cygwin: fix off-by-one in utsname.sysname length

libc-test/tests/cygwin_utsname.rs

@@ -0,0 +1,72 @@
+//! Verify that `libc::uname()` populates the `utsname` struct with each field
+//! starting at the correct C offset on Cygwin.
+//!
+//! Cygwin's `<sys/utsname.h>` declares all six fields as 65 bytes
+//! (`_UTSNAME_LENGTH`). If any field is mis-sized in the Rust declaration,
+//! every subsequent field shifts in the struct and `CStr::from_ptr` reads
+//! that field starting at the wrong byte. The most observable consequence
+//! is that `nodename` no longer matches `gethostname()`, so this test is
+//! the smallest behavioural check that catches such a regression.
+
+#![cfg(target_os = "cygwin")]
+
+use std::ffi::CStr;
+use std::mem::MaybeUninit;
+
+#[test]
+fn uname_nodename_matches_gethostname() {
+    let mut uts = MaybeUninit::<libc::utsname>::uninit();
+    let mut host = [0u8; 256];
+
+    unsafe {
+        assert_ne!(
+            libc::uname(uts.as_mut_ptr()),
+            -1,
+            "uname() failed",
+        );
+        assert_ne!(
+            libc::gethostname(host.as_mut_ptr() as *mut libc::c_char, host.len()),
+            -1,
+            "gethostname() failed",
+        );
+
+        let uts = uts.assume_init();
+        let nodename = CStr::from_ptr(uts.nodename.as_ptr())
+            .to_str()
+            .expect("uname.nodename is not valid UTF-8");
+        let hostname = CStr::from_ptr(host.as_ptr() as *const libc::c_char)
+            .to_str()
+            .expect("gethostname() returned non-UTF-8");
+
+        assert_eq!(
+            nodename, hostname,
+            "uname.nodename and gethostname() must agree",
+        );
+    }
+}
+
+#[test]
+fn uname_fields_are_non_empty() {
+    let mut uts = MaybeUninit::<libc::utsname>::uninit();
+
+    unsafe {
+        assert_ne!(libc::uname(uts.as_mut_ptr()), -1, "uname() failed");
+        let uts = uts.assume_init();
+
+        let read = |buf: &[libc::c_char], name: &str| -> String {
+            CStr::from_ptr(buf.as_ptr())
+                .to_str()
+                .unwrap_or_else(|_| panic!("uname.{name} is not valid UTF-8"))
+                .to_owned()
+        };
+
+        // Every variable-length field must read as a non-empty string.
+        // A leading-zero byte here would mean the field is being read
+        // from a misaligned offset (the bug this test guards against).
+        assert!(!read(&uts.sysname, "sysname").is_empty());
+        assert!(!read(&uts.nodename, "nodename").is_empty());
+        assert!(!read(&uts.release, "release").is_empty());
+        assert!(!read(&uts.version, "version").is_empty());
+        assert!(!read(&uts.machine, "machine").is_empty());
+    }
+}

src/unix/cygwin/mod.rs

@@ -447,7 +447,7 @@ s! {
     }
 
     pub struct utsname {
-        pub sysname: [c_char; 66],
+        pub sysname: [c_char; 65],
         pub nodename: [c_char; 65],
         pub release: [c_char; 65],
         pub version: [c_char; 65],