On Mon, Apr 24, 2023 at 03:29:57PM +0100, Lorenzo Stoakes wrote: > On Mon, Apr 24, 2023 at 10:39:25AM -0300, Jason Gunthorpe wrote: > > On Mon, Apr 24, 2023 at 01:38:49PM +0100, Lorenzo Stoakes wrote: > > > > > I was being fairly conservative in that list, though we certainly need to > > > set the flag for /proc/$pid/mem and ptrace to avoid breaking this > > > functionality (I observed breakpoints breaking without it which obviously > > > is a no go :). I'm not sure if there's a more general way we could check > > > for this though? > > > > More broadly we should make sure these usages of GUP safe somehow so > > that it can reliably write to those types of pages without breaking > > the current FS contract.. > > > > I forget exactly, but IIRC, don't you have to hold some kind of page > > spinlock while writing to the page memory? > > > > I think perhaps you're thinking of the mm->mmap_lock? Which will be held > for the FOLL_GET cases and simply prevent the VMA from disappearing below > us but not do much else. No not mmap_lock, I want to say there is a per-page lock that interacts with the write protect, or at worst this needs to use the page table spinlocks. > I wonder whether we should do this check purely for FOLL_PIN to be honest? > As this indicates medium to long-term access without mmap_lock held. This > would exclude the /proc/$pid/mem and ptrace paths which use gup_remote(). Everything is buggy. FOLL_PIN is part of a someday solution to solve it. > That and a very specific use of uprobes are the only places that use > FOLL_GET in this instance and each of them are careful in any case to > handle setting the dirty page flag. That is actually the bug :) Broadly the bug is to make a page dirty without holding the right locks to actually dirty it. Jason
