(libc::pthread_attr_init(&mut attr), 0);
@@ -51,18 +51,13 @@ impl Thread {
             }
         };
 
-        let ret = libc::pthread_create(
-            &mut native,
-            &attr,
-            thread_start,
-            &mut *p as &mut Box<dyn FnOnce()> as *mut _ as *mut _,
-        );
+        let ret = libc::pthread_create(&mut native, &attr, thread_start, p as *mut _);
         assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
 
         return if ret != 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
-            // safe to manually drop it.
-            mem::ManuallyDrop::drop(&mut p);
+            // safe to reconstruct the box so that it gets deallocated.
+            let _ = Box::from_raw(p);
             Err(io::Error::from_raw_os_error(ret))
         } else {
             Ok(Thread { id: native })
diff --git a/src/libstd/sys/windows/thread.rs b/src/libstd/sys/windows/thread.rs
index a1cad19d0f57e..e39c1c0a13261 100644
--- a/src/libstd/sys/windows/thread.rs
+++ b/src/libstd/sys/windows/thread.rs
@@ -1,6 +1,5 @@
 use crate::ffi::CStr;
 use crate::io;
-use crate::mem;
 use crate::ptr;
 use crate::sys::c;
 use crate::sys::handle::Handle;
@@ -20,7 +19,7 @@ pub struct Thread {
 impl Thread {
     // unsafe: see thread::Builder::spawn_unchecked for safety requirements
     pub unsafe fn new(stack: usize, p: Box<dyn FnOnce()>) -> io::Result<Thread> {
-        let mut p = mem::ManuallyDrop::new(box p);
+        let p = Box::into_raw(box p);
 
         // FIXME On UNIX, we guard against stack sizes that are too small but
         // that's because pthreads enforces that stacks are at least
@@ -34,15 +33,15 @@ impl Thread {
             ptr::null_mut(),
             stack_size,
             thread_start,
-            &mut *p as &mut Box<dyn FnOnce()> as *mut _ as *mut _,
+            p as *mut _,
             c::STACK_SIZE_PARAM_IS_A_RESERVATION,
             ptr::null_mut(),
         );
 
         return if ret as usize == 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
-            // safe to manually drop it.
-            mem::ManuallyDrop::drop(&mut p);
+            // safe to reconstruct the box so that it gets deallocated.
+            let _ = Box::from_raw(p);
             Err(io::Error::last_os_error())
         } else {
             Ok(Thread { handle: Handle::new(ret) })

From baa6d557a7b965ff8277f940a43e0ce3df3b8913 Mon Sep 17 00:00:00 2001
From: Vytautas Astrauskas <astrauv@amazon.com>
Date: Wed, 1 Apr 2020 12:46:14 -0700
Subject: [PATCH 4/9] In Thread::new, add a comment that a panic could cause a
 memory leak.

---
 src/libstd/sys/cloudabi/thread.rs | 5 ++++-
 src/libstd/sys/hermit/thread.rs   | 2 +-
 src/libstd/sys/unix/thread.rs     | 5 ++++-
 src/libstd/sys/vxworks/thread.rs  | 5 ++++-
 src/libstd/sys/windows/thread.rs  | 2 +-
 5 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/src/libstd/sys/cloudabi/thread.rs b/src/libstd/sys/cloudabi/thread.rs
index 9d95a61c3154d..abc15b18e321a 100644
--- a/src/libstd/sys/cloudabi/thread.rs
+++ b/src/libstd/sys/cloudabi/thread.rs
@@ -31,12 +31,15 @@ impl Thread {
         assert_eq!(libc::pthread_attr_setstacksize(&mut attr, stack_size), 0);
 
         let ret = libc::pthread_create(&mut native, &attr, thread_start, p as *mut _);
+        // Note: if the thread creation fails and this assert fails, then p will
+        // be leaked. However, an alternative design could cause double-free
+        // which is clearly worse.
         assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
 
         return if ret != 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
             // safe to reconstruct the box so that it gets deallocated.
-            let _ = Box::from_raw(p);
+            drop(Box::from_raw(p));
             Err(io::Error::from_raw_os_error(ret))
         } else {
             Ok(Thread { id: native })
diff --git a/src/libstd/sys/hermit/thread.rs b/src/libstd/sys/hermit/thread.rs
index 6b00903780541..4f20a6453fc27 100644
--- a/src/libstd/sys/hermit/thread.rs
+++ b/src/libstd/sys/hermit/thread.rs
@@ -61,7 +61,7 @@ impl Thread {
         return if ret != 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
             // safe to reconstruct the box so that it gets deallocated.
-            let _ = Box::from_raw(p);
+            drop(Box::from_raw(p));
             Err(io::Error::new(io::ErrorKind::Other, "Unable to create thread!"))
         } else {
             Ok(Thread { tid: tid })
diff --git a/src/libstd/sys/unix/thread.rs b/src/libstd/sys/unix/thread.rs
index 1cad474e33e63..aab5a92a7ad2a 100644
--- a/src/libstd/sys/unix/thread.rs
+++ b/src/libstd/sys/unix/thread.rs
@@ -64,12 +64,15 @@ impl Thread {
         };
 
         let ret = libc::pthread_create(&mut native, &attr, thread_start, p as *mut _);
+        // Note: if the thread creation fails and this assert fails, then p will
+        // be leaked. However, an alternative design could cause double-free
+        // which is clearly worse.
         assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
 
         return if ret != 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
             // safe to reconstruct the box so that it gets deallocated.
-            let _ = Box::from_raw(p);
+            drop(Box::from_raw(p));
             Err(io::Error::from_raw_os_error(ret))
         } else {
             Ok(Thread { id: native })
diff --git a/src/libstd/sys/vxworks/thread.rs b/src/libstd/sys/vxworks/thread.rs
index 3c9557db94ade..4d0196e4b4de5 100644
--- a/src/libstd/sys/vxworks/thread.rs
+++ b/src/libstd/sys/vxworks/thread.rs
@@ -52,12 +52,15 @@ impl Thread {
         };
 
         let ret = libc::pthread_create(&mut native, &attr, thread_start, p as *mut _);
+        // Note: if the thread creation fails and this assert fails, then p will
+        // be leaked. However, an alternative design could cause double-free
+        // which is clearly worse.
         assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
 
         return if ret != 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
             // safe to reconstruct the box so that it gets deallocated.
-            let _ = Box::from_raw(p);
+            drop(Box::from_raw(p));
             Err(io::Error::from_raw_os_error(ret))
         } else {
             Ok(Thread { id: native })
diff --git a/src/libstd/sys/windows/thread.rs b/src/libstd/sys/windows/thread.rs
index e39c1c0a13261..38839ea5e90ed 100644
--- a/src/libstd/sys/windows/thread.rs
+++ b/src/libstd/sys/windows/thread.rs
@@ -41,7 +41,7 @@ impl Thread {
         return if ret as usize == 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
             // safe to reconstruct the box so that it gets deallocated.
-            let _ = Box::from_raw(p);
+            drop(Box::from_raw(p));
             Err(io::Error::last_os_error())
         } else {
             Ok(Thread { handle: Handle::new(ret) })

From ec8275c364493b938357a3615216e3f30c60a443 Mon Sep 17 00:00:00 2001
From: Vytautas Astrauskas <astrauv@amazon.com>
Date: Thu, 2 Apr 2020 07:15:12 -0700
Subject: [PATCH 5/9] Remove stack overflow handler stub for wasm.

---
 src/libstd/sys/wasm/stack_overflow.rs | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/src/libstd/sys/wasm/stack_overflow.rs b/src/libstd/sys/wasm/stack_overflow.rs
index cbf62b6e5b7e3..32555394cd5a5 100644
--- a/src/libstd/sys/wasm/stack_overflow.rs
+++ b/src/libstd/sys/wasm/stack_overflow.rs
@@ -1,11 +1,3 @@
-pub struct Handler;
-
-impl Handler {
-    pub unsafe fn new() -> Handler {
-        Handler
-    }
-}
-
 pub unsafe fn init() {}
 
 pub unsafe fn cleanup() {}

From 1c1bd957d5c9f5c478a738ae83116c1260bc4896 Mon Sep 17 00:00:00 2001
From: Vytautas Astrauskas <astrauv@amazon.com>
Date: Thu, 2 Apr 2020 07:15:45 -0700
Subject: [PATCH 6/9] Remove unnecessary intermediate pointer cast in
 Thread::new.

---
 src/libstd/sys/hermit/thread.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libstd/sys/hermit/thread.rs b/src/libstd/sys/hermit/thread.rs
index 4f20a6453fc27..beb02d9c8be47 100644
--- a/src/libstd/sys/hermit/thread.rs
+++ b/src/libstd/sys/hermit/thread.rs
@@ -53,7 +53,7 @@ impl Thread {
         let ret = abi::spawn(
             &mut tid as *mut Tid,
             thread_start,
-            p as *mut u8 as usize,
+            p as usize,
             Priority::into(NORMAL_PRIO),
             core_id,
         );

From 53aa5a1113619dda8e1e1b5ebda920ef1adb2395 Mon Sep 17 00:00:00 2001
From: Vytautas Astrauskas <astrauv@amazon.com>
Date: Thu, 2 Apr 2020 16:29:09 -0700
Subject: [PATCH 7/9] Remove unnecessary stack overflow handler stub for sgx.

---
 src/libstd/sys/sgx/stack_overflow.rs | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/src/libstd/sys/sgx/stack_overflow.rs b/src/libstd/sys/sgx/stack_overflow.rs
index a2d13d11849e7..b96652a8330e9 100644
--- a/src/libstd/sys/sgx/stack_overflow.rs
+++ b/src/libstd/sys/sgx/stack_overflow.rs
@@ -1,11 +1,3 @@
-pub struct Handler;
-
-impl Handler {
-    pub unsafe fn new() -> Handler {
-        Handler
-    }
-}
-
 #[cfg_attr(test, allow(dead_code))]
 pub unsafe fn init() {}
 

From d637d6e7a832ac8015ae62cfc51d3e27f387a108 Mon Sep 17 00:00:00 2001
From: Vytautas Astrauskas <astrauv@amazon.com>
Date: Fri, 3 Apr 2020 10:07:40 -0700
Subject: [PATCH 8/9] Delete unnecessary stub stack overflow handler for
 hermit.

---
 src/libstd/sys/hermit/stack_overflow.rs | 8 --------
 src/libstd/sys/hermit/thread.rs         | 4 ----
 2 files changed, 12 deletions(-)

diff --git a/src/libstd/sys/hermit/stack_overflow.rs b/src/libstd/sys/hermit/stack_overflow.rs
index 65a1b17acce9a..121fe42011da5 100644
--- a/src/libstd/sys/hermit/stack_overflow.rs
+++ b/src/libstd/sys/hermit/stack_overflow.rs
@@ -1,11 +1,3 @@
-pub struct Handler;
-
-impl Handler {
-    pub unsafe fn new() -> Handler {
-        Handler
-    }
-}
-
 #[inline]
 pub unsafe fn init() {}
 
diff --git a/src/libstd/sys/hermit/thread.rs b/src/libstd/sys/hermit/thread.rs
index beb02d9c8be47..c7bea168f34d8 100644
--- a/src/libstd/sys/hermit/thread.rs
+++ b/src/libstd/sys/hermit/thread.rs
@@ -5,7 +5,6 @@ use crate::fmt;
 use crate::io;
 use crate::mem;
 use crate::sys::hermit::abi;
-use crate::sys::stack_overflow;
 use crate::time::Duration;
 use core::u32;
 
@@ -69,9 +68,6 @@ impl Thread {
 
         extern "C" fn thread_start(main: usize) {
             unsafe {
-                // Next, set up our stack overflow handler which may get triggered if we run
-                // out of stack.
-                let _handler = stack_overflow::Handler::new();
                 // Finally, let's run some code.
                 Box::from_raw(main as *mut Box<dyn FnOnce()>)();
             }

From d512b22f8bbe50ab944cf59a423ca72ccf8538db Mon Sep 17 00:00:00 2001
From: Vytautas Astrauskas <astrauv@amazon.com>
Date: Fri, 3 Apr 2020 10:13:49 -0700
Subject: [PATCH 9/9] Delete unnecessary stub stack overflow handler for
 cloudabi.

---
 src/libstd/sys/cloudabi/stack_overflow.rs | 8 --------
 src/libstd/sys/cloudabi/thread.rs         | 6 +-----
 2 files changed, 1 insertion(+), 13 deletions(-)

diff --git a/src/libstd/sys/cloudabi/stack_overflow.rs b/src/libstd/sys/cloudabi/stack_overflow.rs
index e97831b2c2855..9339b14373105 100644
--- a/src/libstd/sys/cloudabi/stack_overflow.rs
+++ b/src/libstd/sys/cloudabi/stack_overflow.rs
@@ -1,13 +1,5 @@
 #![cfg_attr(test, allow(dead_code))]
 
-pub struct Handler;
-
-impl Handler {
-    pub unsafe fn new() -> Handler {
-        Handler
-    }
-}
-
 pub unsafe fn init() {}
 
 pub unsafe fn cleanup() {}
diff --git a/src/libstd/sys/cloudabi/thread.rs b/src/libstd/sys/cloudabi/thread.rs
index abc15b18e321a..a15dc8653e83a 100644
--- a/src/libstd/sys/cloudabi/thread.rs
+++ b/src/libstd/sys/cloudabi/thread.rs
@@ -4,7 +4,6 @@ use crate::io;
 use crate::mem;
 use crate::ptr;
 use crate::sys::cloudabi::abi;
-use crate::sys::stack_overflow;
 use crate::sys::time::checked_dur2intervals;
 use crate::time::Duration;
 
@@ -47,10 +46,7 @@ impl Thread {
 
         extern "C" fn thread_start(main: *mut libc::c_void) -> *mut libc::c_void {
             unsafe {
-                // Next, set up our stack overflow handler which may get triggered if we run
-                // out of stack.
-                let _handler = stack_overflow::Handler::new();
-                // Finally, let's run some code.
+                // Let's run some code.
                 Box::from_raw(main as *mut Box<dyn FnOnce()>)();
             }
             ptr::null_mut()