On 6/4/20 2:16 AM, Miklos Szeredi wrote: > On Thu, May 28, 2020 at 11:01 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote: >> >> Well yuck! get_unmapped_area is not part of mm_struct if !CONFIG_MMU. >> >> Miklos, would adding '#ifdef CONFIG_MMU' around the overlayfs code be too >> ugly for you? Another option is to use real_file() in the mmap code as >> done in [1]. > > I think the proper fix is to add an inline helper > (call_get_unmapped_area()?) in linux/mm.h, and make that work properly > for the NOMMU case as well. > I'm ignoring the above issue for now. There may be a more fundamental issue that I do not know how to solve without adding another memeber to file_operations. Why? In order to go from file -> realfile, one needs to do something like the code you provided. while (file->f_op == &ovl_file_operations) file = file->private_data; return file; The problem is that this needs to be called from core kernel code (is_file_hugepages in mmap). ovl_file_operations is obviously only defined in the overlayfs code. Since overlayfs can be built as a module, I do not know of a way to reference ovl_file_operations which will only become available when/if overlayfs is loaded. Is there a way to do that? If there is no other way to do this, then I think we need to add another member to file_operations as is done in the following patch. I hope there is another way, because adding another file_op seems like overkill. >From 6d22c93284263b5ddcc2199adc1bcceb233f1499 Mon Sep 17 00:00:00 2001 From: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Date: Wed, 10 Jun 2020 16:23:46 -0700 Subject: [PATCH v3] ovl: add f_real file_operation and supporting code for overlayfs 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() to identify hugetlbfs file and potentially round up length because hugetlbfs files have different alignment restrictions. This is defined/expected behavior. 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 file operation f_real while will return the underlying file. Only overlayfs provides a function for this operation. - Add a new routine real_file() which can be used by core code get an underlying file. - Update is_file_hugepages to get the real file. - Add get_unmapped_area f_op to oerrlayfs to call underlying routine. [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 | 23 +++++++++++++++++++++++ include/linux/fs.h | 8 ++++++++ include/linux/hugetlb.h | 2 ++ 3 files changed, 33 insertions(+) diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c index 87c362f65448..eb870fc3912f 100644 --- a/fs/overlayfs/file.c +++ b/fs/overlayfs/file.c @@ -757,6 +757,27 @@ 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) +{ + file = real_file(file); + + return (file->f_op->get_unmapped_area ?: + current->mm->get_unmapped_area)(file, uaddr, len, pgoff, flags); +} +#else +#define ovl_get_unmapped_area NULL +#endif + +static struct file *ovl_f_real(struct file *file) +{ + while (file->f_op == &ovl_file_operations) + file = file->private_data; + return file; +} + const struct file_operations ovl_file_operations = { .open = ovl_open, .release = ovl_release, @@ -771,9 +792,11 @@ const struct file_operations ovl_file_operations = { .compat_ioctl = ovl_compat_ioctl, .splice_read = ovl_splice_read, .splice_write = ovl_splice_write, + .f_real = ovl_f_real, .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) diff --git a/include/linux/fs.h b/include/linux/fs.h index 45cc10cdf6dd..59a969549aec 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -1863,6 +1863,7 @@ struct file_operations { struct file *file_out, loff_t pos_out, loff_t len, unsigned int remap_flags); int (*fadvise)(struct file *, loff_t, loff_t, int); + struct file * (*f_real)(struct file *file); } __randomize_layout; struct inode_operations { @@ -1895,6 +1896,13 @@ struct inode_operations { int (*set_acl)(struct inode *, struct posix_acl *, int); } ____cacheline_aligned; +static inline struct file *real_file(struct file *file) +{ + if (unlikely(file->f_op->f_real)) + return file->f_op->f_real(file); + return file; +} + static inline ssize_t call_read_iter(struct file *file, struct kiocb *kio, struct iov_iter *iter) { diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h index 43a1cef8f0f1..528b07145414 100644 --- a/include/linux/hugetlb.h +++ b/include/linux/hugetlb.h @@ -437,6 +437,8 @@ 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; -- 2.25.4
