On Tue, Nov 15, 2022 at 10:22:56PM IST, Dave Marchevsky wrote: > On 11/14/22 2:15 PM, Kumar Kartikeya Dwivedi wrote: > > This commit implements the delayed release logic for bpf_list_push_front > > and bpf_list_push_back. > > > > Once a node has been added to the list, it's pointer changes to > > PTR_UNTRUSTED. However, it is only released once the lock protecting the > > list is unlocked. For such PTR_TO_BTF_ID | MEM_ALLOC with PTR_UNTRUSTED > > set but an active ref_obj_id, it is still permitted to read them as long > > as the lock is held. Writing to them is not allowed. > > > > This allows having read access to push items we no longer own until we > > release the lock guarding the list, allowing a little more flexibility > > when working with these APIs. > > > > Note that enabling write support has fairly tricky interactions with > > what happens inside the critical section. Just as an example, currently, > > bpf_obj_drop is not permitted, but if it were, being able to write to > > the PTR_UNTRUSTED pointer while the object gets released back to the > > memory allocator would violate safety properties we wish to guarantee > > (i.e. not crashing the kernel). The memory could be reused for a > > different type in the BPF program or even in the kernel as it gets > > eventually kfree'd. > > > > Not enabling bpf_obj_drop inside the critical section would appear to > > prevent all of the above, but that is more of an artifical limitation > > right now. Since the write support is tangled with how we handle > > potential aliasing of nodes inside the critical section that may or may > > not be part of the list anymore, it has been deferred to a future patch. > > > > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> > > --- > > Can the two WARN_ON_ONCE in this patch be converted to > verifier-log-and-EFAULT? Looks like they're both in > functions with access to 'env' and are checking for > scenarios that should be considered bugs in the verifier. > Will do. > Aside from that style nit, logic and patch summary updates > here LGTM. > > Acked-by: Dave Marchevsky <davemarchevsky@xxxxxx>
