From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Song Liu <songliubraving@fb.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: Mon, 4 Jan 2021 17:46:25 -0800 [thread overview]
Message-ID: <20210105014625.krtz3uzqtfu4y7m5@ast-mbp> (raw)
In-Reply-To: <09DA43B9-0F6F-45C1-A60D-12E61493C71F@fb.com>
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.
next prev parent reply other threads:[~2021-01-05 1:47 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
2021-01-05 1:46 ` Alexei Starovoitov [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20210105014625.krtz3uzqtfu4y7m5@ast-mbp \
--to=alexei.starovoitov@gmail.com \
--cc=Kernel-team@fb.com \
--cc=andrii@kernel.org \
--cc=ast@kernel.org \
--cc=bpf@vger.kernel.org \
--cc=daniel@iogearbox.net \
--cc=john.fastabend@gmail.com \
--cc=kpsingh@chromium.org \
--cc=netdev@vger.kernel.org \
--cc=songliubraving@fb.com \
--cc=yhs@fb.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).