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Wed, 25 Dec 2019 09:01:06 +0000 Received: from xhacker.debian (124.74.246.114) by TY2PR01CA0028.jpnprd01.prod.outlook.com (2603:1096:404:ce::16) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2581.11 via Frontend Transport; Wed, 25 Dec 2019 09:01:03 +0000 From: Jisheng Zhang To: Masami Hiramatsu CC: "catalin.marinas@arm.com" , "will@kernel.org" , "naveen.n.rao@linux.vnet.ibm.com" , "mark.rutland@arm.com" , "corbet@lwn.net" , "rostedt@goodmis.org" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH v6] arm64: implement KPROBES_ON_FTRACE Thread-Topic: [PATCH v6] arm64: implement KPROBES_ON_FTRACE Thread-Index: AQHVtWtmR90OKS0v9Um1jpVbBvlnOKe/4n0AgAd5EwCAAb3+AIABe0eA Date: Wed, 25 Dec 2019 09:01:06 +0000 Message-ID: <20191225164605.559f4dbb@xhacker.debian> References: <20191218140622.57bbaca5@xhacker.debian> <20191218222550.51f0b681de7bbab7e49b09a9@kernel.org> <20191223153300.30281a93@xhacker.debian> <20191224190916.2e47478445fb179e88f60cc3@kernel.org> In-Reply-To: <20191224190916.2e47478445fb179e88f60cc3@kernel.org> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [124.74.246.114] x-clientproxiedby: TY2PR01CA0028.jpnprd01.prod.outlook.com (2603:1096:404:ce::16) To BYAPR03MB4773.namprd03.prod.outlook.com (2603:10b6:a03:139::21) authentication-results: spf=none (sender IP is ) smtp.mailfrom=Jisheng.Zhang@synaptics.com; x-ms-exchange-messagesentrepresentingtype: 1 x-mailer: Claws Mail 3.17.4 (GTK+ 2.24.32; x86_64-pc-linux-gnu) x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: 3190c22f-5e51-4595-9ff9-08d78918f9bf x-ms-traffictypediagnostic: BYAPR03MB4278: x-microsoft-antispam-prvs: x-ms-oob-tlc-oobclassifiers: OLM:10000; x-forefront-prvs: 02622CEF0A x-forefront-antispam-report: SFV:NSPM;SFS:(10009020)(366004)(376002)(346002)(396003)(39850400004)(136003)(189003)(199004)(16526019)(316002)(7696005)(8936002)(6916009)(8676002)(81166006)(81156014)(4326008)(2906002)(52116002)(966005)(26005)(478600001)(186003)(6506007)(1076003)(71200400001)(86362001)(7416002)(66476007)(54906003)(66556008)(64756008)(66946007)(66446008)(956004)(5660300002)(55016002)(9686003)(39210200001);DIR:OUT;SFP:1101;SCL:1;SRVR:BYAPR03MB4278;H:BYAPR03MB4773.namprd03.prod.outlook.com;FPR:;SPF:None;LANG:en;PTR:InfoNoRecords;MX:1;A:0; received-spf: None (protection.outlook.com: synaptics.com does not designate permitted sender hosts) x-ms-exchange-senderadcheck: 1 x-microsoft-antispam: BCL:0; x-microsoft-antispam-message-info: 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 x-ms-exchange-transport-forked: True Content-Type: text/plain; charset="us-ascii" Content-ID: <61DF09A3DC40AB469F1A241979E79A4C@namprd03.prod.outlook.com> Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: synaptics.com X-MS-Exchange-CrossTenant-Network-Message-Id: 3190c22f-5e51-4595-9ff9-08d78918f9bf X-MS-Exchange-CrossTenant-originalarrivaltime: 25 Dec 2019 09:01:06.1678 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 335d1fbc-2124-4173-9863-17e7051a2a0e X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: rAxMDH+oyLe/tLmWQhFMva7yIrmANLaEpD0c9qjsYDqyk6pJEQf+mdj9t7PNnZtO9hAv7UDlV7Gh7SpYyTHNtA== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR03MB4278 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Tue, 24 Dec 2019 19:09:16 +0900 Masami Hiramatsu wrote: >=20 > Hi Jisheng, >=20 > On Mon, 23 Dec 2019 07:47:24 +0000 > Jisheng Zhang wrote: >=20 > > Hi Masami, > > > > On Wed, 18 Dec 2019 22:25:50 +0900 Masami Hiramatsu wrote: > > > > =20 > > > > > > > > > On Wed, 18 Dec 2019 06:21:35 +0000 > > > Jisheng Zhang wrote: > > > =20 > > > > KPROBES_ON_FTRACE avoids much of the overhead with regular kprobes = as it > > > > eliminates the need for a trap, as well as the need to emulate or > > > > single-step instructions. > > > > > > > > Tested on berlin arm64 platform. > > > > > > > > ~ # mount -t debugfs debugfs /sys/kernel/debug/ > > > > ~ # cd /sys/kernel/debug/ > > > > /sys/kernel/debug # echo 'p _do_fork' > tracing/kprobe_events > > > > > > > > before the patch: > > > > > > > > /sys/kernel/debug # cat kprobes/list > > > > ffffff801009fe28 k _do_fork+0x0 [DISABLED] > > > > > > > > after the patch: > > > > > > > > /sys/kernel/debug # cat kprobes/list > > > > ffffff801009ff54 k _do_fork+0x4 [DISABLED][FTRACE] =20 > > > > > > BTW, it seems this automatically changes the offset without > > > user's intention or any warnings. How would you manage if the user > > > pass a new probe on _do_fork+0x4? =20 > > > > In current implementation, two probes at the same address _do_fork+0x4 = =20 >=20 > OK, that is my point. >=20 > > > IOW, it is still the question who really wants to probe on > > > the _do_fork+"0", if kprobes modifies it automatically, > > > no one can do that anymore. > > > This can be happen if the user want to record LR or SP value > > > at the function call for debug. If kprobe always modifies it, > > > we will lose the way to do it. =20 > > > > arm64's DYNAMIC_FTRACE_WITH_REGS implementation makes use of GCC > > -fpatchable-function-entry=3D2 option to insert two nops. When the func= tion > > is traced, the first nop will be modified to the LR saver, then the > > second nop to "bl ", commit 3b23e4991fb6(" > > arm64: implement ftrace with regs") explains the mechanism. =20 >=20 > So both of the instruction at func+0 and func+4 are replaced. >=20 > > So on arm64(in fact any arch makes use of -fpatchable-function-entry wi= ll > > behave similarly), when DYNAMIC_FTRACE_WITH_REGS is enabled, the ftrace= location > > is always on the first 4 bytes offset. > > > > I think when users want to register a kprobe on _do_fork+0, what he rea= lly want > > is to kprobe on the patched(by -fpatchable-function-entry)_do_fork+4 =20 >=20 > OK, in this case, kprobe should treat the first 2 instructions as a > single virtual instruction. This means, >=20 > - kprobes can probe func+0, but not func+4 if the function is ftraced. > (-EILSEQ must be returned) > - both debugfs/kprobes/list and tracefs/kprobe_events should show the > probed address as func+0. Not func+4. >=20 > Then, user can use kprobes as if there is one big (8-byte) instruction > at the entry of the function. Since probing on func+4 is rejected and > the call-site LR/SP is restored in ftrace, there should be no > contradiction. It should work as if we put a breakpoint on the func + 0. >From https://lkml.org/lkml/2019/6/18/648, Naveen tried to allow probing on any ftrace address, is it acceptable on arm64 as well? I will post patches for this purpose. Thanks for your review