Fix memory unsafety in unistd::getgrouplist

Fixes nix-rust#1541

CHANGELOG.md

@@ -91,9 +91,10 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 - Added more errno definitions for better backwards compatibility with
   Nix 0.21.0.
   (#[1467](https://github.com/nix-rust/nix/pull/1467))
-
 - Fixed potential undefined behavior in `Signal::try_from` on some platforms.
   (#[1484](https://github.com/nix-rust/nix/pull/1484))
+- Fixed buffer overflow in `unistd::getgrouplist`.
+  (#[1542](https://github.com/nix-rust/nix/pull/1542))
 
 ### Removed
 

src/unistd.rs

@@ -1540,8 +1540,7 @@ pub fn getgrouplist(user: &CStr, group: Gid) -> Result<Vec<Gid>> {
         Ok(None) | Err(_) => <c_int>::max_value(),
     };
     use std::cmp::min;
-    let mut ngroups = min(ngroups_max, 8);
-    let mut groups = Vec::<Gid>::with_capacity(ngroups as usize);
+    let mut groups = Vec::<Gid>::with_capacity(min(ngroups_max, 8) as usize);
     cfg_if! {
         if #[cfg(any(target_os = "ios", target_os = "macos"))] {
             type getgrouplist_group_t = c_int;
@@ -1551,6 +1550,7 @@ pub fn getgrouplist(user: &CStr, group: Gid) -> Result<Vec<Gid>> {
     }
     let gid: gid_t = group.into();
     loop {
+        let mut ngroups = groups.capacity() as i32;
         let ret = unsafe {
             libc::getgrouplist(user.as_ptr(),
                                gid as getgrouplist_group_t,