[RFC PATCH bpf-next 02/16] bpf: create /sys/kernel/bpfdump mount file system

[Yonghong Song <yhs@fb.com>]

This patch creates a mount point "bpfdump" under
/sys/kernel. The file system has a single user
mode, i.e., all mount points will be identical.

The magic number I picked for the new file system
is "dump".

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 include/uapi/linux/magic.h |  1 +
 kernel/bpf/Makefile        |  1 +
 kernel/bpf/dump.c          | 79 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 81 insertions(+)
 create mode 100644 kernel/bpf/dump.c

diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
index d78064007b17..4ce3d8882315 100644
--- a/include/uapi/linux/magic.h
+++ b/include/uapi/linux/magic.h
@@ -88,6 +88,7 @@
 #define BPF_FS_MAGIC		0xcafe4a11
 #define AAFS_MAGIC		0x5a3c69f0
 #define ZONEFS_MAGIC		0x5a4f4653
+#define DUMPFS_MAGIC		0x64756d70
 
 /* Since UDF 2.01 is ISO 13346 based... */
 #define UDF_SUPER_MAGIC		0x15013346
diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
index f2d7be596966..4a1376ab2bea 100644
--- a/kernel/bpf/Makefile
+++ b/kernel/bpf/Makefile
@@ -26,6 +26,7 @@ obj-$(CONFIG_BPF_SYSCALL) += reuseport_array.o
 endif
 ifeq ($(CONFIG_SYSFS),y)
 obj-$(CONFIG_DEBUG_INFO_BTF) += sysfs_btf.o
+obj-$(CONFIG_BPF_SYSCALL) += dump.o
 endif
 ifeq ($(CONFIG_BPF_JIT),y)
 obj-$(CONFIG_BPF_SYSCALL) += bpf_struct_ops.o
diff --git a/kernel/bpf/dump.c b/kernel/bpf/dump.c
new file mode 100644
index 000000000000..e0c33486e0e7
--- /dev/null
+++ b/kernel/bpf/dump.c
@@ -0,0 +1,79 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2020 Facebook */
+
+#include <linux/init.h>
+#include <linux/magic.h>
+#include <linux/mount.h>
+#include <linux/anon_inodes.h>
+#include <linux/namei.h>
+#include <linux/fs.h>
+#include <linux/fs_context.h>
+#include <linux/fs_parser.h>
+#include <linux/filter.h>
+#include <linux/bpf.h>
+
+static void bpfdump_free_inode(struct inode *inode)
+{
+	kfree(inode->i_private);
+	free_inode_nonrcu(inode);
+}
+
+static const struct super_operations bpfdump_super_operations = {
+	.statfs		= simple_statfs,
+	.free_inode	= bpfdump_free_inode,
+};
+
+static int bpfdump_fill_super(struct super_block *sb, struct fs_context *fc)
+{
+	static const struct tree_descr files[] = { { "" } };
+	int err;
+
+	err = simple_fill_super(sb, DUMPFS_MAGIC, files);
+	if (err)
+		return err;
+
+	sb->s_op = &bpfdump_super_operations;
+	return 0;
+}
+
+static int bpfdump_get_tree(struct fs_context *fc)
+{
+	return get_tree_single(fc, bpfdump_fill_super);
+}
+
+static const struct fs_context_operations bpfdump_context_ops = {
+	.get_tree	= bpfdump_get_tree,
+};
+
+static int bpfdump_init_fs_context(struct fs_context *fc)
+{
+	fc->ops = &bpfdump_context_ops;
+	return 0;
+}
+
+static struct file_system_type fs_type = {
+	.owner			= THIS_MODULE,
+	.name			= "bpfdump",
+	.init_fs_context	= bpfdump_init_fs_context,
+	.kill_sb		= kill_litter_super,
+};
+
+static int __init bpfdump_init(void)
+{
+	int ret;
+
+	ret = sysfs_create_mount_point(kernel_kobj, "bpfdump");
+	if (ret)
+		return ret;
+
+	ret = register_filesystem(&fs_type);
+	if (ret)
+		goto remove_mount;
+
+	return 0;
+
+remove_mount:
+	sysfs_remove_mount_point(kernel_kobj, "bpfdump");
+	return ret;
+}
+core_initcall(bpfdump_init);
-- 
2.24.1