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 :( > > Right now, we do have helper functions, sysfs_emit(), that know to stop > > the overflow of the buffer size, but porting the whole kernel to them is > > going to take a bunch of churn, for almost no real benefit except a > > potential random driver that might be doing bad things here that we have > > not noticed yet. > > I am not familiar with sysfs, I just got lost in all the indirection but > replacing buffer by the seq_file and operate on that should be possible, > no? sysfs files should be very simple and easy, and have a single value being written to userspace. I guess seq_printf() does handle the issue of "big buffers", but there should not be a big buffer here to worry about in the first place (yes, there was a bug where a driver took unchecked data and sent it to userspace overflowing the buffer which started this whole thread...) I guess Kees wants to change all show functions to use the seq_ api, which now makes a bit more sense, but still seems like a huge overkill. But I now understand the idea here, the buffer management is handled by the core kernel and overflows are impossible. A "simpler" fix is to keep the api the same today, and just "force" everyone to use sysfs_emit() which does the length checking automatically. I don't know, it all depends on how much effort we want to put into the "drivers can not do stupid things because we prevent them from it" type of work here... thanks, greg k-h
