On Thu, Mar 12, 2020 at 09:19:39AM +0000, David Howells wrote:
> Hi Matthew,
>
> Is ext4_dio_read_iter() broken? It calls:
>
> file_accessed(iocb->ki_filp);
>
> at the end of the function - but surely iocb should be expected to have been
> freed when iocb->ki_complete() was called?
>
> In my cachefiles rewrite, I'm seeing the attached kasan dump. The offending
> RIP, ext4_file_read_iter+0x12b is at the above line, where it is trying to
> read iocb->ki_filp.
>
> Here's an excerpt of the relevant bits from my code:
>
> static void cachefiles_read_complete(struct kiocb *iocb, long ret, long ret2)
> {
> struct cachefiles_kiocb *ki =
> container_of(iocb, struct cachefiles_kiocb, iocb);
> struct fscache_io_request *req = ki->req;
> ...
> fput(ki->iocb.ki_filp);
> kfree(ki);
> fscache_end_io_operation(req->cookie);
> ...
> }
>
> int cachefiles_read(struct fscache_object *obj,
> struct fscache_io_request *req,
> struct iov_iter *iter)
> {
> struct cachefiles_object *object =
> container_of(obj, struct cachefiles_object, fscache);
> struct cachefiles_kiocb *ki;
> struct file *file = object->backing_file;
> ssize_t ret = -ENOBUFS;
> ...
> ki = kzalloc(sizeof(struct cachefiles_kiocb), GFP_KERNEL);
> if (!ki)
> goto presubmission_error;
>
> ki->iocb.ki_filp = get_file(file);
> ki->iocb.ki_pos = req->pos;
> ki->iocb.ki_flags = IOCB_DIRECT;
> ki->iocb.ki_hint = ki_hint_validate(file_write_hint(file));
> ki->iocb.ki_ioprio = get_current_ioprio();
> ki->req = req;
>
> if (req->io_done)
> ki->iocb.ki_complete = cachefiles_read_complete;
>
> ret = rw_verify_area(READ, file, &ki->iocb.ki_pos, iov_iter_count(iter));
> if (ret < 0)
> goto presubmission_error_free;
>
> fscache_get_io_request(req);
> trace_cachefiles_read(object, file_inode(file), req);
> ret = call_read_iter(file, &ki->iocb, iter);
> ...
> }
>
> The allocation point, cachefiles_read+0xd0, is the kzalloc() you can see and
> the free point, cachefiles_read_complete+0x86, is the kfree() in the callback
> function.
It looks to me like you are creating your own iocb that you are
doing AIO+DIO on, and you aren't taking a reference count to the
iocb yourself to ensure that it exists for the entire submission
path. i.e. see how the AIO code uses ki_refcnt and iocb_put() to
ensure the iocb does not get destroyed by IO completion before the
submission path has completed.
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
