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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Message-ID: <a2bf7ed9-4977-608c-d5a5-8ee6a520cf52@huawei.com>
Date:   Tue, 20 Dec 2022 10:32:31 +0800
From:   Xu Kuohai <xukuohai@...wei.com>
To:     Pu Lehui <pulehui@...weicloud.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 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.

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