On Wed 23-01-19 13:26:26, Greg KH wrote: > On Wed, Jan 23, 2019 at 01:13:50PM +0100, Michal Hocko wrote: > > On Wed 23-01-19 12:55:35, Greg KH wrote: > > > On Wed, Jan 23, 2019 at 12:06:28PM +0100, Michal Hocko wrote: > > > > On Wed 23-01-19 11:28:14, Greg KH wrote: > > > > > When an error happens, debugfs should return an error pointer value, > > > > > not > > > > > NULL. This will prevent the totally theoretical error where a debugfs > > > > > call fails due to lack of memory, returning NULL, and that dentry > > > > > value > > > > > is then passed to another debugfs call, which would end up succeeding, > > > > > creating a file at the root of the debugfs tree, but would then be > > > > > impossible to remove (because you can not remove the directory NULL). > > > > > > > > > > So, to make everyone happy, always return errors, this makes the users > > > > > of debugfs much simpler (they do not have to ever check the return > > > > > value), and everyone can rest easy. > > > > > > > > How come this is safe at all? Say you are creating a directory by > > > > debugfs_create_dir and then feed the return value to > > > > debugfs_create_files > > > > as a parent. In case of error you are giving it an invalid pointer and > > > > likely blow up unless I miss something. > > > > > > debugfs_create_files checks for invalid parents and will just refuse to > > > create the file. It's always done that. > > > > I must be missing something because debugfs_create_files does > > d_inode(parent)->i_private = data; > > as the very first thing and that means that it dereferences an invalid > > pointer right there. > > debugfs_create_file() -> __debugfs_create_file() -> start_creating() > and that function checks if parent is an error, which it aborts on, or > if it is NULL, it sets parent to a valid value: > > /* If the parent is not specified, we create it in the root. > * We need the root dentry to do this, which is in the super > * block. A pointer to that is in the struct vfsmount that we > * have around. > */ > if (!parent) > parent = debugfs_mount->mnt_root; > > I don't see any line that looks like: > > d_inode(parent)->i_private = data; > in Linus's tree right now, what kernel version are you referring to?
Ohh, my bad. I have looked at debugfs_create_files which is a mq helper around debugfs_create_file. But that is a good example why this patch is dangerous anyway. blk_mq_debugfs_register simply checks for NULL and debugfs_create_files doesn't expect ERR_PTR here. So you would have to check each and every user to make sure you can do that. > > > > I do agree that reporting errors is better than a simple catch all NULL > > > > but this should have been done when introduced rather than now when most > > > > callers simply check for NULL as a failure. > > > > > > I'm fixing up all the "NULL is a failure" callsites in the kernel, see > > > lkml for the first round of those patches. > > > > You are merely removing them, which doesn't really help for this patch. > > It doesn't hurt either, as if you really wanted to handle errors from debugfs > properly, you have to check for IS_ERR() as well, because the filesystem can > be > compiled out (and then it returns an error pointer) I would assume that this would be achieved by a direct config dependency. E.g. BLK_DEBUG_FS. So the code doesn't even get compiled and wouldn't ever encounter the ERR_PTR. Yeah this whole debugfs API is broken and it would take a lot of time to unclutter it but it definitely is not that simple as what this patch does. -- Michal Hocko SUSE Labs