linux-kernel - Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a struct/union

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Message-ID: <20230801113240.4e625020@gandalf.local.home>
Date:   Tue, 1 Aug 2023 11:32:40 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        linux-trace-kernel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        bpf <bpf@...r.kernel.org>, Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a
 struct/union

On Tue, 1 Aug 2023 11:20:36 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> The solution was to come up with ftrace_regs, which just means it has all
> the registers to extract the arguments of a function and nothing more. Most

This isn't 100% true. The ftrace_regs may hold a fully filled pt_regs. As
the FTRACE_WITH_REGS callbacks still get passed a ftrace_regs pointer. They
will do:

	void callback(..., struct ftrace_regs *fregs) {
		struct pt_regs *regs = ftrace_get_regs(fregs);


Where ftrace_get_regs() will return the pt_regs only if it is fully filled.
If it is not, then it returns NULL. This was what the x86 maintainers
agreed with.

-- Steve


> implementations just have a partially filled pt_regs within it, but an API
> needs to be used to get to the argument values.