[LibOS] Fix flag printing in syscall parser

Previously, Gramine assumed the existence of mapping type in mmap flags
on syscall arg parsing. It had assertions on such flags which could lead
to failures in debug mode in case of missing them. Besides, Gramine also
incorrectly parsed file access modes in open flags.

This commit fixes the above issues and makes Gramine fail with the same
error on syscall handling as Linux does (on incorrect flags).

Signed-off-by: Kailun Qin <kailun.qin@intel.com>

libos/src/libos_parser.c

@@ -688,14 +688,21 @@ static int parse_flags(struct print_buf* buf, int flags, const struct flag_table
 static void parse_open_flags(struct print_buf* buf, va_list* ap) {
     int flags = va_arg(*ap, int);
 
-    if (flags & O_WRONLY) {
-        buf_puts(buf, "O_WRONLY");
-        flags &= ~O_WRONLY;
-    } else if (flags & O_RDWR) {
-        buf_puts(buf, "O_RDWR");
-        flags &= ~O_RDWR;
-    } else {
-        buf_puts(buf, "O_RDONLY");
+    switch (flags & O_ACCMODE) {
+        case O_RDONLY:
+            buf_puts(buf, "O_RDONLY");
+            flags &= ~O_RDONLY;
+            break;
+        case O_WRONLY:
+            buf_puts(buf, "O_WRONLY");
+            flags &= ~O_WRONLY;
+            break;
+        case O_RDWR:
+            buf_puts(buf, "O_RDWR");
+            flags &= ~O_RDWR;
+            break;
+        default:
+            break;
     }
 
     if (flags & O_APPEND) {
@@ -833,8 +840,7 @@ static void parse_mmap_flags(struct print_buf* buf, va_list* ap) {
     } else if (flags & MAP_SHARED) {
         buf_puts(buf, "MAP_SHARED");
         flags &= ~MAP_SHARED;
-    } else {
-        assert(flags & MAP_PRIVATE);
+    } else if (flags & MAP_PRIVATE) {
         buf_puts(buf, "MAP_PRIVATE");
         flags &= ~MAP_PRIVATE;
     }