On Wed, Jan 23, 2019 at 02:09:17PM +0100, Michal Hocko wrote: > On Wed 23-01-19 14:00:57, Greg KH wrote: > > On Wed, Jan 23, 2019 at 01:40:24PM +0100, Michal Hocko wrote: > > > 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. > > > > Ah, I already have that patch in my "to add a proper changelog" queue, > > it's below and fixes that problem. > > OK, fair enough. I am just wondering how many more gems like that are > lurking there. Do not get me wrong but a broken error handling is rarely > fixed by removing it.
I think you all are the "lucky" one here :) I did audit the whole kernel tree already, with the exception of the gpu drivers, as they are even more insane than block drivers... > And Cc: stable is completely inappropriate IMNSHO. This is just adding a > risk without a large benefit. What risk? Again, the _ONLY_ way that NULL is returned here is if you are out of memory when you try to create that debugfs file/directory. Given that we usually don't even fail small kmallocs, I doubt this can even ever happen. > Moreover all these changes should be posted in a single patch thread so > that everybody can see the final outcome. No one wants to see a 200+ patch thread and be cc:ed on them all. thanks, greg k-h
