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Date:   Tue, 10 May 2022 10:44:46 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Wang ShaoBo <bobo.shaobowang@huawei.com>,
        cj.chengjian@huawei.com, huawei.libin@huawei.com,
        xiexiuqi@huawei.com, liwei391@huawei.com,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        catalin.marinas@arm.com, will@kernel.org, zengshun.wu@outlook.com,
        Jiri Olsa <jolsa@kernel.org>
Subject: Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically
 allocated trampolines
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On Tue, 10 May 2022 18:10:12 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> >
> > This was suggested by both Peter Zijlstra and Thomas Gleixner when I
> > introduced FTRACE_WITH_ARGS, where all functions can now get the arguments
> > from fregs, but not the full pt_regs.  
> 
> Hmm, I thought the ftrace_get_regs() is the all-or-nothing interface, or
> is there any way to get the arguments from fregs?

Not yet generically. But that can easily be added. If you look at x86 live
patching, since it is arch specific, it just took the regs parameter
directly, knowing that the args were already set up. That is, ftrace_regs is
just a wrapper around pt_regs with just the regs for the arguments and stack
initialized. If you get regs from ftrace_get_regs(fregs) it will return
NULL if it wasn't full set of regs. But we can add generic functions to get
the parameters.

That is, we can create a ftrace_get_kernel_argument() function that takes
fregs instead of pt_regs, and produce the same thing as
regs_get_kernel_argument().

x86 live kernel patching has this:

arch/x86/include/asm/ftrace.h:

 #define ftrace_instruction_pointer_set(fregs, _ip)     \
       do { (fregs)->regs.ip = (_ip); } while (0)


arch/x86/include/asm/livepatch.h:

 static inline void klp_arch_set_pc(struct ftrace_regs *fregs, unsigned long ip)
 {
        ftrace_instruction_pointer_set(fregs, ip);
 }

Where fregs is not a full set of regs.

> 
> > If a ftrace_ops has the REGS flag set
> > (using ftrace_regs_caller), the ftrace_get_regs(fregs) will return the
> > pt_regs, or it will return NULL if ftrace_regs_caller was not used.
> > 
> > This way the same parameter can provide full pt_regs or a subset, and have
> > an generic interface to tell the difference.  
> 
> If it can provide a partial (subset of) pt_regs, that could be good for me
> too, since at least kprobe-events on ftrace can check the traced register
> is in the subset or not and reject it if it doesn't saved.

That's exactly the use case for ftrace_regs.

-- Steve