aufs: loopback-mounted branch

It is ok that the branch is loopback-mounted.
But it had a problem if the backend fs-image is placed on another
branch, and aufs had prohibited such nested branch for a long time due
to the recursive lookup by 'loopN' daemon.
I don't remember when it was, but the daemon stopped such recursive
behaviour, but aufs is still prohibitting the nested branch via
loopback-mount.
Upon the request from users, aufs will allow the loopback-mounted branch
on another branch by another patch (aufs4-loopback.patch in
aufs4-standalone.git).
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

fs/aufs/Kconfig

@@ -138,6 +138,14 @@ config AUFS_SHWH
 	sounds like philosophy or something, but in technically it
 	simply shows the name of whiteout with keeping its behaviour.
 
+config AUFS_BDEV_LOOP
+	bool
+	depends on BLK_DEV_LOOP
+	default y
+	help
+	Automatic configuration for internal use.
+	Convert =[ym] into =y.
+
 config AUFS_DEBUG
 	bool "Debug aufs"
 	help

fs/aufs/Makefile

@@ -24,6 +24,7 @@ aufs-y := module.o sbinfo.o super.o branch.o xino.o sysaufs.o opts.o \
 aufs-$(CONFIG_PROC_FS) += procfs.o plink.o
 aufs-$(CONFIG_SYSFS) += sysfs.o
 aufs-$(CONFIG_DEBUG_FS) += dbgaufs.o
+aufs-$(CONFIG_AUFS_BDEV_LOOP) += loop.o
 aufs-$(CONFIG_AUFS_HNOTIFY) += hnotify.o
 aufs-$(CONFIG_AUFS_HFSNOTIFY) += hfsnotify.o
 aufs-$(CONFIG_AUFS_EXPORT) += export.o

fs/aufs/aufs.h

@@ -35,6 +35,7 @@
 #include "hbl.h"
 #include "inode.h"
 #include "lcnt.h"
+#include "loop.h"
 #include "module.h"
 #include "opts.h"
 #include "rwsem.h"

fs/aufs/branch.c

@@ -102,7 +102,8 @@ int au_br_index(struct super_block *sb, aufs_bindex_t br_id)
 static int test_overlap(struct super_block *sb, struct dentry *h_adding,
 			struct dentry *h_root)
 {
-	if (unlikely(h_adding == h_root))
+	if (unlikely(h_adding == h_root
+		     || au_test_loopback_overlap(sb, h_adding)))
 		return 1;
 	if (h_adding->d_sb != h_root->d_sb)
 		return 0;

fs/aufs/f_op.c

@@ -345,6 +345,13 @@ static ssize_t aufs_read_iter(struct kiocb *kio, struct iov_iter *iov_iter)
 	if (IS_ERR(h_file))
 		goto out;
 
+	if (au_test_loopback_kthread()) {
+		au_warn_loopback(h_file->f_path.dentry->d_sb);
+		if (file->f_mapping != h_file->f_mapping) {
+			file->f_mapping = h_file->f_mapping;
+			smp_mb(); /* unnecessary? */
+		}
+	}
 	fi_read_unlock(file);
 
 	err = au_do_iter(h_file, MAY_READ, kio, iov_iter);

fs/aufs/loop.c

+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2005-2019 Junjiro R. Okajima
+ */
+
+/*
+ * support for loopback block device as a branch
+ */
+
+#include "aufs.h"
+
+/* added into drivers/block/loop.c */
+static struct file *(*backing_file_func)(struct super_block *sb);
+
+/*
+ * test if two lower dentries have overlapping branches.
+ */
+int au_test_loopback_overlap(struct super_block *sb, struct dentry *h_adding)
+{
+	struct super_block *h_sb;
+	struct file *backing_file;
+
+	if (unlikely(!backing_file_func)) {
+		/* don't load "loop" module here */
+		backing_file_func = symbol_get(loop_backing_file);
+		if (unlikely(!backing_file_func))
+			/* "loop" module is not loaded */
+			return 0;
+	}
+
+	h_sb = h_adding->d_sb;
+	backing_file = backing_file_func(h_sb);
+	if (!backing_file)
+		return 0;
+
+	h_adding = backing_file->f_path.dentry;
+	/*
+	 * h_adding can be local NFS.
+	 * in this case aufs cannot detect the loop.
+	 */
+	if (unlikely(h_adding->d_sb == sb))
+		return 1;
+	return !!au_test_subdir(h_adding, sb->s_root);
+}
+
+/* true if a kernel thread named 'loop[0-9].*' accesses a file */
+int au_test_loopback_kthread(void)
+{
+	int ret;
+	struct task_struct *tsk = current;
+	char c, comm[sizeof(tsk->comm)];
+
+	ret = 0;
+	if (tsk->flags & PF_KTHREAD) {
+		get_task_comm(comm, tsk);
+		c = comm[4];
+		ret = ('0' <= c && c <= '9'
+		       && !strncmp(comm, "loop", 4));
+	}
+
+	return ret;
+}
+
+/* ---------------------------------------------------------------------- */
+
+#define au_warn_loopback_step	16
+static int au_warn_loopback_nelem = au_warn_loopback_step;
+static unsigned long *au_warn_loopback_array;
+
+void au_warn_loopback(struct super_block *h_sb)
+{
+	int i, new_nelem;
+	unsigned long *a, magic;
+	static DEFINE_SPINLOCK(spin);
+
+	magic = h_sb->s_magic;
+	spin_lock(&spin);
+	a = au_warn_loopback_array;
+	for (i = 0; i < au_warn_loopback_nelem && *a; i++)
+		if (a[i] == magic) {
+			spin_unlock(&spin);
+			return;
+		}
+
+	/* h_sb is new to us, print it */
+	if (i < au_warn_loopback_nelem) {
+		a[i] = magic;
+		goto pr;
+	}
+
+	/* expand the array */
+	new_nelem = au_warn_loopback_nelem + au_warn_loopback_step;
+	a = au_kzrealloc(au_warn_loopback_array,
+			 au_warn_loopback_nelem * sizeof(unsigned long),
+			 new_nelem * sizeof(unsigned long), GFP_ATOMIC,
+			 /*may_shrink*/0);
+	if (a) {
+		au_warn_loopback_nelem = new_nelem;
+		au_warn_loopback_array = a;
+		a[i] = magic;
+		goto pr;
+	}
+
+	spin_unlock(&spin);
+	AuWarn1("realloc failed, ignored\n");
+	return;
+
+pr:
+	spin_unlock(&spin);
+	pr_warn("you may want to try another patch for loopback file "
+		"on %s(0x%lx) branch\n", au_sbtype(h_sb), magic);
+}
+
+int au_loopback_init(void)
+{
+	int err;
+	struct super_block *sb __maybe_unused;
+
+	BUILD_BUG_ON(sizeof(sb->s_magic) != sizeof(unsigned long));
+
+	err = 0;
+	au_warn_loopback_array = kcalloc(au_warn_loopback_step,
+					 sizeof(unsigned long), GFP_NOFS);
+	if (unlikely(!au_warn_loopback_array))
+		err = -ENOMEM;
+
+	return err;
+}
+
+void au_loopback_fin(void)
+{
+	if (backing_file_func)
+		symbol_put(loop_backing_file);
+	au_kfree_try_rcu(au_warn_loopback_array);
+}

fs/aufs/loop.h

+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2005-2019 Junjiro R. Okajima
+ */
+
+/*
+ * support for loopback mount as a branch
+ */
+
+#ifndef __AUFS_LOOP_H__
+#define __AUFS_LOOP_H__
+
+#ifdef __KERNEL__
+
+struct dentry;
+struct super_block;
+
+#ifdef CONFIG_AUFS_BDEV_LOOP
+/* drivers/block/loop.c */
+struct file *loop_backing_file(struct super_block *sb);
+
+/* loop.c */
+int au_test_loopback_overlap(struct super_block *sb, struct dentry *h_adding);
+int au_test_loopback_kthread(void);
+void au_warn_loopback(struct super_block *h_sb);
+
+int au_loopback_init(void);
+void au_loopback_fin(void);
+#else
+AuStub(struct file *, loop_backing_file, return NULL)
+
+AuStubInt0(au_test_loopback_overlap, struct super_block *sb,
+	   struct dentry *h_adding)
+AuStubInt0(au_test_loopback_kthread, void)
+AuStubVoid(au_warn_loopback, struct super_block *h_sb)
+
+AuStubInt0(au_loopback_init, void)
+AuStubVoid(au_loopback_fin, void)
+#endif /* BLK_DEV_LOOP */
+
+#endif /* __KERNEL__ */
+#endif /* __AUFS_LOOP_H__ */

fs/aufs/module.c

@@ -193,9 +193,12 @@ static int __init aufs_init(void)
 	err = au_wkq_init();
 	if (unlikely(err))
 		goto out_procfs;
-	err = au_hnotify_init();
+	err = au_loopback_init();
 	if (unlikely(err))
 		goto out_wkq;
+	err = au_hnotify_init();
+	if (unlikely(err))
+		goto out_loopback;
 	err = au_sysrq_init();
 	if (unlikely(err))
 		goto out_hin;
@@ -217,6 +220,8 @@ out_sysrq:
 	au_sysrq_fin();
 out_hin:
 	au_hnotify_fin();
+out_loopback:
+	au_loopback_fin();
 out_wkq:
 	au_wkq_fin();
 out_procfs: