examples/ucontext-cp.c: cope with variable SIGSTKSZ

The size of C arrays at file scope must be constant. The following
compiler error occurs with recent upstream glibc (2.33.9000):

  CC ucontext-cp
  ucontext-cp.c:31:23: error: variably modified ‘stack_buf’ at file scope
  31 |         unsigned char stack_buf[SIGSTKSZ];
     |                       ^~~~~~~~~
  make[1]: *** [Makefile:26: ucontext-cp] Error 1

The following glibc commit changed SIGSTKSZ from a constant value to a
variable:

  commit 6c57d320484988e87e446e2e60ce42816bf51d53
  Author: H.J. Lu <hjl.tools@gmail.com>
  Date:   Mon Feb 1 11:00:38 2021 -0800

    sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305]
  ...
  +# define SIGSTKSZ sysconf (_SC_SIGSTKSZ)

Allocate the stack buffer explicitly to avoid declaring an array at file
scope.

Cc: H.J. Lu <hjl.tools@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

examples/ucontext-cp.c

@@ -28,7 +28,7 @@
 
 typedef struct {
 	struct io_uring *ring;
-	unsigned char stack_buf[SIGSTKSZ];
+	unsigned char *stack_buf;
 	ucontext_t ctx_main, ctx_fnew;
 } async_context;
 
@@ -115,8 +115,13 @@ static int setup_context(async_context *pctx, struct io_uring *ring)
 		perror("getcontext");
 		return -1;
 	}
-	pctx->ctx_fnew.uc_stack.ss_sp = &pctx->stack_buf;
-	pctx->ctx_fnew.uc_stack.ss_size = sizeof(pctx->stack_buf);
+	pctx->stack_buf = malloc(SIGSTKSZ);
+	if (!pctx->stack_buf) {
+		perror("malloc");
+		return -1;
+	}
+	pctx->ctx_fnew.uc_stack.ss_sp = pctx->stack_buf;
+	pctx->ctx_fnew.uc_stack.ss_size = SIGSTKSZ;
 	pctx->ctx_fnew.uc_link = &pctx->ctx_main;
 
 	return 0;
@@ -174,6 +179,7 @@ static void copy_file_wrapper(arguments_bundle *pbundle)
 	free(iov.iov_base);
 	close(pbundle->infd);
 	close(pbundle->outfd);
+	free(pbundle->pctx->stack_buf);
 	free(pbundle->pctx);
 	free(pbundle);