linux-kernel - Re: [PATCH v2 bpf-next 1/5] ftrace: allow customized flags for ftrace_direct_multi ftrace

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Message-ID: <0029EF24-6508-4011-B365-3E2175F9FEAB@fb.com>
Date:   Thu, 14 Jul 2022 00:11:53 +0000
From:   Song Liu <songliubraving@...com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     Song Liu <song@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii@...nel.org" <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 1/5] ftrace: allow customized flags for
 ftrace_direct_multi ftrace_ops



> On Jul 13, 2022, at 4:18 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> On Thu, 2 Jun 2022 12:37:02 -0700
> Song Liu <song@...nel.org> wrote:
> 
>> This enables users of ftrace_direct_multi to specify the flags based on
>> the actual use case. For example, some users may not set flag IPMODIFY.
> 
> If we apply this patch without any of the others, then we are relying on
> the caller to get it right?
> 
> That is, can we register a direct function with this function and pick a
> function with IPMODIFY already attached?

Yes, if the direct function follows regs->ip, it works. 

For example, BPF trampoline with only fentry calls will just work with only this patch.

Thanks,
Song

> 
> -- Steve
> 
> 
>> 
>> Signed-off-by: Song Liu <song@...nel.org>
>> ---
>> kernel/trace/ftrace.c | 5 ++---
>> 1 file changed, 2 insertions(+), 3 deletions(-)
>> 
>> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
>> index 2fcd17857ff6..afe782ae28d3 100644
>> --- a/kernel/trace/ftrace.c
>> +++ b/kernel/trace/ftrace.c
>> @@ -5456,8 +5456,7 @@ int modify_ftrace_direct(unsigned long ip,
>> }
>> EXPORT_SYMBOL_GPL(modify_ftrace_direct);
>> 
>> -#define MULTI_FLAGS (FTRACE_OPS_FL_IPMODIFY | FTRACE_OPS_FL_DIRECT | \
>> -             FTRACE_OPS_FL_SAVE_REGS)
>> +#define MULTI_FLAGS (FTRACE_OPS_FL_DIRECT | FTRACE_OPS_FL_SAVE_REGS)
>> 
>> static int check_direct_multi(struct ftrace_ops *ops)
>> {
>> @@ -5547,7 +5546,7 @@ int register_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr)
>>    }
>> 
>>    ops->func = call_direct_funcs;
>> -    ops->flags = MULTI_FLAGS;
>> +    ops->flags |= MULTI_FLAGS;
>>    ops->trampoline = FTRACE_REGS_ADDR;
>> 
>>    err = register_ftrace_function(ops);
>