generic: new case to test getcwd(2)

The following kernel commit introduced a race condition that causes
getcwd(2) to return "/" instead of correct path

232d2d6 dcache: Translating dentry into pathname without taking rename_lock

Jan Stancek hit it once when building ltp and Mikulas Patocka could
hit it by running lvm2 test suite. Please refer to this thread

https://www.mail-archive.com/ltp-list@lists.sourceforge.net/msg17896.html

These commits fixed the bug
ede4ceb prepend_path() needs to reinitialize dentry/vfsmount/mnt on restarts
f650080 __dentry_path() fixes

Cc: Artem Savkov <asavkov@redhat.com>
Cc: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

src/Makefile

@@ -19,7 +19,7 @@ LINUX_TARGETS = xfsctl bstat t_mtab getdevicesize preallo_rw_pattern_reader \
 	bulkstat_unlink_test_modified t_dir_offset t_futimens t_immutable \
 	stale_handle pwrite_mmap_blocked t_dir_offset2 seek_sanity_test \
 	seek_copy_test t_readdir_1 t_readdir_2 fsync-tester nsexec cloner \
-	renameat2
+	renameat2 t_getcwd
 
 SUBDIRS =
 

src/t_getcwd.c

@@ -0,0 +1,102 @@
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#define TIMEOUT 5
+#define BUF_SIZE 256
+
+static sig_atomic_t end;
+
+int test_getcwd(char *init_cwd)
+{
+	int i = 0;
+	char cur_cwd[BUF_SIZE];
+	while (1) {
+		getcwd(cur_cwd, BUF_SIZE);
+		if (strncmp(init_cwd, cur_cwd, BUF_SIZE)) {
+			printf("[%u] %s != %s\n", i, init_cwd, cur_cwd);
+			return 1;
+		}
+		if (end)
+			break;
+		i++;
+	}
+	return 0;
+}
+
+void do_rename(char *prefix)
+{
+	int i = 0;
+	int fd;
+	char c_name[BUF_SIZE];
+	char n_name[BUF_SIZE];
+
+	strncpy(c_name, prefix, BUF_SIZE);
+
+	fd = open(c_name, O_CREAT | O_RDWR, 0644);
+	if (fd < 0) {
+		fprintf(stderr, "failed to create file %s: %s\n",
+			c_name, strerror(errno));
+		exit(1);
+	}
+	close(fd);
+
+	while (1) {
+		i++;
+		snprintf(n_name, BUF_SIZE, "%s%u", prefix, i);
+		rename(c_name, n_name);
+		strncpy(c_name, n_name, BUF_SIZE);
+	}
+}
+
+void sigproc(int sig)
+{
+	end = 1;
+}
+
+int main(int argc, char *argv[])
+{
+	char *init_cwd;
+	pid_t pid;
+	int status;
+	int ret = 1;
+
+	if (argc != 2) {
+		printf("Usage: %s <dir>\n", argv[0]);
+		exit(1);
+	}
+
+	init_cwd = argv[1];
+	ret = chdir(init_cwd);
+	if (ret < 0) {
+		perror("chdir failed");
+		exit(1);
+	}
+
+	if (signal(SIGALRM, sigproc) == SIG_ERR) {
+		perror("signal failed");
+		exit(1);
+	}
+
+	alarm(TIMEOUT);
+
+	pid = fork();
+	if (pid < 0) {
+		perror("fork failed");
+		exit(1);
+	} else if (pid == 0) {
+		do_rename("t_getcwd_testfile");
+	} else {
+		ret = test_getcwd(init_cwd);
+		kill(pid, SIGTERM);
+		waitpid(pid, &status, 0);
+	}
+
+	exit(ret);
+}