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Mon, 21 Oct 2024 08:11:23 +0000 Received: from BYAPR10MB3366.namprd10.prod.outlook.com ([fe80::baf2:dff1:d471:1c9]) by BYAPR10MB3366.namprd10.prod.outlook.com ([fe80::baf2:dff1:d471:1c9%6]) with mapi id 15.20.8069.024; Mon, 21 Oct 2024 08:11:23 +0000 Date: Mon, 21 Oct 2024 09:11:19 +0100 From: Lorenzo Stoakes To: Christian Brauner Cc: Shuah Khan , "Liam R . Howlett" , Suren Baghdasaryan , Vlastimil Babka , pedro.falcato@gmail.com, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, Oliver Sang , John Hubbard Subject: Re: [PATCH v4 2/4] pidfd: add PIDFD_SELF_* sentinels to refer to own thread/process Message-ID: <3a12d0b1-a3c4-453e-a09a-c95f3e545abe@lucifer.local> References: <3bf7f2d8efc768007b5de8122275405afc9942d4.1729198898.git.lorenzo.stoakes@oracle.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3bf7f2d8efc768007b5de8122275405afc9942d4.1729198898.git.lorenzo.stoakes@oracle.com> X-ClientProxiedBy: LO4P123CA0607.GBRP123.PROD.OUTLOOK.COM (2603:10a6:600:314::9) To BYAPR10MB3366.namprd10.prod.outlook.com (2603:10b6:a03:14f::25) Precedence: bulk X-Mailing-List: linux-api@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: 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X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: kQvBa+h4lqtUsrUmhlUD0PCHf5LEetya1Y/jfdmp4bJe3TD+cQajSy7I8ZLPdROGIQ9iI7zEqUMqgtYmwp/1fuft7B909QlqGaaUE3IBSqA= X-MS-Exchange-Transport-CrossTenantHeadersStamped: CO1PR10MB4625 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1051,Hydra:6.0.680,FMLib:17.12.62.30 definitions=2024-10-21_04,2024-10-17_01,2024-09-30_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 adultscore=0 bulkscore=0 malwarescore=0 spamscore=0 mlxlogscore=999 phishscore=0 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2409260000 definitions=main-2410210057 X-Proofpoint-GUID: 6B3LwUegrez2E-susJ5-M1X8suodvrOz X-Proofpoint-ORIG-GUID: 6B3LwUegrez2E-susJ5-M1X8suodvrOz On Thu, Oct 17, 2024 at 10:05:50PM +0100, Lorenzo Stoakes wrote: > It is useful to be able to utilise the pidfd mechanism to reference the > current thread or process (from a userland point of view - thread group > leader from the kernel's point of view). > > Therefore introduce PIDFD_SELF_THREAD to refer to the current thread, and > PIDFD_SELF_THREAD_GROUP to refer to the current thread group leader. > > For convenience and to avoid confusion from userland's perspective we alias > these: > > * PIDFD_SELF is an alias for PIDFD_SELF_THREAD - This is nearly always what > the user will want to use, as they would find it surprising if for > instance fd's were unshared()'d and they wanted to invoke pidfd_getfd() > and that failed. > > * PIDFD_SELF_PROCESS is an alias for PIDFD_SELF_THREAD_GROUP - Most users > have no concept of thread groups or what a thread group leader is, and > from userland's perspective and nomenclature this is what userland > considers to be a process. > > Due to the refactoring of the central __pidfd_get_pid() function we can > implement this functionality centrally, providing the use of this sentinel > in most functionality which utilises pidfd's. > > We need to explicitly adjust kernel_waitid_prepare() to permit this (though > it wouldn't really make sense to use this there, we provide the ability for > consistency). > > We explicitly disallow use of this in setns(), which would otherwise have > required explicit custom handling, as it doesn't make sense to set the > current calling thread to join the namespace of itself. > > As the callers of pidfd_get_pid() expect an increased reference count on > the pid we do so in the self case, reducing churn and avoiding any breakage > from existing logic which decrements this reference count. > > This change implicitly provides PIDFD_SELF_* support in the waitid(P_PIDFS, > ...), process_madvise(), process_mrelease(), pidfd_send_signal(), and > pidfd_getfd() system calls. > > Things such as polling a pidfs and general fd operations are not supported, > this strictly provides the sentinel for APIs which explicitly accept a > pidfd. > We need a: Suggested-by: Pedro Falcato here, will add if respin, otherwise could you add Christian? My apologies Pedro, this was wholly an oversight and my mistake! > Signed-off-by: Lorenzo Stoakes