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Date:   Fri, 22 May 2020 12:05:53 +0200
From:   Miklos Szeredi <miklos@szeredi.hu>
To:     Mike Kravetz <mike.kravetz@oracle.com>
Cc:     Colin Walters <walters@verbum.org>,
        syzbot <syzbot+d6ec23007e951dadf3de@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm <linux-mm@kvack.org>,
        Miklos Szeredi <mszeredi@redhat.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        linux-unionfs@vger.kernel.org
Subject: Re: kernel BUG at mm/hugetlb.c:LINE!
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On Wed, May 20, 2020 at 10:27:15AM -0700, Mike Kravetz wrote:

> I am fairly confident it is all about checking limits and alignment.  The
> filesystem knows if it can/should align to base or huge page size. DAX has
> some interesting additional restrictions, and several 'traditional' filesystems
> check if they are 'on DAX'.


Okay, I haven't looked at DAX vs. overlay.  I'm sure it's going to come up at
some point, if it hasn't already.

> 
> In a previous e-mail, you suggested hugetlb_get_unmapped_area could do the
> length adjustment in hugetlb_get_unmapped_area (generic and arch specific).
> I agree, although there may be the need to add length overflow checks in
> these routines (after round up) as this is done in core code now.  However,
> this can be done as a separate cleanup patch.
> 
> In any case, we need to get the core mmap code to call filesystem specific
> get_unmapped_area if on a union/overlay.  The patch I suggested does this
> by simply calling real_file to determine if there is a filesystem specific
> get_unmapped_area.  The other approach would be to provide an overlayfs
> get_unmapped_area that calls the underlying filesystem get_unmapped_area.

That latter is what's done for all other stacked operations in overlayfs.

Untested patch below.

Thanks,
Miklos

---
 fs/overlayfs/file.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -757,6 +757,17 @@ static loff_t ovl_remap_file_range(struc
 			    remap_flags, op);
 }
 
+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);
+}
+
 const struct file_operations ovl_file_operations = {
 	.open		= ovl_open,
 	.release	= ovl_release,
@@ -774,6 +785,7 @@ const struct file_operations ovl_file_op
 
 	.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)