Messages in this thread | | | Date | Wed, 23 Jan 2019 14:09:17 +0100 | From | Michal Hocko <> | Subject | Re: [PATCH 2/2] debugfs: return error values, not NULL |
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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.
And Cc: stable is completely inappropriate IMNSHO. This is just adding a risk without a large benefit.
Moreover all these changes should be posted in a single patch thread so that everybody can see the final outcome. -- Michal Hocko SUSE Labs
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