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

From: Steven Rostedt
Date: Mon Nov 16 2020 - 16:02:23 EST


On Mon, 16 Nov 2020 15:44:37 -0500
Steven Rostedt <rostedt@xxxxxxxxxxx> 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))