Subject: Re: [PATCH v6 bpf-next 08/11] bpf: introduce BPF_RAW_TRACEPOINT

From: Mathieu Desnoyers <[email-masked]>
Date: Tue, 27 Mar 2018 20:44:12 -0400 (EDT)

----- On Mar 27, 2018, at 8:00 PM, Alexei Starovoitov [email-masked] wrote:

> On 3/27/18 4:13 PM, Mathieu Desnoyers wrote:
>> ----- On Mar 27, 2018, at 6:48 PM, Alexei Starovoitov [email-masked] wrote:
>>
>>> On 3/27/18 2:04 PM, Steven Rostedt wrote:
>>>>
>>>> +#ifdef CONFIG_BPF_EVENTS
>>>> +#define BPF_RAW_TP() . = ALIGN(8);		\
>>
>> Given that the section consists of a 16-bytes structure elements
>> on architectures with 8 bytes pointers, this ". = ALIGN(8)" should
>> be turned into a STRUCT_ALIGN(), especially given that the compiler
>> is free to up-align the structure on 32 bytes.
> 
> STRUCT_ALIGN fixed the 'off by 8' issue with kasan,
> but it fails without kasan too.
> For some reason the whole region __start__bpf_raw_tp - __stop__bpf_raw_tp
> comes inited with cccc:
> [   22.703562] i 1 btp ffffffff8288e530 btp->tp cccccccccccccccc func
> cccccccccccccccc
> [   22.704638] i 2 btp ffffffff8288e540 btp->tp cccccccccccccccc func
> cccccccccccccccc
> [   22.705599] i 3 btp ffffffff8288e550 btp->tp cccccccccccccccc func
> cccccccccccccccc
> [   22.706551] i 4 btp ffffffff8288e560 btp->tp cccccccccccccccc func
> cccccccccccccccc
> [   22.707503] i 5 btp ffffffff8288e570 btp->tp cccccccccccccccc func
> cccccccccccccccc
> [   22.708452] i 6 btp ffffffff8288e580 btp->tp cccccccccccccccc func
> cccccccccccccccc
> [   22.709406] i 7 btp ffffffff8288e590 btp->tp cccccccccccccccc func
> cccccccccccccccc
> [   22.710368] i 8 btp ffffffff8288e5a0 btp->tp cccccccccccccccc func
> cccccccccccccccc
> 
> while gdb shows that everything is good inside vmlinux
> for exactly these addresses.
> Some other linker magic missing?

No, Steven's iteration code is incorrect.

+extern struct bpf_raw_event_map __start__bpf_raw_tp;
+extern struct bpf_raw_event_map __stop__bpf_raw_tp;

That should be:

extern struct bpf_raw_event_map __start__bpf_raw_tp[];
extern struct bpf_raw_event_map __stop__bpf_raw_tp[];

+
+struct bpf_raw_event_map *bpf_find_raw_tracepoint(const char *name)
+{
+        const struct bpf_raw_event_map *btp = &__start__bpf_raw_tp;

const struct bpf_raw_event_map *btp = __start__bpf_raw_tp;

+        int i = 0;
+
+        for (; btp < &__stop__bpf_raw_tp; btp++) {

for (; btp < __stop__bpf_raw_tp; btp++) {

Those start/stop symbols are given their address by the linker
automatically (this is a GNU linker extension). We don't want
pointers to the symbols, but rather the symbols per se to act
as start/stop addresses.

Thanks,

Mathieu

+                i++;
+                if (!strcmp(btp->tp->name, name))
+                        return btp;
+        }
+        return NULL;
+}

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com