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Message-ID: <0039743b-5837-c0d3-7574-7719698f9dc1@huaweicloud.com>
Date:   Wed, 21 Dec 2022 10:36:29 +0800
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Subject: Re: [RFC PATCH RESEND bpf-next 1/4] bpf: Rollback to text_poke when
 arch not supported ftrace direct call
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To:     Xu Kuohai <xukuohai@huawei.com>, bpf@vger.kernel.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Cc:     =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>,
        Luke Nelson <luke.r.nels@gmail.com>,
        Xi Wang <xi.wang@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Pu Lehui <pulehui@huawei.com>
References: <20221220021319.1655871-1-pulehui@huaweicloud.com>
 <20221220021319.1655871-2-pulehui@huaweicloud.com>
 <a2bf7ed9-4977-608c-d5a5-8ee6a520cf52@huawei.com>
From:   Pu Lehui <pulehui@huaweicloud.com>
In-Reply-To: <a2bf7ed9-4977-608c-d5a5-8ee6a520cf52@huawei.com>
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On 2022/12/20 10:32, Xu Kuohai wrote:
> On 12/20/2022 10:13 AM, Pu Lehui wrote:
>> From: Pu Lehui <pulehui@huawei.com>
>>
>> The current bpf trampoline attach to kernel functions via ftrace direct
>> call API, while text_poke is applied for bpf2bpf attach and tail call
>> optimization. For architectures that do not support ftrace direct call,
>> text_poke is still able to attach bpf trampoline to kernel functions.
>> Let's relax it by rollback to text_poke when architecture not supported.
>>
>> Signed-off-by: Pu Lehui <pulehui@huawei.com>
>> ---
>>   kernel/bpf/trampoline.c | 8 ++------
>>   1 file changed, 2 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
>> index d6395215b849..386197a7952c 100644
>> --- a/kernel/bpf/trampoline.c
>> +++ b/kernel/bpf/trampoline.c
>> @@ -228,15 +228,11 @@ static int modify_fentry(struct bpf_trampoline 
>> *tr, void *old_addr, void *new_ad
>>   static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
>>   {
>>       void *ip = tr->func.addr;
>> -    unsigned long faddr;
>>       int ret;
>> -    faddr = ftrace_location((unsigned long)ip);
>> -    if (faddr) {
>> -        if (!tr->fops)
>> -            return -ENOTSUPP;
>> +    if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS) &&
>> +        !!ftrace_location((unsigned long)ip))
>>           tr->func.ftrace_managed = true;
>> -    }
>>
> 
> After this patch, a kernel function with true trace_location will be 
> patched
> by bpf when CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS is disabled, which 
> means
> that a kernel function may be patched by both bpf and ftrace in a mutually
> unaware way. This will cause ftrace and bpf_arch_text_poke to fail in a
> somewhat random way if the function to be patched was already patched
> by the other.

Thanks for your review. And yes, this is a backward compatible solution 
for architectures that not support DYNAMIC_FTRACE_WITH_DIRECT_CALLS.

> 
>>       if (bpf_trampoline_module_get(tr))
>>           return -ENOENT;