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

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>,
	linux-trace-kernel@vger.kernel.org,
	LKML <linux-kernel@vger.kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	bpf <bpf@vger.kernel.org>, Sven Schnelle <svens@linux.ibm.com>,
	Alexei Starovoitov <ast@kernel.org>, Jiri Olsa <jolsa@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Alan Maguire <alan.maguire@oracle.com>
Subject: Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a struct/union
Date: Tue, 1 Aug 2023 15:18:56 -0700	[thread overview]
Message-ID: <CAADnVQ+N7b8_0UhndjwW9-5Vx2wUVvojujFLOCFr648DUv-Y2Q@mail.gmail.com> (raw)
In-Reply-To: <20230801113240.4e625020@gandalf.local.home>

On Tue, Aug 1, 2023 at 8:32 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Tue, 1 Aug 2023 11:20:36 -0400
> Steven Rostedt <rostedt@goodmis.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.

arch/arm64/include/asm/ftrace.h:#define arch_ftrace_get_regs(regs) NULL

Ouch. That's very bad.
We care a lot about bpf running well on arm64.

If you guys decide to convert fprobe to ftrace_regs please
make it depend on kconfig or something.
bpf side needs full pt_regs.
It's not about access to args.
pt_regs is passed from bpf prog further into all kinds of perf event
functions including stack walking.
I think ORC unwinder might depend on availability of all registers.
Other perf helpers might need it too. Like perf_event_output.
bpf progs need to access arguments, no doubt about that.
If ftrace_regs have them exactly in the same offsets as in pt_regs
that might work transparently for bpf progs, but, I'm afraid,
it's not the case on all archs.
So we need full pt_regs to make sure all paths are still working.

Adding Jiri and others.