[PATCH v4 2/2] ovl: call underlying get_unmapped_area() routine. propogate FMODE_HUGETLBFS

[Mike Kravetz]

The core routine get_unmapped_area will call a filesystem specific version
of get_unmapped_area if it exists in file operations.  If a file is on a
union/overlay, overlayfs does not contain a get_unmapped_area f_op and the
underlying filesystem routine may be ignored.  Add an overlayfs f_op to call
the underlying f_op if it exists.

The routine is_file_hugetlbfs() is used to determine if a file is on
hugetlbfs.  This is determined by f_mode & FMODE_HUGETLBFS.  Copy the mode
to the overlayfs file during open so that is_file_hugetlbfs() will work as
intended.

These two issues can result in the BUG as shown in [1].

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

Reported-by: syzbot+d6ec23007e951dadf3de@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Miklos Szeredi <miklos@xxxxxxxxxx>
Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 fs/overlayfs/file.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 87c362f65448..41e5746ba3c6 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -124,6 +124,8 @@ static int ovl_real_fdget(const struct file *file, struct fd *real)
 	return ovl_real_fdget_meta(file, real, false);
 }
 
+#define OVL_F_MODE_TO_UPPER	(FMODE_HUGETLBFS)
+
 static int ovl_open(struct inode *inode, struct file *file)
 {
 	struct file *realfile;
@@ -140,6 +142,9 @@ static int ovl_open(struct inode *inode, struct file *file)
 	if (IS_ERR(realfile))
 		return PTR_ERR(realfile);
 
+	/* Copy modes from underlying file */
+	file->f_mode |= (realfile->f_mode & OVL_F_MODE_TO_UPPER);
+
 	file->private_data = realfile;
 
 	return 0;
@@ -757,6 +762,21 @@ static loff_t ovl_remap_file_range(struct file *file_in, loff_t pos_in,
 			    remap_flags, op);
 }
 
+#ifdef CONFIG_MMU
+static unsigned long ovl_get_unmapped_area(struct file *file,
+				unsigned long uaddr, unsigned long len,
+				unsigned long pgoff, unsigned long flags)
+{
+	struct file *realfile = file->private_data;
+
+	return (realfile->f_op->get_unmapped_area ?:
+		current->mm->get_unmapped_area)(realfile,
+						uaddr, len, pgoff, flags);
+}
+#else
+#define ovl_get_unmapped_area NULL
+#endif
+
 const struct file_operations ovl_file_operations = {
 	.open		= ovl_open,
 	.release	= ovl_release,
@@ -774,6 +794,7 @@ const struct file_operations ovl_file_operations = {
 
 	.copy_file_range	= ovl_copy_file_range,
 	.remap_file_range	= ovl_remap_file_range,
+	.get_unmapped_area	= ovl_get_unmapped_area,
 };
 
 int __init ovl_aio_request_cache_init(void)
-- 
2.25.4