From: Song Liu <songliubraving@fb.com>
To: Yonghong Song <yhs@fb.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.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: Fri, 18 Dec 2020 17:23:25 +0000 [thread overview]
Message-ID: <09DA43B9-0F6F-45C1-A60D-12E61493C71F@fb.com> (raw)
In-Reply-To: <231d0521-62a7-427b-5351-359092e73dde@fb.com>
> 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?
Thanks,
Song
>
>>>>> [1] ff9f47f6f00c ("mm: proc: smaps_rollup: do not stall write attempts on mmap_lock")
>>>>
>>>> Thanks for this link. With "if (mmap_lock_is_contended())" check it should work indeed.
>>>
>>> To make sure we are on the same page: I am using slightly different mechanism in
>>> task_vma_iter, which doesn't require checking mmap_lock_is_contended(). In the
>>> smaps_rollup case, the code only unlock mmap_sem when the lock is contended. In
>>> task_iter, we always unlock mmap_sem between two iterations. This is because we
>>> don't want to hold mmap_sem while calling the BPF program, which may sleep (calling
>>> bpf_d_path).
>> That part is clear. I had to look into mmap_read_lock_killable() implementation
>> to realize that it's checking for lock_is_contended after acquiring
>> and releasing
>> if there is a contention. So it's the same behavior at the end.
next prev parent reply other threads:[~2020-12-18 17:24 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-15 23:36 [PATCH v2 bpf-next 0/4] introduce bpf_iter for task_vma Song Liu
2020-12-15 23:36 ` [PATCH v2 bpf-next 1/4] bpf: introduce task_vma bpf_iter Song Liu
2020-12-16 17:36 ` Yonghong Song
2020-12-16 19:41 ` Song Liu
2020-12-17 0:34 ` Andrii Nakryiko
2020-12-17 1:51 ` Song Liu
2020-12-17 19:03 ` Alexei Starovoitov
2020-12-17 22:08 ` Song Liu
2020-12-18 2:34 ` Alexei Starovoitov
2020-12-18 3:15 ` Yonghong Song
2020-12-18 4:33 ` Song Liu
2020-12-18 5:23 ` Alexei Starovoitov
2020-12-18 16:38 ` Yonghong Song
2020-12-18 17:23 ` Song Liu [this message]
2021-01-05 1:46 ` Alexei Starovoitov
2021-01-05 5:47 ` Song Liu
2021-01-05 16:27 ` Alexei Starovoitov
2021-01-05 17:10 ` Song Liu
2021-01-05 17:27 ` Alexei Starovoitov
2021-01-05 19:38 ` Song Liu
2021-01-05 19:46 ` Alexei Starovoitov
2021-01-05 19:51 ` Song Liu
2020-12-15 23:37 ` [PATCH v2 bpf-next 2/4] bpf: allow bpf_d_path in sleepable bpf_iter program Song Liu
2020-12-16 17:41 ` Yonghong Song
2020-12-16 18:15 ` KP Singh
2020-12-16 18:31 ` KP Singh
2021-01-25 12:52 ` Jiri Olsa
2020-12-15 23:37 ` [PATCH v2 bpf-next 3/4] libbpf: introduce section "iter.s/" for " Song Liu
2020-12-16 17:42 ` Yonghong Song
2020-12-16 18:00 ` KP Singh
2020-12-15 23:37 ` [PATCH v2 bpf-next 4/4] selftests/bpf: add test for bpf_iter_task_vma Song Liu
2020-12-16 18:18 ` Yonghong Song
2020-12-16 23:23 ` Song Liu
2020-12-16 17:00 ` [PATCH v2 bpf-next 0/4] introduce bpf_iter for task_vma Yonghong Song
2020-12-16 17:35 ` Song Liu
2020-12-16 18:31 ` Yonghong Song
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