unix: Don't override existing SIGSEGV/BUS handlers

Although `stack_overflow::init` runs very early in the process, even
before `main`, there may already be signal handlers installed for things
like the address sanitizer. In that case, just leave it alone, and don't
bother trying to allocate our own signal stacks either.

src/libstd/sys/unix/stack_overflow.rs

@@ -13,6 +13,10 @@ impl Handler {
     pub unsafe fn new() -> Handler {
         make_handler()
     }
+
+    fn null() -> Handler {
+        Handler { _data: crate::ptr::null_mut() }
+    }
 }
 
 impl Drop for Handler {
@@ -108,13 +112,20 @@ mod imp {
     }
 
     static mut MAIN_ALTSTACK: *mut libc::c_void = ptr::null_mut();
+    static mut NEED_ALTSTACK: bool = false;
 
     pub unsafe fn init() {
         let mut action: sigaction = mem::zeroed();
-        action.sa_flags = SA_SIGINFO | SA_ONSTACK;
-        action.sa_sigaction = signal_handler as sighandler_t;
-        sigaction(SIGSEGV, &action, ptr::null_mut());
-        sigaction(SIGBUS, &action, ptr::null_mut());
+        for &signal in &[SIGSEGV, SIGBUS] {
+            sigaction(signal, ptr::null_mut(), &mut action);
+            // Configure our signal handler if one is not already set.
+            if action.sa_sigaction == SIG_DFL {
+                action.sa_flags = SA_SIGINFO | SA_ONSTACK;
+                action.sa_sigaction = signal_handler as sighandler_t;
+                sigaction(signal, &action, ptr::null_mut());
+                NEED_ALTSTACK = true;
+            }
+        }
 
         let handler = make_handler();
         MAIN_ALTSTACK = handler._data;
@@ -152,6 +163,9 @@ mod imp {
     }
 
     pub unsafe fn make_handler() -> Handler {
+        if !NEED_ALTSTACK {
+            return Handler::null();
+        }
         let mut stack = mem::zeroed();
         sigaltstack(ptr::null(), &mut stack);
         // Configure alternate signal stack, if one is not already set.
@@ -160,7 +174,7 @@ mod imp {
             sigaltstack(&stack, ptr::null_mut());
             Handler { _data: stack.ss_sp as *mut libc::c_void }
         } else {
-            Handler { _data: ptr::null_mut() }
+            Handler::null()
         }
     }
 
@@ -191,14 +205,12 @@ mod imp {
     target_os = "openbsd"
 )))]
 mod imp {
-    use crate::ptr;
-
     pub unsafe fn init() {}
 
     pub unsafe fn cleanup() {}
 
     pub unsafe fn make_handler() -> super::Handler {
-        super::Handler { _data: ptr::null_mut() }
+        super::Handler::null()
     }
 
     pub unsafe fn drop_handler(_handler: &mut super::Handler) {}

src/test/ui/sanitize/badfree.rs

@@ -0,0 +1,19 @@
+// needs-sanitizer-support
+// only-x86_64
+//
+// compile-flags: -Z sanitizer=address -O
+//
+// run-fail
+// error-pattern: AddressSanitizer: SEGV
+
+use std::ffi::c_void;
+
+extern "C" {
+    fn free(ptr: *mut c_void);
+}
+
+fn main() {
+    unsafe {
+        free(1 as *mut c_void);
+    }
+}