Don't use standard descriptors for logging

The standard descriptors may be redirected.

crtool dumplicates stderr in rlimit.maxfileno-1 and this descriptor
is inherited by all children and will be closed before sigreturn.

Known issues:

 - The logging descriptor may be used by a target process and
   a resume will fail.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>

cr-restore.c

@@ -1623,6 +1623,7 @@ static void sigreturn_restore(pid_t pstree_pid, pid_t pid)
 		task_args->thread_args);
 
 	close_safe(&fd_pstree);
+	deinit_logging();
 
 	/*
 	 * An indirect call to task_restore, note it never resturns

crtools.c

@@ -241,6 +241,9 @@ int main(int argc, char *argv[])
 		{ NULL,		no_argument, NULL, 0 }
 	};
 
+	if (init_logging())
+		return 1;
+
 	BUILD_BUG_ON(PAGE_SIZE != PAGE_IMAGE_SIZE);
 
 	if (argc < 3)

include/util.h

@@ -15,6 +15,8 @@
 #include "compiler.h"
 #include "types.h"
 
+extern int init_logging(void);
+extern void deinit_logging(void);
 extern void printk(const char *format, ...);
 
 #define PREF_SHIFT_OP(pref, op, size)	((size) op (pref ##BYTES_SHIFT))

util.c

@@ -31,6 +31,7 @@
 #include <sys/ptrace.h>
 #include <sys/user.h>
 #include <sys/wait.h>
+#include <sys/resource.h>
 
 #include "compiler.h"
 #include "types.h"
@@ -39,12 +40,38 @@
 
 #include "crtools.h"
 
+static int logfd = STDERR_FILENO;
+
+int init_logging(void)
+{
+	struct rlimit rlimit;
+
+	if (getrlimit(RLIMIT_NOFILE, &rlimit)) {
+		pr_err("can't get rlimit: %m\n");
+		return 1;
+	}
+
+	logfd = rlimit.rlim_cur - 1;
+	if (dup2(2, logfd) < 0) {
+		pr_err("can't duplicate descriptor 2->%d: %m\n", logfd);
+		return 1;
+	}
+
+	return 0;
+}
+
+void deinit_logging(void)
+{
+	close(logfd);
+	logfd = -1;
+}
+
 void printk(const char *format, ...)
 {
 	va_list params;
 
 	va_start(params, format);
-	vfprintf(stdout, format, params);
+	vdprintf(logfd, format, params);
 	va_end(params);
 }