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

From: Song Liu <songliubraving@fb.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Yonghong Song <yhs@fb.com>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"ast@kernel.org" <ast@kernel.org>,
	"daniel@iogearbox.net" <daniel@iogearbox.net>,
	"andrii@kernel.org" <andrii@kernel.org>,
	"john.fastabend@gmail.com" <john.fastabend@gmail.com>,
	"kpsingh@chromium.org" <kpsingh@chromium.org>,
	Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH v2 bpf-next 1/4] bpf: introduce task_vma bpf_iter
Date: Tue, 5 Jan 2021 05:47:34 +0000	[thread overview]
Message-ID: <6E122A14-0F77-46F9-8891-EDF4DB494E37@fb.com> (raw)
In-Reply-To: <20210105014625.krtz3uzqtfu4y7m5@ast-mbp>

> 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. 

Thanks,
Song