vfs: fail write_stat() on file descriptors opened read-only.

Change-Id: I20d586c606c47df6625cc9272f153250a5a621d6
Reviewed-on: https://review.haiku-os.org/706
Reviewed-by: waddlesplash <[email protected]>

src/system/kernel/fs/vfs.cpp

@@ -6562,6 +6562,9 @@ common_write_stat(struct file_descriptor* descriptor, const struct stat* stat,
 	FUNCTION(("common_write_stat(vnode = %p, stat = %p, statMask = %d)\n",
 		vnode, stat, statMask));
 
+	if ((descriptor->open_mode & O_RWMASK) == O_RDONLY)
+		return B_BAD_VALUE;
+
 	if (!HAS_FS_CALL(vnode, write_stat))
 		return B_READ_ONLY_DEVICE;
 

src/tests/system/libroot/posix/truncate.cpp

@@ -6,6 +6,7 @@
 
 #include <ctype.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -48,5 +49,19 @@ main(int argc, char **argv)
 		return 1;
 	}
 
+	int fd = open(argv[1], O_RDONLY);
+	if (fd < 0) {
+		fprintf(stderr, "%s: could open the file read-only \"%s\": %s\n",
+			__progname, argv[1], strerror(errno));
+		return 1;
+	}
+	if (ftruncate(fd, newSize) == 0 || errno != EINVAL) {
+		fprintf(stderr, "%s: could truncate a file opened read-only \"%s\": %s\n",
+			__progname, argv[1], strerror(errno));
+		close(fd);
+		return 1;
+	}
+	close(fd);
+
 	return 0;
 }