On Thu, Apr 25, 2019 at 5:43 AM Jann Horn <jannh@xxxxxxxxxx> wrote: > An interesting effect of this is that it will be possible to set this > on a CoW anon VMA in a fork() child, and then the semantics in the > parent will be subtly different - e.g. if the parent vmsplice()d a > CoWed page into a pipe, then forked an unprivileged child, the child > set MADV_WIPEONRELEASE on its VMA, the parent died somehow, and then > the child died, the page in the pipe would be zeroed out. A child > should not be able to affect its parent like this, I think. If this > was an mmap() flag instead of a madvise() command, that issue could be > avoided. Alternatively, if adding more mmap() flags doesn't work, > perhaps you could scan the VMA and ensure that it contains no pages > yet, or something like that? I /think/ my argument here would be not to do that? I agree that it's unexpected, but I guess the other alternative would be to force a copy on any existing COW pages in the VMA at madvise() time, and maybe also at fork() time (sort of like the behaviour of MADV_WIPEONFORK, but copying the page rather than providing a new zero page) > I think all the callers have a reference to the VMA, so perhaps you > could add a VMA parameter to page_remove_rmap() and then look at the > VMA in there? I'll dig into that, thanks!
