On Tue, Mar 29, 2022 at 4:30 PM Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > On Tue, Mar 29, 2022 at 10:43:42AM -0700, Hao Luo wrote: > > On Tue, Mar 29, 2022 at 2:37 AM Kumar Kartikeya Dwivedi > > <memxor@xxxxxxxxx> wrote: > > > > > > On Mon, Mar 28, 2022 at 11:16:15PM IST, Hao Luo wrote: > > > > On Mon, Mar 28, 2022 at 10:39 AM Hao Luo <haoluo@xxxxxxxxxx> wrote: > > > > > > > > > > Hi Yonghong, > > > > > > > > > > On Fri, Mar 25, 2022 at 12:16 PM Yonghong Song <yhs@xxxxxx> wrote: > > > > > > > > > > > > On 3/24/22 4:41 PM, Hao Luo wrote: > > > > > > > Some map types support mmap operation, which allows userspace to > > > > > > > communicate with BPF programs directly. Currently only arraymap > > > > > > > and ringbuf have mmap implemented. > > > > > > > > > > > > > > However, in some use cases, when multiple program instances can > > > > > > > run concurrently, global mmapable memory can cause race. In that > > > > > > > case, userspace needs to provide necessary synchronizations to > > > > > > > coordinate the usage of mapped global data. This can be a source > > > > > > > of bottleneck. > > > > > > > > > > > > I can see your use case here. Each calling process can get the > > > > > > corresponding bpf program task local storage data through > > > > > > mmap interface. As you mentioned, there is a tradeoff > > > > > > between more memory vs. non-global synchronization. > > > > > > > > > > > > I am thinking that another bpf_iter approach can retrieve > > > > > > the similar result. We could implement a bpf_iter > > > > > > for task local storage map, optionally it can provide > > > > > > a tid to retrieve the data for that particular tid. > > > > > > This way, user space needs an explicit syscall, but > > > > > > does not need to allocate more memory than necessary. > > > > > > > > > > > > WDYT? > > > > > > > > > > > > > > > > Thanks for the suggestion. I have two thoughts about bpf_iter + tid and mmap: > > > > > > > > > > - mmap prevents the calling task from reading other task's value. > > > > > Using bpf_iter, one can pass other task's tid to get their values. I > > > > > assume there are two potential ways of passing tid to bpf_iter: one is > > > > > to use global data in bpf prog, the other is adding tid parameterized > > > > > iter_link. For the first, it's not easy for unpriv tasks to use. For > > > > > the second, we need to create one iter_link object for each interested > > > > > tid. It may not be easy to use either. > > > > > > > > > > - Regarding adding an explicit syscall. I thought about adding > > > > > write/read syscalls for task local storage maps, just like reading > > > > > values from iter_link. Writing or reading task local storage map > > > > > updates/reads the current task's value. I think this could achieve the > > > > > same effect as mmap. > > > > > > > > > > > > > Actually, my use case of using mmap on task local storage is to allow > > > > userspace to pass FDs into bpf prog. Some of the helpers I want to add > > > > need to take an FD as parameter and the bpf progs can run > > > > concurrently, thus using global data is racy. Mmapable task local > > > > storage is the best solution I can find for this purpose. > > > > > > > > Song also mentioned to me offline, that mmapable task local storage > > > > may be useful for his use case. > > > > > > > > I am actually open to other proposals. > > > > > > > > > > You could also use a syscall prog, and use bpf_prog_test_run to update local > > > storage for current. Data can be passed for that specific prog invocation using > > > ctx. You might have to enable bpf_task_storage helpers in it though, since they > > > are not allowed to be called right now. > > > > > > > The loading process needs CAP_BPF to load bpf_prog_test_run. I'm > > thinking of allowing any thread including unpriv ones to be able to > > pass data to the prog and update their own storage. > > If I understand the use case correctly all of this mmap-ing is only to > allow unpriv userspace to access a priv map via unpriv mmap() syscall. > But the map can be accessed as unpriv already. > Pin it with the world read creds and do map_lookup sys_bpf cmd on it. Right, but, if I understand correctly, with sysctl_unprivileged_bpf_disabled, unpriv tasks are not able to make use of __sys_bpf(). Is there anything I missed?
