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Message-ID: <0039743b-5837-c0d3-7574-7719698f9dc1@huaweicloud.com>
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;
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