From patchwork Fri Dec 2 22:07:35 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: David Vernet X-Patchwork-Id: 13063276 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 6ED2DC47088 for ; Fri, 2 Dec 2022 22:07:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234771AbiLBWHr (ORCPT ); Fri, 2 Dec 2022 17:07:47 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53548 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234429AbiLBWHm (ORCPT ); Fri, 2 Dec 2022 17:07:42 -0500 Received: from mail-qt1-f179.google.com (mail-qt1-f179.google.com [209.85.160.179]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D634C12631; Fri, 2 Dec 2022 14:07:40 -0800 (PST) Received: by mail-qt1-f179.google.com with SMTP id y15so7135412qtv.5; Fri, 02 Dec 2022 14:07:40 -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=yfSdIJlgEUCcO+cc0K2PSPr4Zaqt1pUT0K2lkmBRQSQ=; b=Bcl1BzShhcg44VmJfNjfN9PvjImUsZhdln7ssQ+V/ziUNdarR2tbhUYLkWkZg1G+s7 t9JMJ4D1ENPggVcoZJ8v9pAnP3bLwJaEyOpwoSzLIxsFEz7N0AE4Txz4u6cSgoh2JeBt Jlp1RcXl6/B5baSwihVXFqFy323SN9kv4hRCYlvfxZC0NVpPH/zBQnktxluWD+BRNP3T cxN2D2DTsLGkHw2jKtwoEX+X28+X1LOJcgVaT4nySXNSxaTReSoESVUpqviDSGx29eaL cbAD7XFAgbkBMMd369wzFyxBdb7RRtDDgL2rR4UJbDHkXksQYUhCFWsPJckjYlKVbQZW oiSg== X-Gm-Message-State: ANoB5pm6EgDiJPaMDbSMlFMwcZP4Tw7hH/vORozKOlDwHxSW9497AClY AVWXf7/mlKxlyHmvVllwgGRadl5YkII8dZPC X-Google-Smtp-Source: AA0mqf5PU1xPFSsu30JUsU+Cwv7zLkWowlyiruZxoYvNBuTH6D8QzGzKhKszrZt85Vw6bZeTcd/NWw== X-Received: by 2002:ae9:f012:0:b0:6fc:a53a:16a8 with SMTP id l18-20020ae9f012000000b006fca53a16a8mr8770712qkg.88.1670018859595; Fri, 02 Dec 2022 14:07:39 -0800 (PST) Received: from localhost ([2620:10d:c091:480::1:6d2e]) by smtp.gmail.com with ESMTPSA id j10-20020ac806ca000000b003a4f22c6507sm4766155qth.48.2022.12.02.14.07.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 02 Dec 2022 14:07:39 -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 1/2] bpf/docs: Document struct task_struct * kfuncs Date: Fri, 2 Dec 2022 16:07:35 -0600 Message-Id: <20221202220736.521227-2-void@manifault.com> X-Mailer: git-send-email 2.38.1 In-Reply-To: <20221202220736.521227-1-void@manifault.com> References: <20221202220736.521227-1-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(), bpf_task_kptr_get(), 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. Signed-off-by: David Vernet --- Documentation/bpf/kfuncs.rst | 148 +++++++++++++++++++++++++++++++++++ kernel/bpf/helpers.c | 8 +- 2 files changed, 152 insertions(+), 4 deletions(-) diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst index 90774479ab7a..b0c35ad6fad4 100644 --- a/Documentation/bpf/kfuncs.rst +++ b/Documentation/bpf/kfuncs.rst @@ -213,3 +213,151 @@ 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. Here, we just release it. + */ + bpf_task_release(acquired); + return 0; + } + +If you want to acquire a reference to a ``struct task_struct`` kptr that's +already stored in a map, you can use bpf_task_kptr_get(): + +.. kernel-doc:: kernel/bpf/helpers.c + :identifiers: bpf_task_kptr_get + +Here's an example of how it can be used: + +.. code-block:: c + + /* struct containing the struct task_struct kptr which is actually stored in the map. */ + struct __tasks_kfunc_map_value { + struct task_struct __kptr_ref * task; + }; + + /* The map containing struct __tasks_kfunc_map_value entries. */ + struct hash_map { + __uint(type, BPF_MAP_TYPE_HASH); + __type(key, int); + __type(value, struct __tasks_kfunc_map_value); + __uint(max_entries, 1); + } __tasks_kfunc_map SEC(".maps"); + + /* ... */ + + /** + * A simple example tracepoint program showing how a + * struct task_struct kptr that is stored in a map can + * be acquired using the bpf_task_kptr_get() kfunc. + */ + SEC("tp_btf/task_newtask") + int BPF_PROG(task_kptr_get_example, struct task_struct *task, u64 clone_flags) + { + struct task_struct *kptr; + struct __tasks_kfunc_map_value *v; + s32 pid; + long status; + + status = bpf_probe_read_kernel(&pid, sizeof(pid), &task->pid); + if (status) + return status; + + /* Assume a task kptr was previously stored in the map. */ + v = bpf_map_lookup_elem(&__tasks_kfunc_map, &pid); + if (!v) + return -ENOENT; + + /* Acquire a reference to the task kptr that's already stored in the map. */ + kptr = bpf_task_kptr_get(&v->task); + if (!kptr) + /* If no task was present in the map, it's because + * we're racing with another CPU that removed it with + * bpf_kptr_xchg() between the bpf_map_lookup_elem() + * above, and our call to bpf_task_kptr_get(). + * bpf_task_kptr_get() internally safely handles this + * race, and will return NULL if the task is no longer + * present in the map by the time we invoke the kfunc. + */ + return -EBUSY; + + /* Free the reference we just took above. Note that the + * original struct task_struct kptr is still in the map. + * It will be freed either at a later time if another + * context deletes it from the map, or automatically by + * the BPF subsystem if it's still present when the map + * is destroyed. + */ + bpf_task_release(kptr); + + return 0; + } + +Finally, 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) { + /* The pid of the lookup task should 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 a5a511430f2a..004afbc14bbf 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -1868,10 +1868,10 @@ struct task_struct *bpf_task_kptr_get(struct task_struct **pp) } /** - * bpf_task_release - Release the reference acquired on a struct task_struct *. - * If this kfunc is invoked in an RCU read region, the task_struct is - * guaranteed to not be freed until the current grace period has ended, even if - * its refcount drops to 0. + * bpf_task_release - Release the reference acquired on a task. If this kfunc + * is invoked in an RCU read region, the task_struct is guaranteed to not be + * freed until the current grace period has ended, even if its refcount drops + * to 0. * @p: The task on which a reference is being released. */ void bpf_task_release(struct task_struct *p)