Message-ID: <CAKH8qBuPh4aaEz_vv1s2gWYYPRm8e5gMaM-RcuCqg+AeaeZcPg@mail.gmail.com>
Date:   Wed, 15 Jun 2022 08:52:14 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     Quentin Monnet <quentin@...valent.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Harsh Modi <harshmodi@...gle.com>,
        Paul Chaignon <paul@...ium.io>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/2] Revert "bpftool: Use libbpf 1.0 API mode
 instead of RLIMIT_MEMLOCK"

On Wed, Jun 15, 2022 at 6:23 AM Yafang Shao <laoar.shao@...il.com> wrote:
>
> On Tue, Jun 14, 2022 at 10:20 PM Quentin Monnet <quentin@...valent.com> wrote:
> >
> > 2022-06-14 20:37 UTC+0800 ~ Yafang Shao <laoar.shao@...il.com>
> > > On Sat, Jun 11, 2022 at 1:17 AM Stanislav Fomichev <sdf@...gle.com> wrote:
> > >>
> > >> On Fri, Jun 10, 2022 at 10:00 AM Quentin Monnet <quentin@...valent.com> wrote:
> > >>>
> > >>> 2022-06-10 09:46 UTC-0700 ~ Stanislav Fomichev <sdf@...gle.com>
> > >>>> On Fri, Jun 10, 2022 at 9:34 AM Quentin Monnet <quentin@...valent.com> wrote:
> > >>>>>
> > >>>>> 2022-06-10 09:07 UTC-0700 ~ sdf@...gle.com
> > >>>>>> On 06/10, Quentin Monnet wrote:
> > >>>>>>> This reverts commit a777e18f1bcd32528ff5dfd10a6629b655b05eb8.
> > >>>>>>
> > >>>>>>> In commit a777e18f1bcd ("bpftool: Use libbpf 1.0 API mode instead of
> > >>>>>>> RLIMIT_MEMLOCK"), we removed the rlimit bump in bpftool, because the
> > >>>>>>> kernel has switched to memcg-based memory accounting. Thanks to the
> > >>>>>>> LIBBPF_STRICT_AUTO_RLIMIT_MEMLOCK, we attempted to keep compatibility
> > >>>>>>> with other systems and ask libbpf to raise the limit for us if
> > >>>>>>> necessary.
> > >>>>>>
> > >>>>>>> How do we know if memcg-based accounting is supported? There is a probe
> > >>>>>>> in libbpf to check this. But this probe currently relies on the
> > >>>>>>> availability of a given BPF helper, bpf_ktime_get_coarse_ns(), which
> > >>>>>>> landed in the same kernel version as the memory accounting change. This
> > >>>>>>> works in the generic case, but it may fail, for example, if the helper
> > >>>>>>> function has been backported to an older kernel. This has been observed
> > >>>>>>> for Google Cloud's Container-Optimized OS (COS), where the helper is
> > >>>>>>> available but rlimit is still in use. The probe succeeds, the rlimit is
> > >>>>>>> not raised, and probing features with bpftool, for example, fails.
> > >>>>>>
> > >>>>>>> A patch was submitted [0] to update this probe in libbpf, based on what
> > >>>>>>> the cilium/ebpf Go library does [1]. It would lower the soft rlimit to
> > >>>>>>> 0, attempt to load a BPF object, and reset the rlimit. But it may induce
> > >>>>>>> some hard-to-debug flakiness if another process starts, or the current
> > >>>>>>> application is killed, while the rlimit is reduced, and the approach was
> > >>>>>>> discarded.
> > >>>>>>
> > >>>>>>> As a workaround to ensure that the rlimit bump does not depend on the
> > >>>>>>> availability of a given helper, we restore the unconditional rlimit bump
> > >>>>>>> in bpftool for now.
> > >>>>>>
> > >>>>>>> [0]
> > >>>>>>> https://lore.kernel.org/bpf/20220609143614.97837-1-quentin@isovalent.com/
> > >>>>>>> [1] https://github.com/cilium/ebpf/blob/v0.9.0/rlimit/rlimit.go#L39
> > >>>>>>
> > >>>>>>> Cc: Yafang Shao <laoar.shao@...il.com>
> > >>>>>>> Signed-off-by: Quentin Monnet <quentin@...valent.com>
> > >>>>>>> ---
> > >>>>>>>   tools/bpf/bpftool/common.c     | 8 ++++++++
> > >>>>>>>   tools/bpf/bpftool/feature.c    | 2 ++
> > >>>>>>>   tools/bpf/bpftool/main.c       | 6 +++---
> > >>>>>>>   tools/bpf/bpftool/main.h       | 2 ++
> > >>>>>>>   tools/bpf/bpftool/map.c        | 2 ++
> > >>>>>>>   tools/bpf/bpftool/pids.c       | 1 +
> > >>>>>>>   tools/bpf/bpftool/prog.c       | 3 +++
> > >>>>>>>   tools/bpf/bpftool/struct_ops.c | 2 ++
> > >>>>>>>   8 files changed, 23 insertions(+), 3 deletions(-)
> > >>>>>>
> > >>>>>>> diff --git a/tools/bpf/bpftool/common.c b/tools/bpf/bpftool/common.c
> > >>>>>>> index a45b42ee8ab0..a0d4acd7c54a 100644
> > >>>>>>> --- a/tools/bpf/bpftool/common.c
> > >>>>>>> +++ b/tools/bpf/bpftool/common.c
> > >>>>>>> @@ -17,6 +17,7 @@
> > >>>>>>>   #include <linux/magic.h>
> > >>>>>>>   #include <net/if.h>
> > >>>>>>>   #include <sys/mount.h>
> > >>>>>>> +#include <sys/resource.h>
> > >>>>>>>   #include <sys/stat.h>
> > >>>>>>>   #include <sys/vfs.h>
> > >>>>>>
> > >>>>>>> @@ -72,6 +73,13 @@ static bool is_bpffs(char *path)
> > >>>>>>>       return (unsigned long)st_fs.f_type == BPF_FS_MAGIC;
> > >>>>>>>   }
> > >>>>>>
> > >>>>>>> +void set_max_rlimit(void)
> > >>>>>>> +{
> > >>>>>>> +    struct rlimit rinf = { RLIM_INFINITY, RLIM_INFINITY };
> > >>>>>>> +
> > >>>>>>> +    setrlimit(RLIMIT_MEMLOCK, &rinf);
> > >>>>>>
> > >>>>>> Do you think it might make sense to print to stderr some warning if
> > >>>>>> we actually happen to adjust this limit?
> > >>>>>>
> > >>>>>> if (getrlimit(MEMLOCK) != RLIM_INFINITY) {
> > >>>>>>     fprintf(stderr, "Warning: resetting MEMLOCK rlimit to
> > >>>>>>     infinity!\n");
> > >>>>>>     setrlimit(RLIMIT_MEMLOCK, &rinf);
> > >>>>>> }
> > >>>>>>
> > >>>>>> ?
> > >>>>>>
> > >>>>>> Because while it's nice that we automatically do this, this might still
> > >>>>>> lead to surprises for some users. OTOH, not sure whether people
> > >>>>>> actually read those warnings? :-/
> > >>>>>
> > >>>>> I'm not strictly opposed to a warning, but I'm not completely sure this
> > >>>>> is desirable.
> > >>>>>
> > >>>>> Bpftool has raised the rlimit for a long time, it changed only in April,
> > >>>>> so I don't think it would come up as a surprise for people who have used
> > >>>>> it for a while. I think this is also something that several other
> > >>>>> BPF-related applications (BCC I think?, bpftrace, Cilium come to mind)
> > >>>>> have been doing too.
> > >>>>
> > >>>> In this case ignore me and let's continue doing that :-)
> > >>>>
> > >>>> Btw, eventually we'd still like to stop doing that I'd presume?
> > >>>
> > >>> Agreed. I was thinking either finding a way to improve the probe in
> > >>> libbpf, or waiting for some more time until 5.11 gets old, but this may
> > >>> take years :/
> > >>>
> > >>>> Should
> > >>>> we at some point follow up with something like:
> > >>>>
> > >>>> if (kernel_version >= 5.11) { don't touch memlock; }
> > >>>>
> > >>>> ?
> > >>>>
> > >>>> I guess we care only about <5.11 because of the backports, but 5.11+
> > >>>> kernels are guaranteed to have memcg.
> > >>>
> > >>> You mean from uname() and parsing the release? Yes I suppose we could do
> > >>> that, can do as a follow-up.
> > >>
> > >> Yeah, uname-based, I don't think we can do better? Given that probing
> > >> is problematic as well :-(
> > >> But idk, up to you.
> > >>
> > >
> > > Agreed with the uname-based solution. Another possible solution is to
> > > probe the member 'memcg' in struct bpf_map, in case someone may
> > > backport memcg-based  memory accounting, but that will be a little
> > > over-engineering. The uname-based solution is simple and can work.
> > >
> >
> > Thanks! Yes, memcg would be more complex: the struct is not exposed to
> > user space, and BTF is not a hard dependency for bpftool. I'll work on
> > the uname-based test as a follow-up to this set.
> >
>
> After a second thought, the uname-based test may not work, because
> CONFIG_MEMCG_KMEM can be disabled.

Does it matter? Regardless of whether there is memcg or not, we
shouldn't touch ulimit on 5.11+
If there is no memcg, there is no bpf memory enforcement.

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