> On Jan 5, 2021, at 8:27 AM, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > On Mon, Jan 4, 2021 at 9:47 PM Song Liu <songliubraving@xxxxxx> wrote: >> >> >> >>> On Jan 4, 2021, at 5:46 PM, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: >>> >>> On Fri, Dec 18, 2020 at 05:23:25PM +0000, Song Liu wrote: >>>> >>>> >>>>> On Dec 18, 2020, at 8:38 AM, Yonghong Song <yhs@xxxxxx> wrote: >>>>> >>>>> >>>>> >>>>> On 12/17/20 9:23 PM, Alexei Starovoitov wrote: >>>>>> On Thu, Dec 17, 2020 at 8:33 PM Song Liu <songliubraving@xxxxxx> wrote: >>>>>>>> >>>>>>>> ahh. I missed that. Makes sense. >>>>>>>> vm_file needs to be accurate, but vm_area_struct should be accessed as ptr_to_btf_id. >>>>>>> >>>>>>> Passing pointer of vm_area_struct into BPF will be tricky. For example, shall we >>>>>>> allow the user to access vma->vm_file? IIUC, with ptr_to_btf_id the verifier will >>>>>>> allow access of vma->vm_file as a valid pointer to struct file. However, since the >>>>>>> vma might be freed, vma->vm_file could point to random data. >>>>>> I don't think so. The proposed patch will do get_file() on it. >>>>>> There is actually no need to assign it into a different variable. >>>>>> Accessing it via vma->vm_file is safe and cleaner. >>>>> >>>>> I did not check the code but do you have scenarios where vma is freed but old vma->vm_file is not freed due to reference counting, but >>>>> freed vma area is reused so vma->vm_file could be garbage? >>>> >>>> AFAIK, once we unlock mmap_sem, the vma could be freed and reused. I guess ptr_to_btf_id >>>> or probe_read would not help with this? >>> >>> Theoretically we can hack the verifier to treat some ptr_to_btf_id as "less >>> valid" than the other ptr_to_btf_id, but the user experience will not be great. >>> Reading such bpf prog will not be obvious. I think it's better to run bpf prog >>> in mmap_lock then and let it access vma->vm_file. After prog finishes the iter >>> bit can do if (mmap_lock_is_contended()) before iterating. That will deliver >>> better performance too. Instead of task_vma_seq_get_next() doing >>> mmap_lock/unlock at every vma. No need for get_file() either. And no >>> __vm_area_struct exposure. >> >> I think there might be concern calling BPF program with mmap_lock, especially that >> the program is sleepable (for bpf_d_path). It shouldn't be a problem for common >> cases, but I am not 100% sure for corner cases (many instructions in BPF + sleep). >> Current version is designed to be very safe for the workload, which might be too >> conservative. > > I know and I agree with all that, but how do you propose to fix the > vm_file concern > without holding the lock while prog is running? I couldn't come up with a way. I guess the gap here is that I don't see why __vm_area_struct exposure is an issue. It is similar to __sk_buff, and simpler (though we had more reasons to introduce __sk_buff back when there wasn't BTF). If we can accept __vm_area_struct, current version should work, as it doesn't have problem with vm_file. Thanks, Song
