Re: Re: [PATCH bpf v2] bpf: Fix deadlock when freeing cgroup storage

[Alexei Starovoitov]

On Sun, Jan 26, 2025 at 1:31 AM Abel Wu <wuyun.abel@xxxxxxxxxxxxx> wrote:
>
> On 1/25/25 4:20 AM, Martin KaFai Lau Wrote:
> > On 12/20/24 10:10 PM, Abel Wu wrote:
> >> The following commit
> >> bc235cdb423a ("bpf: Prevent deadlock from recursive bpf_task_storage_[get|delete]")
> >> first introduced deadlock prevention for fentry/fexit programs attaching
> >> on bpf_task_storage helpers. That commit also employed the logic in map
> >> free path in its v6 version.
> >>
> >> Later bpf_cgrp_storage was first introduced in
> >> c4bcfb38a95e ("bpf: Implement cgroup storage available to non-cgroup-attached bpf progs")
> >> which faces the same issue as bpf_task_storage, instead of its busy
> >> counter, NULL was passed to bpf_local_storage_map_free() which opened
> >> a window to cause deadlock:
> >>
> >>     <TASK>
> >>         (acquiring local_storage->lock)
> >>     _raw_spin_lock_irqsave+0x3d/0x50
> >>     bpf_local_storage_update+0xd1/0x460
> >>     bpf_cgrp_storage_get+0x109/0x130
> >>     bpf_prog_a4d4a370ba857314_cgrp_ptr+0x139/0x170
> >>     ? __bpf_prog_enter_recur+0x16/0x80
> >>     bpf_trampoline_6442485186+0x43/0xa4
> >>     cgroup_storage_ptr+0x9/0x20
> >>         (holding local_storage->lock)
> >>     bpf_selem_unlink_storage_nolock.constprop.0+0x135/0x160
> >>     bpf_selem_unlink_storage+0x6f/0x110
> >>     bpf_local_storage_map_free+0xa2/0x110
> >>     bpf_map_free_deferred+0x5b/0x90
> >>     process_one_work+0x17c/0x390
> >>     worker_thread+0x251/0x360
> >>     kthread+0xd2/0x100
> >>     ret_from_fork+0x34/0x50
> >>     ret_from_fork_asm+0x1a/0x30
> >>     </TASK>
> >>
> >> Progs:
> >>   - A: SEC("fentry/cgroup_storage_ptr")
> >
> > The v1 thread has suggested using notrace in a few functions. I didn't see any counterarguments that wouldn't be sufficient.
> >
> > imo, that should be a better option instead of having more unnecessary failures in all other normal use cases which will not be interested in tracing cgroup_storage_ptr().

Martin,

task_storage_map_free() is doing this busy inc/dec already,
in that sense doing the same in cgroup_storage_map_free() fits.

fentry/cgroup_storage_ptr use case is certainly convoluted.

I don't buy that adding notrace or doing CFALGS_REMOVE_
will hurt observability,
but this patch seems ok.

wdyt?