linux-kernel - Re: [PATCH v2 bpf-next 3/5] ftrace: introduce FTRACE_OPS_FL_SHARE

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Date:   Fri, 15 Jul 2022 17:42:55 +0000
From:   Song Liu <songliubraving@...com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     Song Liu <song@...nel.org>, Networking <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, lkml <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>
Subject: Re: [PATCH v2 bpf-next 3/5] ftrace: introduce
 FTRACE_OPS_FL_SHARE_IPMODIFY

Hi Steven,

> On Jul 14, 2022, at 7:50 PM, Song Liu <songliubraving@...com> wrote:
> 
> 
> 
>> On Jul 14, 2022, at 7:46 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>> 
>> On Fri, 15 Jul 2022 02:04:33 +0000
>> Song Liu <songliubraving@...com> wrote:
>> 
>>>> What I'm suggesting is that a DIRECT ops will never set IPMODIFY.  
>>> 
>>> Aha, this the point I misunderstood. I thought DIRECT ops would always
>>> set IPMODIFY (as it does now). 
>> 
>> My fault. I was probably not being clear when I was suggesting that
>> DIRECT should *act* like an IPMODIFY, but never explicitly stated that
>> it should not set the IPMODIFY flag.
>> 
>> The only reason it does today was to make it easy to act like an
>> IPMODIFY (because it set the flag). But I'm now suggesting to get rid
>> of that and just make DIRECT act like an IPMDOFIY as there can only be
>> one of them on a function, but now we have some cases where DIRECT can
>> work with IPMODIFY via the callbacks.
> 
> Thanks for the clarification. I think we are finally on the same page on
> this. :)

A quick update and ask for feedback/clarification.

Based on my understanding, you recommended calling ops_func() from 
__ftrace_hash_update_ipmodify() and in ops_func() the direct trampoline
may make changes to the trampoline. Did I get this right?

I am going towards this direction, but hit some issue. Specifically, in 
__ftrace_hash_update_ipmodify(), ftrace_lock is already locked, so the 
direct trampoline cannot easily make changes with 
modify_ftrace_direct_multi(), which locks both direct_mutex and 
ftrace_mutex. 

One solution would be have no-lock version of all the functions called
by modify_ftrace_direct_multi(), but that's a lot of functions and the
code will be pretty ugly. The alternative would be the logic in v2: 
__ftrace_hash_update_ipmodify() returns -EAGAIN, and we make changes to 
the direct trampoline in other places: 

1) if DIRECT ops attached first, the trampoline is updated in 
   prepare_direct_functions_for_ipmodify(), see 3/5 of v2;

2) if IPMODIFY ops attached first, the trampoline is updated in
   bpf_trampoline_update(), see "goto again" path in 5/5 of v2. 

Overall, I think this way is still cleaner. What do you think about this?

Thanks,
Song