Date:   Wed, 14 Jul 2021 16:59:58 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v6 bpf-next 00/11] bpf: Introduce BPF timers.

On Tue, Jul 13, 2021 at 6:05 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> From: Alexei Starovoitov <ast@...nel.org>
>
> The first request to support timers in bpf was made in 2013 before sys_bpf syscall
> was added. That use case was periodic sampling. It was address with attaching
> bpf programs to perf_events. Then during XDP development the timers were requested
> to do garbage collection and health checks. They were worked around by implementing
> timers in user space and triggering progs with BPF_PROG_RUN command.
> The user space timers and perf_event+bpf timers are not armed by the bpf program.
> They're done asynchronously vs program execution. The XDP program cannot send a
> packet and arm the timer at the same time. The tracing prog cannot record an
> event and arm the timer right away. This large class of use cases remained
> unaddressed. The jiffy based and hrtimer based timers are essential part of the
> kernel development and with this patch set the hrtimer based timers will be
> available to bpf programs.
>
> TLDR: bpf timers is a wrapper of hrtimers with all the extra safety added
> to make sure bpf progs cannot crash the kernel.
>
> v5->v6:
> - address code review feedback from Martin and add his Acks.
> - add usercnt > 0 check to bpf_timer_init and remove timers_cancel_and_free
> second loop in map_free callbacks.
> - add cond_resched_rcu.
>
> v4->v5:
> - Martin noticed the following issues:
> . prog could be reallocated bpf_patch_insn_data().
> Fixed by passing 'aux' into bpf_timer_set_callback, since 'aux' is stable
> during insn patching.
> . Added missing rcu_read_lock.
> . Removed redundant record_map.
> - Discovered few bugs with stress testing:
> . One cpu does htab_free_prealloced_timers->bpf_timer_cancel_and_free->hrtimer_cancel
> while another is trying to do something with the timer like bpf_timer_start/set_callback.
> Those ops try to acquire bpf_spin_lock that is already taken by bpf_timer_cancel_and_free,
> so both cpus spin forever. The same problem existed in bpf_timer_cancel().
> One bpf prog on one cpu might call bpf_timer_cancel and wait, while another cpu is in
> the timer callback that tries to do bpf_timer_*() helper on the same timer.
> The fix is to do drop_prog_refcnt() and unlock. And only then hrtimer_cancel.
> Because of this had to add callback_fn != NULL check to bpf_timer_cb().
> Also removed redundant bpf_prog_inc/put from bpf_timer_cb() and replaced
> with rcu_dereference_check similar to recent rcu_read_lock-removal from drivers.
> bpf_timer_cb is in softirq.
> . Managed to hit refcnt==0 while doing bpf_prog_put from bpf_timer_cancel_and_free().
> That exposed the issue that bpf_prog_put wasn't ready to be called from irq context.
> Fixed similar to bpf_map_put which is irq ready.
> - Refactored BPF_CALL_1(bpf_spin_lock) into __bpf_spin_lock_irqsave() to
> make the main logic more clear, since Martin and Yonghong brought up this concern.
>
> v3->v4:
> 1.
> Split callback_fn from bpf_timer_start into bpf_timer_set_callback as
> suggested by Martin. That makes bpf timer api match one to one to
> kernel hrtimer api and provides greater flexibility.
> 2.
> Martin also discovered the following issue with uref approach:
> bpftool prog load xdp_timer.o /sys/fs/bpf/xdp_timer type xdp
> bpftool net attach xdpgeneric pinned /sys/fs/bpf/xdp_timer dev lo
> rm /sys/fs/bpf/xdp_timer
> nc -6 ::1 8888
> bpftool net detach xdpgeneric dev lo
> The timer callback stays active in the kernel though the prog was detached
> and map usercnt == 0.
> It happened because 'bpftool prog load' pinned the prog only.
> The map usercnt went to zero. Subsequent attach and runs didn't
> affect map usercnt. The timer was able to start and bpf_prog_inc itself.
> When the prog was detached the prog stayed active.
> To address this issue added
> if (!atomic64_read(&(t->map->usercnt))) return -EPERM;
> to the first patch.
> Which means that timers are allowed only in the maps that are held
> by user space with open file descriptor or maps pinned in bpffs.
> 3.
> Discovered that timers in inner maps were broken.
> The inner map pointers are dynamic. Therefore changed bpf_timer_init()
> to accept explicit map pointer supplied by the program instead
> of hidden map pointer supplied by the verifier.
> To make sure that pointer to a timer actually belongs to that map
> added the verifier check in patch 3.
> 4.
> Addressed Yonghong's feedback. Improved comments and added
> dynamic in_nmi() check.
> Added Acks.
>
> Acked-by: Toke Høiland-Jørgensen <toke@...hat.com> # for the feature
>
> v2->v3:
> The v2 approach attempted to bump bpf_prog refcnt when bpf_timer_start is
> called to make sure callback code doesn't disappear when timer is active and
> drop refcnt when timer cb is done. That led to a ton of race conditions between
> callback running and concurrent bpf_timer_init/start/cancel on another cpu,
> and concurrent bpf_map_update/delete_elem, and map destroy.
>
> Then v2.5 approach skipped prog refcnt altogether. Instead it remembered all
> timers that bpf prog armed in a link list and canceled them when prog refcnt
> went to zero. The race conditions disappeared, but timers in map-in-map could
> not be supported cleanly, since timers in inner maps have inner map's life time
> and don't match prog's life time.
>
> This v3 approach makes timers to be owned by maps. It allows timers in inner
> maps to be supported from the start. This apporach relies on "user refcnt"
> scheme used in prog_array that stores bpf programs for bpf_tail_call. The
> bpf_timer_start() increments prog refcnt, but unlike 1st approach the timer
> callback does decrement the refcnt. The ops->map_release_uref is
> responsible for cancelling the timers and dropping prog refcnt when user space
> reference to a map is dropped. That addressed all the races and simplified
> locking.
>
> Andrii presented a use case where specifying callback_fn in bpf_timer_init()
> is inconvenient vs specifying in bpf_timer_start(). The bpf_timer_init()
> typically is called outside for timer callback, while bpf_timer_start() most
> likely will be called from the callback.
> timer_cb() { ... bpf_timer_start(timer_cb); ...} looks like recursion and as
> infinite loop to the verifier. The verifier had to be made smarter to recognize
> such async callbacks. Patches 7,8,9 addressed that.
>
> Patch 1 and 2 refactoring.
> Patch 3 implements bpf timer helpers and locking.
> Patch 4 implements map side of bpf timer support.
> Patch 5 prevent pointer mismatch in bpf_timer_init.
> Patch 6 adds support for BTF in inner maps.
> Patch 7 teaches check_cfg() pass to understand async callbacks.
> Patch 8 teaches do_check() pass to understand async callbacks.
> Patch 9 teaches check_max_stack_depth() pass to understand async callbacks.
> Patches 10 and 11 are the tests.
>
> v1->v2:
> - Addressed great feedback from Andrii and Toke.
> - Fixed race between parallel bpf_timer_*() ops.
> - Fixed deadlock between timer callback and LRU eviction or bpf_map_delete/update.
> - Disallowed mmap and global timers.
> - Allow spin_lock and bpf_timer in an element.
> - Fixed memory leaks due to map destruction and LRU eviction.
> - A ton more tests.
>
> Alexei Starovoitov (11):
>   bpf: Prepare bpf_prog_put() to be called from irq context.
>   bpf: Factor out bpf_spin_lock into helpers.
>   bpf: Introduce bpf timers.
>   bpf: Add map side support for bpf timers.
>   bpf: Prevent pointer mismatch in bpf_timer_init.
>   bpf: Remember BTF of inner maps.
>   bpf: Relax verifier recursion check.
>   bpf: Implement verifier support for validation of async callbacks.
>   bpf: Teach stack depth check about async callbacks.
>   selftests/bpf: Add bpf_timer test.
>   selftests/bpf: Add a test with bpf_timer in inner map.
>
>  include/linux/bpf.h                           |  47 ++-
>  include/linux/bpf_verifier.h                  |  19 +-
>  include/linux/btf.h                           |   1 +
>  include/uapi/linux/bpf.h                      |  73 ++++
>  kernel/bpf/arraymap.c                         |  21 ++
>  kernel/bpf/btf.c                              |  77 +++-
>  kernel/bpf/hashtab.c                          | 104 +++++-
>  kernel/bpf/helpers.c                          | 341 +++++++++++++++++-
>  kernel/bpf/local_storage.c                    |   4 +-
>  kernel/bpf/map_in_map.c                       |   8 +
>  kernel/bpf/syscall.c                          |  53 ++-
>  kernel/bpf/verifier.c                         | 307 +++++++++++++++-
>  kernel/trace/bpf_trace.c                      |   2 +-
>  scripts/bpf_doc.py                            |   2 +
>  tools/include/uapi/linux/bpf.h                |  73 ++++
>  .../testing/selftests/bpf/prog_tests/timer.c  |  55 +++
>  .../selftests/bpf/prog_tests/timer_mim.c      |  69 ++++
>  tools/testing/selftests/bpf/progs/timer.c     | 297 +++++++++++++++
>  tools/testing/selftests/bpf/progs/timer_mim.c |  88 +++++
>  .../selftests/bpf/progs/timer_mim_reject.c    |  74 ++++
>  20 files changed, 1651 insertions(+), 64 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/timer.c
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/timer_mim.c
>  create mode 100644 tools/testing/selftests/bpf/progs/timer.c
>  create mode 100644 tools/testing/selftests/bpf/progs/timer_mim.c
>  create mode 100644 tools/testing/selftests/bpf/progs/timer_mim_reject.c
>
> --
> 2.30.2
>

It all looks good to me overall:

Acked-by: Andrii Nakryiko <andrii@...nel.org>

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