On Mon, Jun 05, 2023 at 04:10:21PM +0000, Edgecombe, Rick P wrote: > On Mon, 2023-06-05 at 11:11 +0300, Mike Rapoport wrote: > > On Sun, Jun 04, 2023 at 10:52:44PM -0400, Steven Rostedt wrote: > > > On Thu, 1 Jun 2023 16:54:36 -0700 > > > Nadav Amit <nadav.amit@xxxxxxxxx> wrote: > > > > > > > > The way text_poke() is used here, it is creating a new writable > > > > > alias > > > > > and flushing it for *each* write to the module (like for each > > > > > write of > > > > > an individual relocation, etc). I was just thinking it might > > > > > warrant > > > > > some batching or something. > > > > > > I am not advocating to do so, but if you want to have many > > > > efficient > > > > writes, perhaps you can just disable CR0.WP. Just saying that if > > > > you > > > > are about to write all over the memory, text_poke() does not > > > > provide > > > > too much security for the poking thread. > > > > Heh, this is definitely and easier hack to implement :) > > I don't know the details, but previously there was some strong dislike > of CR0.WP toggling. And now there is also the problem of CET. Setting > CR0.WP=0 will #GP if CR4.CET is 1 (as it currently is for kernel IBT). > I guess you might get away with toggling them both in some controlled > situation, but it might be a lot easier to hack up then to be made > fully acceptable. It does sound much more efficient though. I don't think we'd really want that, especially looking at WARN_ONCE(bits_missing, "CR0 WP bit went missing!?\n"); at native_write_cr0(). > > > Batching does exist, which is what the text_poke_queue() thing > > > does. > > > > For module loading text_poke_queue() will still be much slower than a > > bunch > > of memset()s for no good reason because we don't need all the > > complexity of > > text_poke_bp_batch() for module initialization because we are sure we > > are > > not patching live code. > > > > What we'd need here is a new batching mode that will create a > > writable > > alias mapping at the beginning of apply_relocate_*() and > > module_finalize(), > > then it will use memcpy() to that writable alias and will tear the > > mapping > > down in the end. > > It's probably only a tiny bit faster than keeping a separate writable > allocation and text_poking it in at the end. Right, but it still will be faster than text_poking every relocation. > > Another option is to teach alternatives to update a writable copy > > rather > > than do in place changes like Song suggested. My feeling is that it > > will be > > more intrusive change though. > > You mean keeping a separate RW allocation and then text_poking() the > whole thing in when you are done? That is what I was trying to say at > the beginning of this thread. The other benefit is you don't make the > intermediate loading states of the module, executable. > > I tried this technique previously [0], and I thought it was not too > bad. In most of the callers it looks similar to what you have in > do_text_poke(). Sometimes less, sometimes more. It might need > enlightening of some of the stuff currently using text_poke() during > module loading, like jump labels. So that bit is more intrusive, yea. > But it sounds so much cleaner and well controlled. Did you have a > particular trouble spot in mind? Nothing in particular, except the intrusive part. Except the changes in modules.c we'd need to teach alternatives to deal with a writable copy. > [0] > https://lore.kernel.org/lkml/20201120202426.18009-5-rick.p.edgecombe@xxxxxxxxx/ -- Sincerely yours, Mike.
