On Thu, 17 Jan 2019 17:15:27 -0800 Nadav Amit <nadav.amit@xxxxxxxxx> wrote: > > On Jan 17, 2019, at 3:58 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote: > > > > On 1/16/19 11:54 PM, Masami Hiramatsu wrote: > >> On Wed, 16 Jan 2019 16:32:59 -0800 > >> Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> wrote: > >> > >>> From: Nadav Amit <namit@xxxxxxxxxx> > >>> > >>> It seems dangerous to allow code modifications to take place > >>> concurrently with module unloading. So take the text_mutex while the > >>> memory of the module is freed. > >> > >> At that point, since the module itself is removed from module list, > >> it seems no actual harm. Or would you have any concern? > > > > The issue isn't the module list, but rather when it is safe to free the > > contents, so we don't clobber anything. We absolutely need to enforce > > that we can't text_poke() something that might have already been freed. > > > > That being said, we *also* really would prefer to enforce that we can't > > text_poke() memory that doesn't actually contain code; as far as I can > > tell we don't currently do that check. > > Yes, that what the mutex was supposed to achieve. It’s not supposed just > to check whether it is a code page, but also that it is the same code > page that you wanted to patch. > > > This, again, is a good use for a separate mm context. We can enforce > > that that context will only ever contain valid page mappings for actual > > code pages. > > This will not tell you that you have the *right* code-page. The module > notifiers help to do so, since they synchronize the text poking with > the module removal. > > > (Note: in my proposed algorithm, with a separate mm, replace INVLPG with > > switching CR3 if we have to do a rollback or roll forward in the > > breakpoint handler.) > > I really need to read your patches more carefully to see what you mean. > > Anyhow, so what do you prefer? I’m ok with either one: > 1. Keep this patch > 2. Remove this patch and change into a comment on text_poke() > 3. Just drop the patch I would prefer 2. so at least we should add a comment to text_poke(). Thank you, -- Masami Hiramatsu <mhiramat@xxxxxxxxxx>
