Re: [PATCH v2 bpf-next 1/4] bpf: introduce task_vma bpf_iter

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Jan 5, 2021 at 9:11 AM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Jan 5, 2021, at 8:27 AM, Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> >
> > On Mon, Jan 4, 2021 at 9:47 PM Song Liu <songliubraving@fb.com> wrote:
> >>
> >>
> >>
> >>> On Jan 4, 2021, at 5:46 PM, Alexei Starovoitov <alexei.starovoitov@gmail.com> 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@fb.com> wrote:
> >>>>>
> >>>>>
> >>>>>
> >>>>> On 12/17/20 9:23 PM, Alexei Starovoitov wrote:
> >>>>>> On Thu, Dec 17, 2020 at 8:33 PM Song Liu <songliubraving@fb.com> 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.