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Fri, 26 Jun 2020 22:45:39 +0000 From: Song Liu To: Andrii Nakryiko CC: bpf , Networking , open list , Peter Ziljstra , "Alexei Starovoitov" , Daniel Borkmann , "Kernel Team" , john fastabend , "KP Singh" Subject: Re: [PATCH v2 bpf-next 2/4] bpf: introduce helper bpf_get_task_stak() Thread-Topic: [PATCH v2 bpf-next 2/4] bpf: introduce helper bpf_get_task_stak() Thread-Index: AQHWS06r1sTuO7iWZUmEIP/wOjIJvqjrVxKAgAApV4A= Date: Fri, 26 Jun 2020 22:45:39 +0000 Message-ID: References: <20200626001332.1554603-1-songliubraving@fb.com> <20200626001332.1554603-3-songliubraving@fb.com> In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-mailer: Apple Mail (2.3608.80.23.2.2) authentication-results: gmail.com; dkim=none (message not signed) header.d=none;gmail.com; dmarc=none action=none header.from=fb.com; x-originating-ip: [2620:10d:c090:400::5:1a00] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: 3f890e48-7a57-48f8-ab79-08d81a22a68b x-ms-traffictypediagnostic: BYAPR15MB3366: x-ms-exchange-transport-forked: True x-microsoft-antispam-prvs: x-fb-source: Internal x-ms-oob-tlc-oobclassifiers: OLM:9508; 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charset="us-ascii" Content-ID: Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: BYAPR15MB2999.namprd15.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: 3f890e48-7a57-48f8-ab79-08d81a22a68b X-MS-Exchange-CrossTenant-originalarrivaltime: 26 Jun 2020 22:45:39.7047 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: nm0O3qPoHcbn88kx0dexvNdPw0Z0utEcKHgdwzreUnup3DaXGQcmlfiSt3Ac75q5hkWZK1FB34w1rEjA54gkyw== X-MS-Exchange-Transport-CrossTenantHeadersStamped: BYAPR15MB3366 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.216,18.0.687 definitions=2020-06-26_12:2020-06-26,2020-06-26 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 mlxscore=0 lowpriorityscore=0 cotscore=-2147483648 spamscore=0 suspectscore=0 adultscore=0 impostorscore=0 mlxlogscore=854 priorityscore=1501 clxscore=1015 malwarescore=0 phishscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2004280000 definitions=main-2006260160 X-FB-Internal: deliver Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Jun 26, 2020, at 1:17 PM, Andrii Nakryiko = wrote: >=20 > On Thu, Jun 25, 2020 at 5:14 PM Song Liu wrote: >>=20 >> Introduce helper bpf_get_task_stack(), which dumps stack trace of given >> task. This is different to bpf_get_stack(), which gets stack track of >> current task. One potential use case of bpf_get_task_stack() is to call >> it from bpf_iter__task and dump all /proc//stack to a seq_file. >>=20 >> bpf_get_task_stack() uses stack_trace_save_tsk() instead of >> get_perf_callchain() for kernel stack. The benefit of this choice is tha= t >> stack_trace_save_tsk() doesn't require changes in arch/. The downside of >> using stack_trace_save_tsk() is that stack_trace_save_tsk() dumps the >> stack trace to unsigned long array. For 32-bit systems, we need to >> translate it to u64 array. >>=20 >> Signed-off-by: Song Liu >> --- >=20 > Looks great, I just think that there are cases where user doesn't > necessarily has valid task_struct pointer, just pid, so would be nice > to not artificially restrict such cases by having extra helper. >=20 > Acked-by: Andrii Nakryiko Thanks! >=20 >> include/linux/bpf.h | 1 + >> include/uapi/linux/bpf.h | 35 ++++++++++++++- >> kernel/bpf/stackmap.c | 79 ++++++++++++++++++++++++++++++++-- >> kernel/trace/bpf_trace.c | 2 + >> scripts/bpf_helpers_doc.py | 2 + >> tools/include/uapi/linux/bpf.h | 35 ++++++++++++++- >> 6 files changed, 149 insertions(+), 5 deletions(-) >>=20 >=20 > [...] >=20 >> + /* stack_trace_save_tsk() works on unsigned long array, while >> + * perf_callchain_entry uses u64 array. For 32-bit systems, it i= s >> + * necessary to fix this mismatch. >> + */ >> + if (__BITS_PER_LONG !=3D 64) { >> + unsigned long *from =3D (unsigned long *) entry->ip; >> + u64 *to =3D entry->ip; >> + int i; >> + >> + /* copy data from the end to avoid using extra buffer */ >> + for (i =3D entry->nr - 1; i >=3D (int)init_nr; i--) >> + to[i] =3D (u64)(from[i]); >=20 > doing this forward would be just fine as well, no? First iteration > will cast and overwrite low 32-bits, all the subsequent iterations > won't even overlap. I think first iteration will write zeros to higher 32 bits, no? >=20 >> + } >> + >> +exit_put: >> + put_callchain_entry(rctx); >> + >> + return entry; >> +} >> + >=20 > [...] >=20 >> +BPF_CALL_4(bpf_get_task_stack, struct task_struct *, task, void *, buf, >> + u32, size, u64, flags) >> +{ >> + struct pt_regs *regs =3D task_pt_regs(task); >> + >> + return __bpf_get_stack(regs, task, buf, size, flags); >> +} >=20 >=20 > So this takes advantage of BTF and having a direct task_struct > pointer. But for kprobes/tracepoint I think it would also be extremely > helpful to be able to request stack trace by PID. How about one more > helper which will wrap this one with get/put task by PID, e.g., > bpf_get_pid_stack(int pid, void *buf, u32 size, u64 flags)? Would that > be a problem? That should work. Let me add that in a follow up patch.=20