netdev - Re: [PATCH v6 bpf-next 08/11] bpf: introduce BPF_RAW

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Date:   Tue, 27 Mar 2018 17:51:55 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:     rostedt <rostedt@...dmis.org>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        netdev <netdev@...r.kernel.org>,
        kernel-team <kernel-team@...com>,
        linux-api <linux-api@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v6 bpf-next 08/11] bpf: introduce BPF_RAW_TRACEPOINT

On 3/27/18 5:44 PM, Mathieu Desnoyers wrote:
> ----- On Mar 27, 2018, at 8:00 PM, Alexei Starovoitov ast@...com wrote:
>
>> On 3/27/18 4:13 PM, Mathieu Desnoyers wrote:
>>> ----- On Mar 27, 2018, at 6:48 PM, Alexei Starovoitov ast@...com 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.

right. that part I fixed first.

Turned out it was in init.data section and got poisoned.
this fixes it:
@@ -258,6 +258,7 @@
         LIKELY_PROFILE()                                                \
         BRANCH_PROFILE()                                                \
         TRACE_PRINTKS()                                                 \
+       BPF_RAW_TP()                                                    \
         TRACEPOINT_STR()

  /*
@@ -585,7 +586,6 @@
         *(.init.rodata)                                                 \
         FTRACE_EVENTS()                                                 \
         TRACE_SYSCALLS()                                                \
-       BPF_RAW_TP()                                                    \
         KPROBE_BLACKLIST()                                              \
         ERROR_INJECT_WHITELIST()                                        \
         MEM_DISCARD(init.rodata)                                        \

and it works :)
I will clean few other nits I found while debugging and respin.