stack: synthesize AT_RANDOM when the kernel omits it

src/stack.rs

@@ -119,12 +119,25 @@ impl<'a, A: AsRef<CStr>, E: AsRef<CStr>> StackBuilder<'a, A, E> {
             let at_platform = unsafe { CStr::from_ptr(at_platform) };
             Some(self.push_str(at_platform))
         };
-        let at_random = unsafe {
+        // AT_RANDOM is only provided by Linux >= 2.6.29. On older kernels it is
+        // absent, so synthesize 16 bytes for the interpreter we are about to
+        // hand control to (it uses them for the stack canary / pointer guard).
+        let at_random_bytes: [u8; 16] = unsafe {
             let ptr = getauxval(AT_RANDOM) as *const u8;
-            assert!(!ptr.is_null());
-            std::slice::from_raw_parts(ptr, 16)
+            let mut buf = [0u8; 16];
+            if !ptr.is_null() {
+                std::ptr::copy_nonoverlapping(ptr, buf.as_mut_ptr(), 16);
+            } else if let Ok(mut urandom) = std::fs::File::open("/dev/urandom") {
+                use std::io::Read;
+                // On a short or failed read the remaining bytes stay zero. An
+                // all-zero canary is the accepted worst case here: it is no
+                // worse than what glibc itself does when its AT_RANDOM is
+                // absent, and the alternative on these kernels is a panic.
+                let _ = urandom.read_exact(&mut buf);
+            }
+            buf
         };
-        let at_random_addr = self.push_bytes(at_random);
+        let at_random_addr = self.push_bytes(&at_random_bytes);
 
         // Align argc at bottom
         while (self.stack_reversed.len()