[PATCH] vfs: tiny sample programs for open_tree() and move_mount()

[Alan Jenkins]

A couple of utility commands for fd-based mounts.  They were useful to
reproduce three different issues, in the original implementation of the
system calls.

Also add .gitignore for all the vfs samples.

Signed-off-by: Alan Jenkins <alan.christopher.jenkins@xxxxxxxxx>
---
 samples/vfs/.gitignore        |  4 +++
 samples/vfs/Makefile          |  4 +++
 samples/vfs/move_mount.c      | 42 ++++++++++++++++++++++
 samples/vfs/open_tree_clone.c | 65 +++++++++++++++++++++++++++++++++++
 4 files changed, 115 insertions(+)
 create mode 100644 samples/vfs/.gitignore
 create mode 100644 samples/vfs/move_mount.c
 create mode 100644 samples/vfs/open_tree_clone.c

diff --git a/samples/vfs/.gitignore b/samples/vfs/.gitignore
new file mode 100644
index 000000000000..242ed23f90d2
--- /dev/null
+++ b/samples/vfs/.gitignore
@@ -0,0 +1,4 @@
+open_tree_clone
+move_mount
+test-fsmount
+test-statx
diff --git a/samples/vfs/Makefile b/samples/vfs/Makefile
index 4ac9690fb3c4..3a5bb48d21a0 100644
--- a/samples/vfs/Makefile
+++ b/samples/vfs/Makefile
@@ -1,10 +1,14 @@
 # List of programs to build
 hostprogs-$(CONFIG_SAMPLE_VFS) := \
+	open_tree_clone \
+	move_mount \
 	test-fsmount \
 	test-statx
 
 # Tell kbuild to always build the programs
 always := $(hostprogs-y)
 
+HOSTCFLAGS_open_tree_clone.o += -I$(objtree)/usr/include
+HOSTCFLAGS_move_mount.o += -I$(objtree)/usr/include
 HOSTCFLAGS_test-fsmount.o += -I$(objtree)/usr/include
 HOSTCFLAGS_test-statx.o += -I$(objtree)/usr/include
diff --git a/samples/vfs/move_mount.c b/samples/vfs/move_mount.c
new file mode 100644
index 000000000000..2cc9d876078a
--- /dev/null
+++ b/samples/vfs/move_mount.c
@@ -0,0 +1,42 @@
+/* fd-based mount utility.
+ *
+ * Copyright (C) 2018 Alan Jenkins (alan.christopher.jenkins@xxxxxxxxx).
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public Licence
+ * as published by the Free Software Foundation; either version
+ * 2 of the Licence, or (at your option) any later version.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/fs.h>
+#include <linux/unistd.h>
+
+static inline int move_mount(int from_dfd, const char *from_pathname,
+			     int to_dfd, const char *to_pathname,
+			     unsigned int flags)
+{
+	return syscall(__NR_move_mount,
+		       from_dfd, from_pathname,
+		       to_dfd, to_pathname, flags);
+}
+
+int main(int argc, char *argv[])
+{
+	if (argc != 1) {
+		fprintf(stderr, "Usage: move_mount 3</from/path 4</to/path\n");
+		exit(2);
+	}
+
+	if (move_mount(3, "", 4, "", MOVE_MOUNT_F_EMPTY_PATH |
+		                     MOVE_MOUNT_T_EMPTY_PATH) < 0) {
+		perror("move_mount");
+		exit(1);
+	}
+
+	exit(0);
+}
diff --git a/samples/vfs/open_tree_clone.c b/samples/vfs/open_tree_clone.c
new file mode 100644
index 000000000000..3025d101addc
--- /dev/null
+++ b/samples/vfs/open_tree_clone.c
@@ -0,0 +1,65 @@
+/* fd-based mount utility.
+ *
+ * Inspired by http://skarnet.org/software/execline/redirfd.html etc.
+ *
+ * Copyright (C) 2018 Alan Jenkins (alan.christopher.jenkins@xxxxxxxxx).
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public Licence
+ * as published by the Free Software Foundation; either version
+ * 2 of the Licence, or (at your option) any later version.
+ */
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <ctype.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/fs.h>
+#include <linux/unistd.h>
+
+#ifndef AT_RECURSIVE
+#define AT_RECURSIVE 0x8000
+#endif
+
+#define E(x) do { if ((x) == -1) { perror(#x); exit(1); } } while(0)
+
+static inline int open_tree(int dfd, const char *pathname, unsigned flags)
+{
+	return syscall(__NR_open_tree, dfd, pathname, flags);
+}
+
+int main(int argc, char *argv[])
+{
+	int fd_number;
+	char **command;
+	int mfd;
+
+	if (argc < 3 || !isdigit(argv[1][0])) {
+		fprintf(stderr, "Usage: 3</from/path open_tree_clone FD_NUMBER COMMAND...\n");
+		fprintf(stderr, "Clone the tree mounted at /from/path.\n");
+		fprintf(stderr, "Then execute COMMAND, passing the cloned tree as file FD_NUMBER.\n");
+		fprintf(stderr, "\n");
+		fprintf(stderr, "hint:  3</from/path 4</to/path open_tree_clone 3 move_mount\n");
+		exit(2);
+	}
+	fd_number = atoi(argv[1]);
+	command = argv + 2;
+
+	mfd = open_tree(3, "", AT_EMPTY_PATH | OPEN_TREE_CLONE | AT_RECURSIVE);
+	if (mfd == -1) {
+		perror("open_tree");
+		exit(1);
+	}
+
+	if (fd_number != mfd) {
+		E( dup2(mfd, fd_number) );
+		E( close(mfd) );
+	}
+
+	execvp(command[0], command);
+	perror("exec");
+	exit(1);
+}
-- 
2.17.2


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