coroutine: support SafeStack in ucontext backend

LLVM's SafeStack instrumentation does not yet support programs that make
use of the APIs in ucontext.h
With the current implementation of coroutine-ucontext, the resulting
binary is incorrect, with different coroutines sharing the same unsafe
stack and producing undefined behavior at runtime.
This fix allocates an additional unsafe stack area for each coroutine,
and sets the new unsafe stack pointer before calling swapcontext() in
qemu_coroutine_new.
This is the only place where the pointer needs to be manually updated,
since sigsetjmp/siglongjmp are already instrumented by LLVM to properly
support SafeStack.
The additional stack is then freed in qemu_coroutine_delete.

Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
Message-id: 20200529205122.714-2-dbuono@linux.vnet.ibm.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

include/qemu/coroutine_int.h

@@ -28,6 +28,11 @@
 #include "qemu/queue.h"
 #include "qemu/coroutine.h"
 
+#ifdef CONFIG_SAFESTACK
+/* Pointer to the unsafe stack, defined by the compiler */
+extern __thread void *__safestack_unsafe_stack_ptr;
+#endif
+
 #define COROUTINE_STACK_SIZE (1 << 20)
 
 typedef enum {

util/coroutine-ucontext.c

@@ -45,6 +45,11 @@ typedef struct {
     Coroutine base;
     void *stack;
     size_t stack_size;
+#ifdef CONFIG_SAFESTACK
+    /* Need an unsafe stack for each coroutine */
+    void *unsafe_stack;
+    size_t unsafe_stack_size;
+#endif
     sigjmp_buf env;
 
     void *tsan_co_fiber;
@@ -179,6 +184,10 @@ Coroutine *qemu_coroutine_new(void)
     co = g_malloc0(sizeof(*co));
     co->stack_size = COROUTINE_STACK_SIZE;
     co->stack = qemu_alloc_stack(&co->stack_size);
+#ifdef CONFIG_SAFESTACK
+    co->unsafe_stack_size = COROUTINE_STACK_SIZE;
+    co->unsafe_stack = qemu_alloc_stack(&co->unsafe_stack_size);
+#endif
     co->base.entry_arg = &old_env; /* stash away our jmp_buf */
 
     uc.uc_link = &old_uc;
@@ -203,6 +212,22 @@ Coroutine *qemu_coroutine_new(void)
             COROUTINE_YIELD,
             &fake_stack_save,
             co->stack, co->stack_size, co->tsan_co_fiber);
+
+#ifdef CONFIG_SAFESTACK
+        /*
+         * Before we swap the context, set the new unsafe stack
+         * The unsafe stack grows just like the normal stack, so start from
+         * the last usable location of the memory area.
+         * NOTE: we don't have to re-set the usp afterwards because we are
+         * coming back to this context through a siglongjmp.
+         * The compiler already wrapped the corresponding sigsetjmp call with
+         * code that saves the usp on the (safe) stack before the call, and
+         * restores it right after (which is where we return with siglongjmp).
+         */
+        void *usp = co->unsafe_stack + co->unsafe_stack_size;
+        __safestack_unsafe_stack_ptr = usp;
+#endif
+
         swapcontext(&old_uc, &uc);
     }
 
@@ -235,6 +260,9 @@ void qemu_coroutine_delete(Coroutine *co_)
 #endif
 
     qemu_free_stack(co->stack, co->stack_size);
+#ifdef CONFIG_SAFESTACK
+    qemu_free_stack(co->unsafe_stack, co->unsafe_stack_size);
+#endif
     g_free(co);
 }