From patchwork Wed Dec 7 20:49:10 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: David Vernet X-Patchwork-Id: 13067604 X-Patchwork-Delegate: bpf@iogearbox.net Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 557C1C63708 for ; Wed, 7 Dec 2022 20:51:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230137AbiLGUvG (ORCPT ); Wed, 7 Dec 2022 15:51:06 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60588 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230125AbiLGUub (ORCPT ); Wed, 7 Dec 2022 15:50:31 -0500 Received: from mail-qt1-f178.google.com (mail-qt1-f178.google.com [209.85.160.178]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CCC8981DB8; Wed, 7 Dec 2022 12:49:41 -0800 (PST) Received: by mail-qt1-f178.google.com with SMTP id r7so4524281qtx.6; Wed, 07 Dec 2022 12:49:41 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=Ge7ArlanrmSDH6ZAo9C+yxNluKmqau1XfrzR0bl/RY0=; b=XR8vEEX/1dAW/NXDUs6JLr5bOtJoL5kdDQnd32jBv3A03jNTAcfOyGXMDHUckmlV17 FcDDeIIZX+ehzLiP9+SVR1S1r329Z8kkZ5WKw6IRCr/IOh3SP7Klsk6dlzAZXMGUE7+R ptP9ZklC+0G6wSBK1bkYflYxikjmNTSjzX3bGF1SPSyuIMPfaJjVIdzOrSaiAQjbTT53 NrWLWQ1u3c/2tDH2lGXCyScJsDKBVdvyWCJzSZxQkC0MxhSu0pX3F2db0hLE2B7dmmd4 8b2O8bGMIJzRXFSGShhKBp1waJ0gV4MrVf9HDk5+34we4hWcJd3sqzWT8v56QTbT/773 3kPg== X-Gm-Message-State: ANoB5pl1ASjycCFpEDB63kyrFuhB2PR0eMOfwVFHhZgD9/GkHbFO/+AT rE7q1rWyq+qo5gkZHVYAxzFK92+vBrlnjE56 X-Google-Smtp-Source: AA0mqf5zbsK/tT/nX9T8oJencDTpWzQWCEO1GAmeZhOT8abMtHhNR/xBPY49dsFNNobWldGoh6xmzQ== X-Received: by 2002:ac8:6882:0:b0:3a7:e484:ebd3 with SMTP id m2-20020ac86882000000b003a7e484ebd3mr11807017qtq.285.1670446180300; Wed, 07 Dec 2022 12:49:40 -0800 (PST) Received: from localhost ([2620:10d:c091:480::1:629d]) by smtp.gmail.com with ESMTPSA id y26-20020ac87c9a000000b003a7ef7a758dsm2707417qtv.59.2022.12.07.12.49.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 07 Dec 2022 12:49:39 -0800 (PST) From: David Vernet To: bpf@vger.kernel.org Cc: ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, song@kernel.org, yhs@meta.com, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@google.com, haoluo@google.com, jolsa@kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: [PATCH bpf-next v3 1/2] bpf/docs: Document struct task_struct * kfuncs Date: Wed, 7 Dec 2022 14:49:10 -0600 Message-Id: <20221207204911.873646-2-void@manifault.com> X-Mailer: git-send-email 2.38.1 In-Reply-To: <20221207204911.873646-1-void@manifault.com> References: <20221207204911.873646-1-void@manifault.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org X-Patchwork-Delegate: bpf@iogearbox.net bpf_task_acquire(), bpf_task_release(), and bpf_task_from_pid() are kfuncs that were recently added to kernel/bpf/helpers.c. These are "core" kfuncs in that they're available for use for any tracepoint or struct_ops BPF program. Though they have no ABI stability guarantees, we should still document them. This patch adds a new Core kfuncs section to the BPF kfuncs doc, and adds entries for all of these task kfuncs. Note that bpf_task_kptr_get() is not documented, as it still returns NULL while we're working to resolve how it can use RCU to ensure struct task_struct * lifetime. Signed-off-by: David Vernet --- Documentation/bpf/kfuncs.rst | 83 ++++++++++++++++++++++++++++++++++++ kernel/bpf/helpers.c | 2 +- 2 files changed, 84 insertions(+), 1 deletion(-) diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst index b027fe16ee66..24ed109afc98 100644 --- a/Documentation/bpf/kfuncs.rst +++ b/Documentation/bpf/kfuncs.rst @@ -222,3 +222,86 @@ type. An example is shown below:: return register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &bpf_task_kfunc_set); } late_initcall(init_subsystem); + +3. Core kfuncs +============== + +The BPF subsystem provides a number of "core" kfuncs that are potentially +applicable to a wide variety of different possible use cases and programs. +Those kfuncs are documented here. + +3.1 struct task_struct * kfuncs +------------------------------- + +There are a number of kfuncs that allow ``struct task_struct *`` objects to be +used as kptrs: + +.. kernel-doc:: kernel/bpf/helpers.c + :identifiers: bpf_task_acquire bpf_task_release + +These kfuncs are useful when you want to acquire or release a reference to a +``struct task_struct *`` that was passed as e.g. a tracepoint arg, or a +struct_ops callback arg. For example: + +.. code-block:: c + + /** + * A trivial example tracepoint program that shows how to + * acquire and release a struct task_struct * pointer. + */ + SEC("tp_btf/task_newtask") + int BPF_PROG(task_acquire_release_example, struct task_struct *task, u64 clone_flags) + { + struct task_struct *acquired; + + acquired = bpf_task_acquire(task); + + /* + * In a typical program you'd do something like store + * the task in a map, and the map will automatically + * release it later. Here, we release it manually. + */ + bpf_task_release(acquired); + return 0; + } + +---- + +A BPF program can also look up a task from a pid. This can be useful if the +caller doesn't have a trusted pointer to a ``struct task_struct *`` object that +it can acquire a reference on with bpf_task_acquire(). + +.. kernel-doc:: kernel/bpf/helpers.c + :identifiers: bpf_task_from_pid + +Here is an example of it being used: + +.. code-block:: c + + SEC("tp_btf/task_newtask") + int BPF_PROG(task_get_pid_example, struct task_struct *task, u64 clone_flags) + { + struct task_struct *lookup; + + lookup = bpf_task_from_pid(task->pid); + if (!lookup) + /* A task should always be found, as %task is a tracepoint arg. */ + return -ENOENT; + + if (lookup->pid != task->pid) { + /* bpf_task_from_pid() looks up the task via its + * globally-unique pid from the init_pid_ns. Thus, + * the pid of the lookup task should always be the + * same as the input task. + */ + bpf_task_release(lookup); + return -EINVAL; + } + + /* bpf_task_from_pid() returns an acquired reference, + * so it must be dropped before returning from the + * tracepoint handler. + */ + bpf_task_release(lookup); + return 0; + } diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index 284b3ffdbe48..1e4bf466b08f 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -1904,7 +1904,7 @@ struct task_struct *bpf_task_kptr_get(struct task_struct **pp) } /** - * bpf_task_release - Release the reference acquired on a struct task_struct *. + * bpf_task_release - Release the reference acquired on a task. * @p: The task on which a reference is being released. */ void bpf_task_release(struct task_struct *p)