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

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Date:   Fri, 26 Jun 2020 13:06:01 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Song Liu <songliubraving@...com>, 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 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?

> -       if (!entry)
> -               goto exit_put;
> -
>         ctx.entry     = entry;
>         ctx.max_stack = max_stack;
>         ctx.nr        = entry->nr = init_nr;