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

[Song Liu <songliubraving@fb.com>]

> On Jan 5, 2021, at 9:27 AM, Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> 
> 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.

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