From nobody Tue Dec 16 16:41:07 2025 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 11EC6C001DF for ; Wed, 16 Aug 2023 10:32:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243889AbjHPKbh (ORCPT ); Wed, 16 Aug 2023 06:31:37 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35708 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243918AbjHPKbM (ORCPT ); Wed, 16 Aug 2023 06:31:12 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 107F21BD4 for ; Wed, 16 Aug 2023 03:31:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=Content-Type:MIME-Version:Message-ID: Subject:Cc:To:From:Date:Sender:Reply-To:Content-Transfer-Encoding:Content-ID: Content-Description:In-Reply-To:References; bh=D34RT3Lur+N9/zCG68ltkoNZKq0tMJJ4VHitpo+qvt8=; b=rwFCyKhSkb1ggUhxZeHprZmc+G vwCyROAbsuqqmEnyLLMINDeKfppUN2vNsdA6TERV/UtvSBY5WSjL5cFyW6jPwpwWd89eJYhE3fIt9 AjQ56Tuib+YFAr8RPqbeeE45IVvsRn5BBEcQ3PWn4RqWiEsPcvRtAG0JQBaGVVtnAtGiYkDUuRMbe xWmYQIAaet7XwaqdMZ8Fl/GdtxMAwW2e+LKbMcGNfThr914rjQ8l0RupICJ5f4gW6eGQRdLcNTAp0 kg8cIxHmeNX4wxRPADjb8DggCRIDQa2UYt/tbEPmSOtZuaqUTqbmEj8lIuZl9/1VPy2gu2IiiOiJq F5oTx3UA==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1qWDnq-00E6jY-Tf; Wed, 16 Aug 2023 10:31:03 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 6EC18300137; Wed, 16 Aug 2023 12:31:02 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 580AB2C8679C6; Wed, 16 Aug 2023 12:31:02 +0200 (CEST) Date: Wed, 16 Aug 2023 12:31:02 +0200 From: Peter Zijlstra To: Linus Torvalds , Greg Kroah-Hartman , dan.j.williams@intel.com Cc: linux-kernel@vger.kernel.org, linux@rasmusvillemoes.dk, Nick Desaulniers Subject: [PATCH v2] cleanup: Make no_free_ptr() __must_check Message-ID: <20230816103102.GF980931@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Disposition: inline Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" recent discussion brought about the realization that it makes sense for no_free_ptr() to have __must_check semantics in order to avoid leaking the resource. Additionally, add a few comments to clarify why/how things work. All credit to Linus on how to combine __must_check and the statement expression. Signed-off-by: Peter Zijlstra (Intel) --- include/linux/cleanup.h | 39 ++++++++++++++++++++++++++++++++++++--- 1 file changed, 36 insertions(+), 3 deletions(-) --- a/include/linux/cleanup.h +++ b/include/linux/cleanup.h @@ -7,8 +7,9 @@ /* * DEFINE_FREE(name, type, free): * simple helper macro that defines the required wrapper for a __free() - * based cleanup function. @free is an expression using '_T' to access - * the variable. + * based cleanup function. @free is an expression using '_T' to access the + * variable. @free should typically include a NULL test before calling a + * function, see the example below. * * __free(name): * variable attribute to add a scoped based cleanup to the variable. @@ -17,6 +18,9 @@ * like a non-atomic xchg(var, NULL), such that the cleanup function will * be inhibited -- provided it sanely deals with a NULL value. * + * NOTE: this has __must_check semantics so that it is harder to accidenta= lly + * leak the resource. + * * return_ptr(p): * returns p while inhibiting the __free(). * @@ -24,6 +28,8 @@ * * DEFINE_FREE(kfree, void *, if (_T) kfree(_T)) * + * void *alloc_obj(...) + * { * struct obj *p __free(kfree) =3D kmalloc(...); * if (!p) * return NULL; @@ -32,6 +38,24 @@ * return NULL; * * return_ptr(p); + * } + * + * NOTE: the DEFINE_FREE()'s @free expression includes a NULL test even th= ough + * kfree() is fine to be called with a NULL value. This is on purpose. Thi= s way + * the compiler sees the end of our alloc_obj() function as: + * + * tmp =3D p; + * p =3D NULL; + * if (p) + * kfree(p); + * return tmp; + * + * And through the magic of value-propagation and dead-code-elimination, it + * eliminates the actual cleanup call and compiles into: + * + * return p; + * + * Without the NULL test it turns into a mess and the compiler can't help = us. */ =20 #define DEFINE_FREE(_name, _type, _free) \ @@ -39,8 +63,17 @@ =20 #define __free(_name) __cleanup(__free_##_name) =20 +#define __get_and_null_ptr(p) \ + ({ __auto_type __ptr =3D &(p); \ + __auto_type __val =3D *__ptr; \ + *__ptr =3D NULL; __val; }) + +static inline __must_check +const volatile void * __must_check_fn(const volatile void *val) +{ return val; } + #define no_free_ptr(p) \ - ({ __auto_type __ptr =3D (p); (p) =3D NULL; __ptr; }) + ((typeof(p)) __must_check_fn(__get_and_null_ptr(p))) =20 #define return_ptr(p) return no_free_ptr(p)