On Wed, Mar 17, 2021 at 04:20:52PM +0100, Michal Hocko wrote: > On Wed 17-03-21 15:56:44, Greg KH wrote: > > On Wed, Mar 17, 2021 at 03:44:16PM +0100, Michal Hocko wrote: > > > On Wed 17-03-21 14:34:27, Greg KH wrote: > > > > On Wed, Mar 17, 2021 at 01:08:21PM +0100, Michal Hocko wrote: > > > > > Btw. I still have problems with the approach. seq_file is intended to > > > > > provide safe way to dump values to the userspace. Sacrificing > > > > > performance just because of some abuser seems like a wrong way to go as > > > > > Al pointed out earlier. Can we simply stop the abuse and disallow to > > > > > manipulate the buffer directly? I do realize this might be more tricky > > > > > for reasons mentioned in other emails but this is definitely worth > > > > > doing. > > > > > > > > We have to provide a buffer to "write into" somehow, so what is the best > > > > way to stop "abuse" like this? > > > > > > What is wrong about using seq_* interface directly? > > > > Right now every show() callback of sysfs would have to be changed :( > > Is this really the case? Would it be too ugly to have an intermediate > buffer and then seq_puts it into the seq file inside sysfs_kf_seq_show. Oh, good idea. > Sure one copy more than necessary but it this shouldn't be a hot path or > even visible on small strings. So that might be worth destroying an > inherently dangerous seq API (seq_get_buf). I'm all for that, let me see if I can carve out some time tomorrow to try this out. But, you don't get rid of the "ability" to have a driver write more than a PAGE_SIZE into the buffer passed to it. I guess I could be paranoid and do some internal checks (allocate a bunch of memory and check for overflow by hand), if this is something to really be concerned about... thanks, greg k-h
