linux-kernel - Re: [PATCH bpf-next v4 0/7] Introduce BPF_MODIFY

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Date:   Wed, 4 Mar 2020 14:17:31 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     KP Singh <kpsingh@...omium.org>
Cc:     linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Paul Turner <pjt@...gle.com>, Jann Horn <jannh@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>
Subject: Re: [PATCH bpf-next v4 0/7] Introduce BPF_MODIFY_RET tracing progs

On Wed, Mar 04, 2020 at 08:18:46PM +0100, KP Singh wrote:
> 
> Here is an example of how a fmod_ret program behaves:
> 
> int func_to_be_attached(int a, int b)
V> {  <--- do_fentry
> 
> do_fmod_ret:
>    <update ret by calling fmod_ret>
>    if (ret != 0)
>         goto do_fexit;
> 
> original_function:
> 
>     <side_effects_happen_here>
> 
> }  <--- do_fexit
> 
> ALLOW_ERROR_INJECTION(func_to_be_attached, ERRNO)
> 
> The fmod_ret program attached to this function can be defined as:
> 
> SEC("fmod_ret/func_to_be_attached")
> int BPF_PROG(func_name, int a, int b, int ret)
> {
>         // This will skip the original function logic.
>         return -1;
> }

Applied to bpf-next. Thanks.

I think it sets up a great base to parallelize further work.

1. I'm rebasing my sleepable BPF patches on top.
It's necessary to read enviroment variables without the
'opportunistic copy before hand' hack I saw in your github tree
to do bpf_get_env_var() helper.

2. please continue on LSM_HOOK patches to go via security tree.

3. we need a volunteer to generalize bpf_sk_storage to task and inode structs.
This work will be super useful for all bpf tracing too.
Sleepable progs are useful for tracing as well.