> On Jan 5, 2021, at 9:27 AM, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > On Tue, Jan 5, 2021 at 9:11 AM Song Liu <songliubraving@xxxxxx> wrote: >> >> >> >>> 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 > > True. The problem with __vm_area_struct is that it is a hard coded > uapi with little to none > extensibility. In this form vma iterator is not really useful beyond > the example in selftest. With __vm_area_struct, we can still probe_read the page table, so we can cover more use cases than the selftest. But I agree that it is not as extensible as feeding real vm_area_struct with BTF to the BPF program. Let me try calling BPF program with mmap_lock. Thanks, Song
