Auto merge of #1411 - acfoltzer:ucontext-compat, r=gnzlbg

Remove new field from ucontext_t for compatibility with earlier glibc versions

Per discussion in #1410 with @gnzlbg, this is necessary to avoid struct size mismatches between Rust and C on systems with glibc < 2.28.

Author: bors

libc-test/build.rs

@@ -2098,6 +2098,12 @@ fn test_linux(target: &str) {
             // FIXME: musl version using by mips build jobs 1.0.15 is ancient:
             "ifmap" | "ifreq" | "ifconf" if mips32_musl => true,
 
+            // FIXME: remove once Ubuntu 20.04 LTS is released, somewhere in 2020.
+            // ucontext_t added a new field as of glibc 2.28; our struct definition is
+            // conservative and omits the field, but that means the size doesn't match for newer
+            // glibcs (see https://github.com/rust-lang/libc/issues/1410)
+            "ucontext_t" if gnu => true,
+
             _ => false,
         }
     });

src/unix/linux_like/linux/gnu/b64/x86_64/mod.rs

@@ -285,7 +285,9 @@ s_no_extra_traits! {
         pub uc_mcontext: mcontext_t,
         pub uc_sigmask: ::sigset_t,
         __private: [u8; 512],
-        __ssp: [::c_ulonglong; 4],
+        // FIXME: the shadow stack field requires glibc >= 2.28.
+        // Re-add once we drop compatibility with glibc versions older than 2.28.
+        // __ssp: [::c_ulonglong; 4],
     }
 }