linux-kernel - Re: [PATCH RFC bpf-next 0/2] Mmapable task local storage.

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Message-ID: <20220329232956.gbsr65jdbe4lw2m6@ast-mbp>
Date:   Tue, 29 Mar 2022 16:29:56 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Hao Luo <haoluo@...gle.com>
Cc:     Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Yonghong Song <yhs@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        KP Singh <kpsingh@...nel.org>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC bpf-next 0/2] Mmapable task local storage.

On Tue, Mar 29, 2022 at 10:43:42AM -0700, Hao Luo wrote:
> On Tue, Mar 29, 2022 at 2:37 AM Kumar Kartikeya Dwivedi
> <memxor@...il.com> wrote:
> >
> > On Mon, Mar 28, 2022 at 11:16:15PM IST, Hao Luo wrote:
> > > On Mon, Mar 28, 2022 at 10:39 AM Hao Luo <haoluo@...gle.com> wrote:
> > > >
> > > > Hi Yonghong,
> > > >
> > > > On Fri, Mar 25, 2022 at 12:16 PM Yonghong Song <yhs@...com> wrote:
> > > > >
> > > > > On 3/24/22 4:41 PM, Hao Luo wrote:
> > > > > > Some map types support mmap operation, which allows userspace to
> > > > > > communicate with BPF programs directly. Currently only arraymap
> > > > > > and ringbuf have mmap implemented.
> > > > > >
> > > > > > However, in some use cases, when multiple program instances can
> > > > > > run concurrently, global mmapable memory can cause race. In that
> > > > > > case, userspace needs to provide necessary synchronizations to
> > > > > > coordinate the usage of mapped global data. This can be a source
> > > > > > of bottleneck.
> > > > >
> > > > > I can see your use case here. Each calling process can get the
> > > > > corresponding bpf program task local storage data through
> > > > > mmap interface. As you mentioned, there is a tradeoff
> > > > > between more memory vs. non-global synchronization.
> > > > >
> > > > > I am thinking that another bpf_iter approach can retrieve
> > > > > the similar result. We could implement a bpf_iter
> > > > > for task local storage map, optionally it can provide
> > > > > a tid to retrieve the data for that particular tid.
> > > > > This way, user space needs an explicit syscall, but
> > > > > does not need to allocate more memory than necessary.
> > > > >
> > > > > WDYT?
> > > > >
> > > >
> > > > Thanks for the suggestion. I have two thoughts about bpf_iter + tid and mmap:
> > > >
> > > > - mmap prevents the calling task from reading other task's value.
> > > > Using bpf_iter, one can pass other task's tid to get their values. I
> > > > assume there are two potential ways of passing tid to bpf_iter: one is
> > > > to use global data in bpf prog, the other is adding tid parameterized
> > > > iter_link. For the first, it's not easy for unpriv tasks to use. For
> > > > the second, we need to create one iter_link object for each interested
> > > > tid. It may not be easy to use either.
> > > >
> > > > - Regarding adding an explicit syscall. I thought about adding
> > > > write/read syscalls for task local storage maps, just like reading
> > > > values from iter_link. Writing or reading task local storage map
> > > > updates/reads the current task's value. I think this could achieve the
> > > > same effect as mmap.
> > > >
> > >
> > > Actually, my use case of using mmap on task local storage is to allow
> > > userspace to pass FDs into bpf prog. Some of the helpers I want to add
> > > need to take an FD as parameter and the bpf progs can run
> > > concurrently, thus using global data is racy. Mmapable task local
> > > storage is the best solution I can find for this purpose.
> > >
> > > Song also mentioned to me offline, that mmapable task local storage
> > > may be useful for his use case.
> > >
> > > I am actually open to other proposals.
> > >
> >
> > You could also use a syscall prog, and use bpf_prog_test_run to update local
> > storage for current. Data can be passed for that specific prog invocation using
> > ctx. You might have to enable bpf_task_storage helpers in it though, since they
> > are not allowed to be called right now.
> >
> 
> The loading process needs CAP_BPF to load bpf_prog_test_run. I'm
> thinking of allowing any thread including unpriv ones to be able to
> pass data to the prog and update their own storage.

If I understand the use case correctly all of this mmap-ing is only to
allow unpriv userspace to access a priv map via unpriv mmap() syscall.
But the map can be accessed as unpriv already.
Pin it with the world read creds and do map_lookup sys_bpf cmd on it.