Fix vdso crashes on current Ubuntu.

- Remove noops from several vdso stubs to make them shorter.

- Allow new stubs to be a little longer than what they replace if
  it fits given 8-byte alignment.

reverie-ptrace/src/lib.rs

@@ -30,6 +30,7 @@
 #![deny(rustdoc::broken_intra_doc_links)]
 #![feature(internal_output_capture)]
 #![feature(never_type)]
+#![feature(int_roundings)]
 
 mod children;
 mod cp;

reverie-ptrace/src/vdso.rs

@@ -39,8 +39,6 @@ mod vdso_syms {
         0xb8, 0xe4, 0x00, 0x00, 0x00, // mov SYS_clock_gettime, %eax
         0x0f, 0x05, // syscall
         0xc3, // retq
-        0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, // nopl 0x0(%rax, %rax, 1)
-        0x00,
     ];
 
     pub const getcpu: &[u8] = &[
@@ -59,16 +57,12 @@ mod vdso_syms {
         0xb8, 0x60, 0x00, 0x00, 0x00, // mov SYS_gettimeofday, %eax
         0x0f, 0x05, // syscall
         0xc3, // retq
-        0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, // nopl 0x0(%rax, %rax, 1)
-        0x00,
     ];
 
     pub const clock_getres: &[u8] = &[
         0xb8, 0xe5, 0x00, 0x00, 0x00, // mov SYS_clock_getres, %eax
         0x0f, 0x05, // syscall
         0xc3, // retq
-        0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, // nopl 0x0(%rax, %rax, 1)
-        0x00,
     ];
 }
 
@@ -160,11 +154,11 @@ lazy_static! {
         for (k, v) in VDSO_SYMBOLS {
             if let Some(&(base, size)) = info.get(*k) {
                 assert!(
-                    v.len() <= size,
+                    v.len().div_ceil(8) <= size.div_ceil(8),
                     "vdso symbol {}'s real size is {} bytes, but trying to replace it with {} bytes",
                     k,
-                    size,
-                    v.len()
+                    size.div_ceil(8),
+                    v.len().div_ceil(8)
                 );
                 res.insert(*k, (base, size, *v));
             }
@@ -239,10 +233,9 @@ where
 
         for (name, (offset, size, bytes)) in VDSO_PATCH_INFO.iter() {
             let start = vdso.address.0 + offset;
-            assert!(bytes.len() <= *size);
+            assert!(bytes.len().div_ceil(8) <= (*size).div_ceil(8));
             let rptr = AddrMut::from_raw(start as usize).unwrap();
             memory.write_exact(rptr, bytes)?;
-            assert!(*size >= bytes.len());
             if *size > bytes.len() {
                 let fill: Vec<u8> = std::iter::repeat(0x90u8).take(size - bytes.len()).collect();
                 memory.write_exact(unsafe { rptr.add(bytes.len()) }, &fill)?;