[PATCH] ovl: provide real_file() for use by hugetlb and mmap

[Mike Kravetz]

If a file is on a union/overlay, then the 'struct file *' will have
overlayfs file operations.  The routine is_file_hugepages() compares
f->f_op to hugetlbfs_file_operations to determine if it is a hugetlbfs
file.  If a hugetlbfs file is on a union/overlay, this comparison is
false and is_file_hugepages() incorrectly indicates the underlying
file is not hugetlbfs.  One result of this is a BUG as shown in [1].

mmap uses is_file_hugepages() because hugetlbfs files have different
alignment restrictions.  In addition, mmap code would like to use the
filesystem specific get_unmapped_area() routine if one is defined.

To address this issue, add a new routine real_file() which will return
the underlying file.  Update is_file_hugepages and mmap code to get the
real file.

[1] https://lore.kernel.org/linux-mm/000000000000b4684e05a2968ca6@xxxxxxxxxx/

Reported-by: syzbot+d6ec23007e951dadf3de@xxxxxxxxxxxxxxxxxxxxxxxxx
Suggested-by: Miklos Szeredi <miklos@xxxxxxxxxx>
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 include/linux/hugetlb.h   |  3 +++
 include/linux/overlayfs.h | 27 +++++++++++++++++++++++++++
 mm/mmap.c                 |  2 ++
 3 files changed, 32 insertions(+)
 create mode 100644 include/linux/overlayfs.h

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 43a1cef8f0f1..fb22c0a7474a 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -9,6 +9,7 @@
 #include <linux/cgroup.h>
 #include <linux/list.h>
 #include <linux/kref.h>
+#include <linux/overlayfs.h>
 #include <asm/pgtable.h>
 
 struct ctl_table;
@@ -437,6 +438,8 @@ struct file *hugetlb_file_setup(const char *name, size_t size, vm_flags_t acct,
 
 static inline bool is_file_hugepages(struct file *file)
 {
+	file = real_file(file);
+
 	if (file->f_op == &hugetlbfs_file_operations)
 		return true;
 
diff --git a/include/linux/overlayfs.h b/include/linux/overlayfs.h
new file mode 100644
index 000000000000..eecdfda0286f
--- /dev/null
+++ b/include/linux/overlayfs.h
@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_OVERLAYFS_H
+#define _LINUX_OVERLAYFS_H
+
+#include <linux/fs.h>
+
+extern const struct file_operations ovl_file_operations;
+
+#ifdef CONFIG_OVERLAY_FS
+/*
+ * If file is on a union/overlay, then return the underlying real file.
+ * Otherwise return the file itself.
+ */
+static inline struct file *real_file(struct file *file)
+{
+	while (unlikely(file->f_op == &ovl_file_operations))
+		file = file->private_data;
+	return file;
+}
+#else
+static inline struct file *real_file(struct file *file)
+{
+	return file;
+}
+#endif
+
+#endif /* _LINUX_OVERLAYFS_H */
diff --git a/mm/mmap.c b/mm/mmap.c
index f609e9ec4a25..7f45a4057a15 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -47,6 +47,7 @@
 #include <linux/pkeys.h>
 #include <linux/oom.h>
 #include <linux/sched/mm.h>
+#include <linux/overlayfs.h>
 
 #include <linux/uaccess.h>
 #include <asm/cacheflush.h>
@@ -2203,6 +2204,7 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
 
 	get_area = current->mm->get_unmapped_area;
 	if (file) {
+		file = real_file(file);
 		if (file->f_op->get_unmapped_area)
 			get_area = file->f_op->get_unmapped_area;
 	} else if (flags & MAP_SHARED) {
-- 
2.25.4