On Fri, Oct 2, 2020 at 11:18 AM Michel Lespinasse <walken@xxxxxxxxxx> wrote: > On Thu, Oct 1, 2020 at 6:25 PM Jann Horn <jannh@xxxxxxxxxx> wrote: > > Until now, the mmap lock of the nascent mm was ordered inside the mmap lock > > of the old mm (in dup_mmap() and in UML's activate_mm()). > > A following patch will change the exec path to very broadly lock the > > nascent mm, but fine-grained locking should still work at the same time for > > the new mm. > > To do this in a way that lockdep is happy about, let's turn around the lock > > ordering in both places that currently nest the locks. > > Since SINGLE_DEPTH_NESTING is normally used for the inner nesting layer, > > make up our own lock subclass MMAP_LOCK_SUBCLASS_NASCENT and use that > > instead. > > > > The added locking calls in exec_mmap() are temporary; the following patch > > will move the locking out of exec_mmap(). > > Thanks for doing this. > > This is probably a silly question, but I am not sure exactly where we > lock the old MM while bprm is creating the new MM ? I am guessing this > would be only in setup_arg_pages(), copying the args and environment > from the old the the new MM ? If that is correct, then wouldn't it be > sufficient to use mmap_write_lock_nested in setup_arg_pages() ? Or, is > the issue that we'd prefer to have a killable version of it there ? We're also implicitly locking the old MM anytime we take page faults before exec_mmap(), which basically means the various userspace memory accesses in do_execveat_common(). This happens after bprm_mm_init(), so we've already set bprm->vma at that point. > Also FYI I was going to play with these patches a bit to help answer > these questions on my own, but wasn't able to easily apply them as > they came lightly mangled (whitespace issues) when I saved them. Uuugh, dammit, I see what happened. Sorry about the trouble. Thanks for telling me, guess I'll go back to sending patches the way I did it before. :/ I guess I'll go make a v2 of this with some extra comment about where the old MM is accessed, as Jason suggested, and without the whitespace issues?
