SGX target: handle empty user buffers correctly

src/libstd/sys/sgx/abi/usercalls/alloc.rs

@@ -537,7 +537,12 @@ impl UserRef<super::raw::ByteBuffer> {
     pub fn copy_user_buffer(&self) -> Vec<u8> {
         unsafe {
             let buf = self.to_enclave();
-            User::from_raw_parts(buf.data as _, buf.len).to_enclave()
+            if buf.len > 0 {
+                User::from_raw_parts(buf.data as _, buf.len).to_enclave()
+            } else {
+                // Mustn't look at `data` or call `free` if `len` is `0`.
+                Vec::with_capacity(0)
+            }
         }
     }
 }

src/libstd/sys/sgx/abi/usercalls/mod.rs

@@ -22,7 +22,8 @@ pub fn read(fd: Fd, buf: &mut [u8]) -> IoResult<usize> {
 #[unstable(feature = "sgx_platform", issue = "56975")]
 pub fn read_alloc(fd: Fd) -> IoResult<Vec<u8>> {
     unsafe {
-        let mut userbuf = alloc::User::<ByteBuffer>::uninitialized();
+        let userbuf = ByteBuffer { data: ::ptr::null_mut(), len: 0 };
+        let mut userbuf = alloc::User::new_from_enclave(&userbuf);
         raw::read_alloc(fd, userbuf.as_raw_mut_ptr()).from_sgx_result()?;
         Ok(userbuf.copy_user_buffer())
     }