add fixmes for the ucontext_t shadow stack field

Author: acfoltzer

libc-test/build.rs

@@ -2098,6 +2098,7 @@ 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)

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

@@ -285,6 +285,9 @@ s_no_extra_traits! {
         pub uc_mcontext: mcontext_t,
         pub uc_sigmask: ::sigset_t,
         __private: [u8; 512],
+        // 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],
     }
 }