linux-kernel - Re: [RFC PATCH RESEND bpf-next 1/4] bpf: Rollback to text_poke when arch not supported ftrace direct call

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Date:   Wed, 21 Dec 2022 10:36:29 +0800
From:   Pu Lehui <pulehui@...weicloud.com>
To:     Xu Kuohai <xukuohai@...wei.com>, bpf@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Björn Töpel <bjorn@...nel.org>,
        Luke Nelson <luke.r.nels@...il.com>,
        Xi Wang <xi.wang@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Pu Lehui <pulehui@...wei.com>
Subject: Re: [RFC PATCH RESEND bpf-next 1/4] bpf: Rollback to text_poke when
 arch not supported ftrace direct call



On 2022/12/20 10:32, Xu Kuohai wrote:
> On 12/20/2022 10:13 AM, Pu Lehui wrote:
>> From: Pu Lehui <pulehui@...wei.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@...wei.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;