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

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Date:   Fri, 26 Jun 2020 21:29:38 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        linux-kernel <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>,
        "kpsingh@...omium.org" <kpsingh@...omium.org>
Subject: Re: [PATCH v2 bpf-next 1/4] perf: export get/put_chain_entry()



> On Jun 26, 2020, at 4:00 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?

I will fold this in the next version. 

Thanks,
Song

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