linux-kernel - Re: [PATCH v2 bpf-next 1/4] perf: export get/put_chain

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Date:   Fri, 26 Jun 2020 21:38:42 +0000
From:   Song Liu <songliubraving@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     Peter Zijlstra <peterz@...radead.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "john fastabend" <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH v2 bpf-next 1/4] perf: export get/put_chain_entry()



> On Jun 26, 2020, at 1:06 PM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> 
> On Fri, Jun 26, 2020 at 5:10 AM Peter Zijlstra <peterz@...radead.org> wrote:
>> 
>> On Thu, Jun 25, 2020 at 05:13:29PM -0700, Song Liu wrote:
>>> This would be used by bpf stack mapo.
>> 
>> Would it make sense to sanitize the API a little before exposing it?
>> 
>> diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
>> index 334d48b16c36..016894b0d2c2 100644
>> --- a/kernel/events/callchain.c
>> +++ b/kernel/events/callchain.c
>> @@ -159,8 +159,10 @@ static struct perf_callchain_entry *get_callchain_entry(int *rctx)
>>                return NULL;
>> 
>>        entries = rcu_dereference(callchain_cpus_entries);
>> -       if (!entries)
>> +       if (!entries) {
>> +               put_recursion_context(this_cpu_ptr(callchain_recursion), rctx);
>>                return NULL;
>> +       }
>> 
>>        cpu = smp_processor_id();
>> 
>> @@ -183,12 +185,9 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
>>        int rctx;
>> 
>>        entry = get_callchain_entry(&rctx);
>> -       if (rctx == -1)
>> +       if (!entry || rctx == -1)
>>                return NULL;
>> 
> 
> isn't rctx == -1 check here not necessary anymore? Seems like
> get_callchain_entry() will always return NULL if rctx == -1?

Yes, looks like we only need to check entry. 

Thanks,
Song