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

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Date:   Fri, 26 Jun 2020 13:00:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        kernel-team@...com, john.fastabend@...il.com, kpsingh@...omium.org
Subject: Re: [PATCH v2 bpf-next 1/4] perf: export get/put_chain_entry()

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