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Mon, 30 May 2022 05:38:34 -0700 (PDT) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Mon, 30 May 2022 14:38:31 +0200 To: Masami Hiramatsu Cc: Mark Rutland , Steven Rostedt , Wang ShaoBo , 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, bpf@vger.kernel.org Subject: Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically allocated trampolines Message-ID: References: <20220505121538.04773ac98e2a8ba17f675d39@kernel.org> <20220509142203.6c4f2913@gandalf.local.home> <20220510181012.d5cba23a2547f14d14f016b9@kernel.org> <20220510104446.6d23b596@gandalf.local.home> <20220511233450.40136cdf6a53eb32cd825be8@kernel.org> <20220511111207.25d1a693@gandalf.local.home> <20220512210231.f9178a98f20a37981b1e89e3@kernel.org> <20220530100310.c22c36df4ea9324cb9cb3515@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20220530100310.c22c36df4ea9324cb9cb3515@kernel.org> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220530_053839_073570_420A734D X-CRM114-Status: GOOD ( 38.30 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, May 30, 2022 at 10:03:10AM +0900, Masami Hiramatsu wrote: > (Cc: BPF ML) > > On Wed, 25 May 2022 13:17:30 +0100 > Mark Rutland wrote: > > > On Thu, May 12, 2022 at 09:02:31PM +0900, Masami Hiramatsu wrote: > > > On Wed, 11 May 2022 11:12:07 -0400 > > > Steven Rostedt wrote: > > > > > > > On Wed, 11 May 2022 23:34:50 +0900 > > > > Masami Hiramatsu wrote: > > > > > > > > > OK, so fregs::regs will have a subset of pt_regs, and accessibility of > > > > > the registers depends on the architecture. If we can have a checker like > > > > > > > > > > ftrace_regs_exist(fregs, reg_offset) > > > > > > > > Or something. I'd have to see the use case. > > > > > > > > > > > > > > kprobe on ftrace or fprobe user (BPF) can filter user's requests. > > > > > I think I can introduce a flag for kprobes so that user can make a > > > > > kprobe handler only using a subset of registers. > > > > > Maybe similar filter code is also needed for BPF 'user space' library > > > > > because this check must be done when compiling BPF. > > > > > > > > Is there any other case without full regs that the user would want anything > > > > other than the args, stack pointer and instruction pointer? > > > > > > For the kprobes APIs/events, yes, it needs to access to the registers > > > which is used for local variables when probing inside the function body. > > > However at the function entry, I think almost no use case. (BTW, pstate > > > is a bit special, that may show the actual processor-level status > > > (context), so for the debugging, user might want to read it.) > > > > As before, if we really need PSTATE we *must* take an exception to get a > > reliable snapshot (or to alter the value). So I'd really like to split this > > into two cases: > > > > * Where users *really* need PSTATE (or arbitrary GPRs), they use kprobes. That > > always takes an exception and they can have a complete, real struct pt_regs. > > > > * Where users just need to capture a function call boundary, they use ftrace. > > That uses a trampoline without taking an exception, and they get the minimal > > set of registers relevant to the function call boundary (which does not > > include PSTATE or most GPRs). > > I totally agree with this idea. The x86 is a special case, since the > -fentry option puts a call on the first instruction of the function entry, > I had to reuse the ftrace instead of swbp for kprobes. > But on arm64 (and other RISCs), we can use them properly. > > My concern is that the eBPF depends on kprobe (pt_regs) interface, thus > I need to ask them that it is OK to not accessable to some part of > pt_regs (especially, PSTATE) if they puts probes on function entry > with ftrace (fprobe in this case.) > > (Jiri and BPF developers) > Currently fprobe is only enabled on x86 for "multiple kprobes" BPF > interface, but in the future, it will be enabled on arm64. And at > that point, it will be only accessible to the regs for function > arguments. Is that OK for your use case? And will the BPF compiler I guess from practical POV registers for arguments and ip should be enough, but whole pt_regs was already exposed to programs, so people can already use any of them.. not sure it's good idea to restrict it > be able to restrict the user program to access only those registers > when using the "multiple kprobes"? pt-regs pointer is provided to kprobe programs, I guess we could provide copy of that with just available values jirka _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel