Add a helper to allocate a statmount buffer and call statmount(). This
helper will be shared by multiple test suites that need to query mount
information via statmount().

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../selftests/filesystems/statmount/statmount.h    | 27 ++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/tools/testing/selftests/filesystems/statmount/statmount.h b/tools/testing/selftests/filesystems/statmount/statmount.h
index e1cba4bfd8d9..4ef23e54212c 100644
--- a/tools/testing/selftests/filesystems/statmount/statmount.h
+++ b/tools/testing/selftests/filesystems/statmount/statmount.h
@@ -3,10 +3,14 @@
 #ifndef __STATMOUNT_H
 #define __STATMOUNT_H
 
+#include <errno.h>
 #include <stdint.h>
+#include <stdlib.h>
 #include <linux/mount.h>
 #include <asm/unistd.h>
 
+#define STATMOUNT_BUFSIZE (1 << 15)
+
 #ifndef __NR_statmount
 	#if defined __alpha__
 		#define __NR_statmount 567
@@ -84,4 +88,27 @@ static inline ssize_t listmount(uint64_t mnt_id, uint64_t mnt_ns_id,
 	return syscall(__NR_listmount, &req, list, num, flags);
 }
 
+static inline struct statmount *statmount_alloc(uint64_t mnt_id, uint64_t mnt_ns_id, uint64_t mask)
+{
+	struct statmount *buf;
+	size_t bufsize = STATMOUNT_BUFSIZE;
+	int ret;
+
+	for (;;) {
+		buf = malloc(bufsize);
+		if (!buf)
+			return NULL;
+
+		ret = statmount(mnt_id, mnt_ns_id, 0, mask, buf, bufsize, 0);
+		if (ret == 0)
+			return buf;
+
+		free(buf);
+		if (errno != EOVERFLOW)
+			return NULL;
+
+		bufsize <<= 1;
+	}
+}
+
 #endif /* __STATMOUNT_H */

-- 
2.47.3