From patchwork Tue Sep 3 15:06:35 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: =?utf-8?q?Matthias_M=C3=A4nnich?= X-Patchwork-Id: 11128191 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id F0E531920 for ; Tue, 3 Sep 2019 15:08:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CDD5520674 for ; Tue, 3 Sep 2019 15:08:10 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="OehgC/Qp" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729911AbfICPHv (ORCPT ); Tue, 3 Sep 2019 11:07:51 -0400 Received: from mail-yw1-f73.google.com ([209.85.161.73]:43250 "EHLO mail-yw1-f73.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729876AbfICPHv (ORCPT ); Tue, 3 Sep 2019 11:07:51 -0400 Received: by mail-yw1-f73.google.com with SMTP id a12so13552831ywm.10 for ; Tue, 03 Sep 2019 08:07:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:in-reply-to:message-id:mime-version:references:subject:from:to :cc; bh=PXDMDCBfc2xecCOj15IRjy6gG7tFhf1boblcojHoIqc=; b=OehgC/QpGVW5WHRtYYFEBPQjfIK+N42Odb/gz465atQlpSIk1lvdVNY2fEt5bT5y/i ncnP5Uzjks7oK05vFxfW7R4g/0AHAm6ZfhdpT9wXG6QB9n3Z0n3y2ZjV4jnYLOuSPB+V X3WZHM6Z6AEL0iTYXZ2q8rc3cx61dR3VH7hA6VsD64SSclKjEvBcfHz00aeGDU4u9Wg0 kO46LRoLWUpD93faDcaTwE3uYYBlaEGBRH9xLFbyVy1nOz7+8Ec5oTMVlw5Xr2i9V05w 0afYMRX4Wu/UypBQBMVOR/dlbL/g92c6Fkmc05h9ztxwfejrJ3k5dAKR3nVZAllXy1E2 4rlw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=PXDMDCBfc2xecCOj15IRjy6gG7tFhf1boblcojHoIqc=; b=PbnnUl1M7u5GD9jZv56sXX/Yyys8N11fYqHRhCy/zKwIoo6m7T7LEi7C533xcXxJcA FsLPz3Zxr0L7j1WVyYmqXRJer3+PERIcLKSM2p1Tvbjo0H43mohUztyy48E5pyPI6dZz i1shY9002cAHQ9vXBeXqzvVkAQqA+CIgNuLRFLM8Oueo5WdvkTY7SVtV2MfT4venfzlA AoJjXQkS/vGursZjaRMRNURZ0ZHDBNoQQoPlirq4ZtwK0ZCtCi4axvEk5LtgecczBSiG vYTWvtDScfLprCiTPhU1vR9LYpw17JQout56v3+obT0Arb15VS4Vp3cG2m4PBIwA8T87 tsEA== X-Gm-Message-State: APjAAAWNlspVu1GSAeyG9NuPvDRSS084TlS0TKB+lnzhK1uQP7cLKlkQ UkobE+dr9nElFM6uta9rkq8wfXqoFIT14g== X-Google-Smtp-Source: APXvYqylPWFHf2hpNQhFQu2/5vQMLUmJtcBW5//m7WkaHo/DcTGkmE7DrAleumJF4actayeB+yWtnVWCxpa+8g== X-Received: by 2002:a25:6191:: with SMTP id v139mr25130367ybb.396.1567523270130; Tue, 03 Sep 2019 08:07:50 -0700 (PDT) Date: Tue, 3 Sep 2019 16:06:35 +0100 In-Reply-To: <20190903150638.242049-1-maennich@google.com> Message-Id: <20190903150638.242049-10-maennich@google.com> Mime-Version: 1.0 References: <20180716122125.175792-1-maco@android.com> <20190903150638.242049-1-maennich@google.com> X-Mailer: git-send-email 2.23.0.187.g17f5b7556c-goog Subject: [PATCH v4 09/12] docs: Add documentation for Symbol Namespaces From: Matthias Maennich To: linux-kernel@vger.kernel.org Cc: kernel-team@android.com, maennich@google.com, arnd@arndb.de, gregkh@linuxfoundation.org, jeyu@kernel.org, joel@joelfernandes.org, lucas.de.marchi@gmail.com, maco@android.com, sspatil@google.com, will@kernel.org, yamada.masahiro@socionext.com, linux-kbuild@vger.kernel.org, linux-modules@vger.kernel.org, linux-usb@vger.kernel.org, usb-storage@lists.one-eyed-alien.net, linux-watchdog@vger.kernel.org Sender: owner-linux-modules@vger.kernel.org Precedence: bulk List-ID: Describe using Symbol Namespaces from a perspective of a user. I.e. module authors or subsystem maintainers. Signed-off-by: Matthias Maennich Reviewed-by: Greg Kroah-Hartman --- Documentation/kbuild/namespaces.rst | 154 +++++++++++++++++++++++ Documentation/kernel-hacking/hacking.rst | 18 +++ 2 files changed, 172 insertions(+) create mode 100644 Documentation/kbuild/namespaces.rst diff --git a/Documentation/kbuild/namespaces.rst b/Documentation/kbuild/namespaces.rst new file mode 100644 index 000000000000..982ed7b568ac --- /dev/null +++ b/Documentation/kbuild/namespaces.rst @@ -0,0 +1,154 @@ +================= +Symbol Namespaces +================= + +The following document describes how to use Symbol Namespaces to structure the +export surface of in-kernel symbols exported through the family of +EXPORT_SYMBOL() macros. + +.. Table of Contents + + === 1 Introduction + === 2 How to define Symbol Namespaces + --- 2.1 Using the EXPORT_SYMBOL macros + --- 2.2 Using the DEFAULT_SYMBOL_NAMESPACE define + === 3 How to use Symbols exported in Namespaces + === 4 Loading Modules that use namespaced Symbols + === 5 Automatically creating MODULE_IMPORT_NS statements + +1. Introduction +=============== + +Symbol Namespaces have been introduced as a means to structure the export +surface of the in-kernel API. It allows subsystem maintainers to partition +their exported symbols into separate namespaces. That is useful for +documentation purposes (think of the SUBSYSTEM_DEBUG namespace) as well as for +limiting the availability of a set of symbols for use in other parts of the +kernel. As of today, modules that make use of symbols exported into namespaces, +are required to import the namespace. Otherwise the kernel will, depending on +its configuration, reject loading the module or warn about a missing import. + +2. How to define Symbol Namespaces +================================== + +Symbols can be exported into namespace using different methods. All of them are +changing the way EXPORT_SYMBOL and friends are instrumented to create ksymtab +entries. + +2.1 Using the EXPORT_SYMBOL macros +================================== + +In addition to the macros EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL(), that allow +exporting of kernel symbols to the kernel symbol table, variants of these are +available to export symbols into a certain namespace: EXPORT_SYMBOL_NS() and +EXPORT_SYMBOL_NS_GPL(). They take one additional argument: the namespace. +Please note that due to macro expansion that argument needs to be a +preprocessor symbol. E.g. to export the symbol `usb_stor_suspend` into the +namespace `USB_STORAGE`, use:: + + EXPORT_SYMBOL_NS(usb_stor_suspend, USB_STORAGE); + +The corresponding ksymtab entry struct `kernel_symbol` will have the member +`namespace` set accordingly. A symbol that is exported without a namespace will +refer to `NULL`. There is no default namespace if none is defined. `modpost` +and kernel/module.c make use the namespace at build time or module load time, +respectively. + +2.2 Using the DEFAULT_SYMBOL_NAMESPACE define +============================================= + +Defining namespaces for all symbols of a subsystem can be very verbose and may +become hard to maintain. Therefore a default define (DEFAULT_SYMBOL_NAMESPACE) +is been provided, that, if set, will become the default for all EXPORT_SYMBOL() +and EXPORT_SYMBOL_GPL() macro expansions that do not specify a namespace. + +There are multiple ways of specifying this define and it depends on the +subsystem and the maintainer's preference, which one to use. The first option +is to define the default namespace in the `Makefile` of the subsystem. E.g. to +export all symbols defined in usb-common into the namespace USB_COMMON, add a +line like this to drivers/usb/common/Makefile:: + + ccflags-y += -DDEFAULT_SYMBOL_NAMESPACE=USB_COMMON + +That will affect all EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL() statements. A +symbol exported with EXPORT_SYMBOL_NS() while this definition is present, will +still be exported into the namespace that is passed as the namespace argument +as this argument has preference over a default symbol namespace. + +A second option to define the default namespace is directly in the compilation +unit as preprocessor statement. The above example would then read:: + + #undef DEFAULT_SYMBOL_NAMESPACE + #define DEFAULT_SYMBOL_NAMESPACE USB_COMMON + +within the corresponding compilation unit before any EXPORT_SYMBOL macro is +used. + +3. How to use Symbols exported in Namespaces +============================================ + +In order to use symbols that are exported into namespaces, kernel modules need +to explicitly import these namespaces. Otherwise the kernel might reject to +load the module. The module code is required to use the macro MODULE_IMPORT_NS +for the namespaces it uses symbols from. E.g. a module using the +usb_stor_suspend symbol from above, needs to import the namespace USB_STORAGE +using a statement like:: + + MODULE_IMPORT_NS(USB_STORAGE); + +This will create a `modinfo` tag in the module for each imported namespace. +This has the side effect, that the imported namespaces of a module can be +inspected with modinfo:: + + $ modinfo drivers/usb/storage/ums-karma.ko + [...] + import_ns: USB_STORAGE + [...] + + +It is advisable to add the MODULE_IMPORT_NS() statement close to other module +metadata definitions like MODULE_AUTHOR() or MODULE_LICENSE(). Refer to section +5. for a way to create missing import statements automatically. + +4. Loading Modules that use namespaced Symbols +============================================== + +At module loading time (e.g. `insmod`), the kernel will check each symbol +referenced from the module for its availability and whether the namespace it +might be exported to has been imported by the module. The default behaviour of +the kernel is to reject loading modules that don't specify sufficient imports. +An error will be logged and loading will be failed with EINVAL. In order to +allow loading of modules that don't satisfy this precondition, a configuration +option is available: Setting MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS=y will +enable loading regardless, but will emit a warning. + +5. Automatically creating MODULE_IMPORT_NS statements +===================================================== + +Missing namespaces imports can easily be detected at build time. In fact, +modpost will emit a warning if a module uses a symbol from a namespace +without importing it. +MODULE_IMPORT_NS() statements will usually be added at a definite location +(along with other module meta data). To make the life of module authors (and +subsystem maintainers) easier, a script and make target is available to fixup +missing imports. Fixing missing imports can be done with:: + + $ make nsdeps + +A typical scenario for module authors would be:: + + - write code that depends on a symbol from a not imported namespace + - `make` + - notice the warning of modpost telling about a missing import + - run `make nsdeps` to add the import to the correct code location + +For subsystem maintainers introducing a namespace, the steps are very similar. +Again, `make nsdeps` will eventually add the missing namespace imports for +in-tree modules:: + + - move or add symbols to a namespace (e.g. with EXPORT_SYMBOL_NS()) + - `make` (preferably with an allmodconfig to cover all in-kernel + modules) + - notice the warning of modpost telling about a missing import + - run `make nsdeps` to add the import to the correct code location + diff --git a/Documentation/kernel-hacking/hacking.rst b/Documentation/kernel-hacking/hacking.rst index 5891a701a159..a3ddb213a5e1 100644 --- a/Documentation/kernel-hacking/hacking.rst +++ b/Documentation/kernel-hacking/hacking.rst @@ -594,6 +594,24 @@ internal implementation issue, and not really an interface. Some maintainers and developers may however require EXPORT_SYMBOL_GPL() when adding any new APIs or functionality. +:c:func:`EXPORT_SYMBOL_NS()` +---------------------------- + +Defined in ``include/linux/export.h`` + +This is the variant of `EXPORT_SYMBOL()` that allows specifying a symbol +namespace. Symbol Namespaces are documented in +``Documentation/kbuild/namespaces.rst``. + +:c:func:`EXPORT_SYMBOL_NS_GPL()` +-------------------------------- + +Defined in ``include/linux/export.h`` + +This is the variant of `EXPORT_SYMBOL_GPL()` that allows specifying a symbol +namespace. Symbol Namespaces are documented in +``Documentation/kbuild/namespaces.rst``. + Routines and Conventions ========================