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

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Date:   Tue, 27 Mar 2018 19:13:37 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Alexei Starovoitov <ast@...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 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.

This could explain the kasan splat you are experiencing.

Thanks,

Mathieu


>> +			 VMLINUX_SYMBOL(__start__bpf_raw_tp) = .;	\
>> +			 KEEP(*(__bpf_raw_tp_map))			\
>> +			 VMLINUX_SYMBOL(__stop__bpf_raw_tp) = .;
> 
> that looks to be correct, but something wrong with it.
> 
> Can you try your mini test with kasan on ?
> 
> I'm seeing this crash:
> test_stacktrace_[   18.760662] start ffffffff84642438 stop ffffffff84644f60
> map_raw_tp:PASS:[   18.761467] i 1 btp->tp cccccccccccccccc
> prog_load raw tp[   18.762064] kasan: CONFIG_KASAN_INLINE enabled
>  0 nsec
> [   18.762704] kasan: GPF could be caused by NULL-ptr deref or user
> memory access
> [   18.765125] general protection fault: 0000 [#1] SMP KASAN PTI
> [   18.765830] Modules linked in:
> [   18.778358] Call Trace:
> [   18.778674]  bpf_raw_tracepoint_open.isra.27+0x92/0x380
> 
> for some reason the start_bpf_raw_tp is off by 8.
> Not sure how it works for you.
> 
> (gdb) p &__bpf_trace_tp_map_sys_exit
> $10 = (struct bpf_raw_event_map *) 0xffffffff84642440
> <__bpf_trace_tp_map_sys_exit>
> 
> (gdb)  p &__start__bpf_raw_tp
> $7 = (<data variable, no debug info> *) 0xffffffff84642438
> 
> (gdb)  p (void*)(&__start__bpf_raw_tp)+8
> $11 = (void *) 0xffffffff84642440 <__bpf_trace_tp_map_sys_exit>

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