Re: [PATCH] bpf: don't fail kmalloc while releasing raw_tp

From: Steven Rostedt <rostedt@goodmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: paulmck <paulmck@kernel.org>, Matt Mullins <mmullins@mmlx.us>,
	Ingo Molnar <mingo@redhat.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Dmitry Vyukov <dvyukov@google.com>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andriin@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	netdev <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>
Subject: Re: [PATCH] bpf: don't fail kmalloc while releasing raw_tp
Date: Mon, 16 Nov 2020 16:02:18 -0500	[thread overview]
Message-ID: <20201116160218.3b705345@gandalf.local.home> (raw)
In-Reply-To: <20201116154437.254a8b97@gandalf.local.home>

On Mon, 16 Nov 2020 15:44:37 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> If you use a stub function, it shouldn't affect anything. And the worse
> that would happen is that you have a slight overhead of calling the stub
> until you can properly remove the callback.

Something like this:

(haven't compiled it yet, I'm about to though).

-- Steve

diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 3f659f855074..8eab40f9d388 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -53,10 +53,16 @@ struct tp_probes {
 	struct tracepoint_func probes[];
 };
 
-static inline void *allocate_probes(int count)
+/* Called in removal of a func but failed to allocate a new tp_funcs */
+static void tp_stub_func(void)
+{
+	return;
+}
+
+static inline void *allocate_probes(int count, gfp_t extra_flags)
 {
 	struct tp_probes *p  = kmalloc(struct_size(p, probes, count),
-				       GFP_KERNEL);
+				       GFP_KERNEL | extra_flags);
 	return p == NULL ? NULL : p->probes;
 }
 
@@ -150,7 +156,7 @@ func_add(struct tracepoint_func **funcs, struct tracepoint_func *tp_func,
 		}
 	}
 	/* + 2 : one for new probe, one for NULL func */
-	new = allocate_probes(nr_probes + 2);
+	new = allocate_probes(nr_probes + 2, 0);
 	if (new == NULL)
 		return ERR_PTR(-ENOMEM);
 	if (old) {
@@ -188,8 +194,9 @@ static void *func_remove(struct tracepoint_func **funcs,
 	/* (N -> M), (N > 1, M >= 0) probes */
 	if (tp_func->func) {
 		for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
-			if (old[nr_probes].func == tp_func->func &&
-			     old[nr_probes].data == tp_func->data)
+			if ((old[nr_probes].func == tp_func->func &&
+			     old[nr_probes].data == tp_func->data) ||
+			    old[nr_probes].func == tp_stub_func)
 				nr_del++;
 		}
 	}
@@ -207,15 +214,20 @@ static void *func_remove(struct tracepoint_func **funcs,
 		int j = 0;
 		/* N -> M, (N > 1, M > 0) */
 		/* + 1 for NULL */
-		new = allocate_probes(nr_probes - nr_del + 1);
-		if (new == NULL)
-			return ERR_PTR(-ENOMEM);
-		for (i = 0; old[i].func; i++)
-			if (old[i].func != tp_func->func
-					|| old[i].data != tp_func->data)
-				new[j++] = old[i];
-		new[nr_probes - nr_del].func = NULL;
-		*funcs = new;
+		new = allocate_probes(nr_probes - nr_del + 1, __GFP_NOFAIL);
+		if (new) {
+			for (i = 0; old[i].func; i++)
+				if (old[i].func != tp_func->func
+				    || old[i].data != tp_func->data)
+					new[j++] = old[i];
+			new[nr_probes - nr_del].func = NULL;
+		} else {
+			for (i = 0; old[i].func; i++)
+				if (old[i].func == tp_func->func &&
+				    old[i].data == tp_func->data)
+					old[i].func = tp_stub_func;
+		}
+		*funcs = old;
 	}
 	debug_print_probes(*funcs);
 	return old;
@@ -300,6 +312,10 @@ static int tracepoint_remove_func(struct tracepoint *tp,
 		return PTR_ERR(old);
 	}
 
+	if (tp_funcs == old)
+		/* Failed allocating new tp_funcs, replaced func with stub */
+		return 0;
+
 	if (!tp_funcs) {
 		/* Removed last function */
 		if (tp->unregfunc && static_key_enabled(&tp->key))
