Re: [BUG] hitting bug when running spinlock test

On Tue, Mar 09, 2021 at 10:36:06AM -0800, Yonghong Song wrote:

On 3/9/21 10:21 AM, Roman Gushchin wrote:

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On Mon, Mar 08, 2021 at 09:44:08PM -0800, Yonghong Song wrote:

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On 3/5/21 1:10 PM, Yonghong Song wrote:

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On 3/5/21 12:38 PM, Roman Gushchin wrote:

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On Thu, Mar 04, 2021 at 08:03:33PM +0100, Jiri Olsa wrote:

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hi,
I'm getting attached BUG/crash when running in parralel selftests, like:

    while :; do ./test_progs -t spinlock; done
    while :; do ./test_progs ; done

it's the latest bpf-next/master, I can send the .config if needed,
but I don't think there's anything special about it, because I saw
the bug on other servers with different generic configs

it looks like it's related to cgroup local storage, for some reason
the storage deref returns NULL

I'm bit lost in this code, so any help would be great ;-)

Hi!

I think the patch to blame is df1a2cb7c74b ("bpf/test_run: fix
unkillable BPF_PROG_TEST_RUN").

Thanks, Roman, I did some experiments and found the reason of NULL
storage deref is because a tracing program (mostly like a kprobe) is run
after bpf_cgroup_storage_set() is called but before bpf program calls
bpf_get_local_storage(). Note that trace_call_bpf() macro
BPF_PROG_RUN_ARRAY_CHECK does call bpf_cgroup_storage_set().

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Prior to it, we were running the test program in the
preempt_disable() && rcu_read_lock()
section:

preempt_disable();
rcu_read_lock();
bpf_cgroup_storage_set(storage);
ret = BPF_PROG_RUN(prog, ctx);
rcu_read_unlock();
preempt_enable();

So, a percpu variable with a cgroup local storage pointer couldn't
go away.

I think even with using preempt_disable(), if the bpf program calls map
lookup and there is a kprobe bpf on function htab_map_lookup_elem(), we
will have the issue as BPF_PROG_RUN_ARRAY_CHECK will call
bpf_cgroup_storage_set() too. I need to write a test case to confirm
this though.

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After df1a2cb7c74b we can temporarily enable the preemption, so
nothing prevents
another program to call into bpf_cgroup_storage_set() on the same cpu.
I guess it's exactly what happens here.

It is. I confirmed.

Actually, the failure is not due to breaking up preempt_disable(). Even with
adding cond_resched(), bpf_cgroup_storage_set() still happens
inside the preempt region. So it is okay. What I confirmed is that
changing migration_{disable/enable} to preempt_{disable/enable} fixed
the issue.

Hm, how so? If preemption is enabled, another task/bpf program can start
executing on the same cpu and set their cgroup storage. I guess it's harder
to reproduce or it will result in the (bpf map) memory corruption instead
of a panic, but I don't think it's safe.

The code has been refactored recently. The following is the code right
before refactoring to make it easy to understand:

        rcu_read_lock();
        migrate_disable();
        time_start = ktime_get_ns();
        for (i = 0; i < repeat; i++) {
                bpf_cgroup_storage_set(storage);

                if (xdp)
                        *retval = bpf_prog_run_xdp(prog, ctx);
                else
                        *retval = BPF_PROG_RUN(prog, ctx);

                if (signal_pending(current)) {
                        ret = -EINTR;
                        break;
                }

                if (need_resched()) {
                        time_spent += ktime_get_ns() - time_start;
                        migrate_enable();
                        rcu_read_unlock();

                        cond_resched();

                        rcu_read_lock();
                        migrate_disable();
                        time_start = ktime_get_ns();
                }
        }
        time_spent += ktime_get_ns() - time_start;
        migrate_enable();
        rcu_read_unlock();

bpf_cgroup_storage_set() is called inside migration_disable/enable().
Previously it is called inside preempt_disable/enable(), so it should be
fine.

Ah, gotcha, thank you for the explanation!

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So migration_{disable/enable} is the issue since any other process (and its
bpf program) and preempted the current process/bpf program and run.

Oh, I didn't know about the preempt_{disable/enable}/migration_{disable/enable}
change. It's definitely not safe from a cgroup local storage perspective.

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Currently for each program, we will set the local storage before the
program run and each program may access to multiple local storage
maps. So we might need something similar sk_local_storage.
Considering possibility of deep nested migration_{disable/enable},
the cgroup local storage has to be preserved in prog/map data
structure and not as a percpu variable as it will be very hard
to save and restore percpu virable content as migration can
happen anywhere. I don't have concrete design yet. Just throw some
idea here.

Initially I thought about saving this pointer on stack, but then we need
some sort of gcc/assembly magic to get this pointer from the stack outside
of the current scope. At that time we didn't have sleepable programs,
so the percpu approach looked simpler and more reliable. Maybe it's time
to review it.

Indeed this is the time.

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BTW, I send a patch to prevent tracing programs to mess up
with cgroup local storage:
    https://lore.kernel.org/bpf/20210309052638.400562-1-yhs@fb.com/T/#u (local)
we now all programs access cgroup local storage should be in
process context and we don't need to worry about kprobe-induced
percpu local storage access.

Thank you! My only issue is that the commit log looks like an optimization
(like we're calling for set_cgroup_storage() for no good reason), where if
I understand it correctly, it prevents some class of problems.

Yes, it prevents real problems as well. The reason I did not say it is
because the patch does not really fix fundamental issue. But it does
prevent some issues. Let me reword the commit message.

Thank you!