Re: [PATCH v2] seq_file: Unconditionally use vmalloc for buffer

From: Greg Kroah-Hartman
Date: Wed Mar 17 2021 - 09:35:20 EST

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?

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.

Other than that, suggestions are welcome!


greg k-h