On 5/12/20 11:11 AM, Mike Kravetz wrote: > On 5/12/20 8:04 AM, Miklos Szeredi wrote: >> On Tue, Apr 7, 2020 at 12:06 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote: >>> On 4/5/20 8:06 PM, syzbot wrote: >>> >>> The routine is_file_hugepages() is just comparing the file ops to huegtlbfs: >>> >>> if (file->f_op == &hugetlbfs_file_operations) >>> return true; >>> >>> Since the file is in an overlayfs, file->f_op == ovl_file_operations. >>> Therefore, length will not be rounded up to huge page size and we create a >>> mapping with incorrect size which leads to the BUG. >>> >>> Because of the code in mmap, the hugetlbfs mmap() routine assumes length is >>> rounded to a huge page size. I can easily add a check to hugetlbfs mmap >>> to validate length and return -EINVAL. However, I think we really want to >>> do the 'round up' earlier in mmap. This is because the man page says: >>> >>> Huge page (Huge TLB) mappings >>> For mappings that employ huge pages, the requirements for the arguments >>> of mmap() and munmap() differ somewhat from the requirements for map‐ >>> pings that use the native system page size. >>> >>> For mmap(), offset must be a multiple of the underlying huge page size. >>> The system automatically aligns length to be a multiple of the underly‐ >>> ing huge page size. >>> >>> Since the location for the mapping is chosen BEFORE getting to the hugetlbfs >>> mmap routine, we can not wait until then to round up the length. Is there a >>> defined way to go from a struct file * to the underlying filesystem so we >>> can continue to do the 'round up' in early mmap code? >> >> That's easy enough: >> >> static inline struct file *real_file(struct file *file) >> { >> return file->f_op != ovl_file_operations ? file : file->private_data; >> } >> >> But adding more filesystem specific code to generic code does not >> sound like the cleanest way to solve this... > > We can incorporate the above 'real_file' functionality in the filesystem > specific routine is_file_hugepages(), and I think that would address this > specific issue. I'll code that up. > >>> One other thing I noticed with overlayfs is that it does not contain a >>> specific get_unmapped_area file_operations routine. I would expect it to at >>> least check for and use the get_unmapped_area of the underlying filesystem? >>> Can someone comment if this is by design? >> >> Not sure. What exactly is f_op->get_unmapped_area supposed to do? >> > > IIUC, filesystems can define their own routines to get addresses for mmap > operations. Quite a few filesystems define get_unmapped_area. > > The generic mmap code does the following, > > get_area = current->mm->get_unmapped_area; > if (file) { > if (file->f_op->get_unmapped_area) > get_area = file->f_op->get_unmapped_area; > } else if (flags & MAP_SHARED) { > /* > * mmap_region() will call shmem_zero_setup() to create a file, > * so use shmem's get_unmapped_area in case it can be huge. > * do_mmap_pgoff() will clear pgoff, so match alignment. > */ > pgoff = 0; > get_area = shmem_get_unmapped_area; > } > > addr = get_area(file, addr, len, pgoff, flags); > > If the filesystem provides a get_unmapped_area, it will use it. I beleive > overlayfs prevents this from happening for the underlying filesystem. > > Perhaps we do need to add something like a call 'real_file' to this generic > code? I can't think of any other way to get to the underlying filesystem > get_unmapped_area here. I started going down the path of creating a get_unmapped_area f_op for overlayfs. That is pretty straight forward and works well. But that did not take care of the is_file_hugepages() routine. Recall that is_file_hugepages simply does if (file->f_op == &hugetlbfs_file_operations). I suppose I could add a specific overlayfs check like real_file here. But, that does not seem like a clean solution. I also discovered other routines doing comparisons like if (file->f_op == <expected_fops>), they are: is_dma_buf_file() is_file_shm_hugepages() get_pipe_info() is_file_epoll() So, it seems that these routines are also impacted if operating on files in an overlayfs? Any suggestions on how to move forward? It seems like there may be the need for a real_file() routine? I see a d_real dentry_op was added to deal with this issue for dentries. Might we need something similiar for files (f_real)? Looking for suggestions as I do not normally work with this code. -- Mike Kravetz