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

[Steven Rostedt]

On Mon, 16 Nov 2020 15:44:37 -0500
Steven Rostedt <rost...@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))