From mboxrd@z Thu Jan 1 00:00:00 1970 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 lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 88A61C48292 for ; Mon, 5 Feb 2024 17:42:00 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rX2y1-0006zD-27; Mon, 05 Feb 2024 12:41:13 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rX2xz-0006yv-8T for qemu-devel@nongnu.org; Mon, 05 Feb 2024 12:41:11 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rX2xx-0000Ut-7q for qemu-devel@nongnu.org; Mon, 05 Feb 2024 12:41:11 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1707154867; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=fdVhtIG6LceAs3TEVflqIDsV0QFMoYZpZ/zHEMf5vtk=; b=ZrYJ5Av8ARgxnxknddhrAjrSOxgtq9990ZRaXnPLCLwomUgCyZqIyZTRyHfW0Ew0ZV5Ouy CO6WSOQ6/tcXjLxDEn1HN/Oncu73ZTcarvpy3bvpEQEWpgHoXtvJEDeQF1JkHvw8k2l5LF aaosBSGoKQNY7p3sno77cnYN03fYDoA= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-665-sIr1I87AOXyOQd99gwRA1w-1; Mon, 05 Feb 2024 12:41:05 -0500 X-MC-Unique: sIr1I87AOXyOQd99gwRA1w-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 69415185A782; Mon, 5 Feb 2024 17:41:05 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.69]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 2934C2026D66; Mon, 5 Feb 2024 17:41:04 +0000 (UTC) Date: Mon, 5 Feb 2024 17:41:02 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, Hanna Czenczek , Markus Armbruster , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Peter Maydell , Richard Henderson , Thomas Huth , Alex =?utf-8?Q?Benn=C3=A9e?= Subject: Re: [PATCH] docs/style: allow C99 mixed declarations Message-ID: References: <20240205171819.474283-1-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20240205171819.474283-1-stefanha@redhat.com> User-Agent: Mutt/2.2.12 (2023-09-09) X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.4 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -23 X-Spam_score: -2.4 X-Spam_bar: -- X-Spam_report: (-2.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.285, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Mon, Feb 05, 2024 at 12:18:19PM -0500, Stefan Hajnoczi wrote: > C99 mixed declarations support interleaving of local variable > declarations and code. > > The coding style "generally" forbids C99 mixed declarations with some > exceptions to the rule. This rule is not checked by checkpatch.pl and > naturally there are violations in the source tree. > > While contemplating adding another exception, I came to the conclusion > that the best location for declarations depends on context. Let the > programmer declare variables where it is best for legibility. Don't try > to define all possible scenarios/exceptions. IIRC, we had a discussion on this topic sometime last year, but can't remember what the $SUBJECT was, so I'll just repeat what I said then. Combining C99 mixed declarations with 'goto' is dangerous. Consider this program: $ cat jump.c #include int main(int argc, char**argv) { if (getenv("SKIP")) goto target; char *foo = malloc(30); target: free(foo); } $ gcc -Wall -Wuninitialized -o jump jump.c $ SKIP=1 ./jump free(): invalid pointer Aborted (core dumped) -> The programmer thinks they have initialized 'foo' -> GCC thinks the programmer has initialized 'foo' -> Yet 'foo' is not guaranteed to be initialized at 'target:' Given that QEMU makes heavy use of 'goto', allowing C99 mixed declarations exposes us to significant danger. Full disclosure, GCC fails to diagnmose this mistake, even with a decl at start of 'main', but at least the mistake is now more visible to the programmer. Fortunately with -fanalyzer GCC can diagnose this: $ gcc -fanalyzer -Wall -o jump jump.c jump.c: In function ‘main’: jump.c:12:3: warning: use of uninitialized value ‘foo’ [CWE-457] [-Wanalyzer-use-of-uninitialized-value] 12 | free(foo); | ^~~~~~~~~ ‘main’: events 1-5 | | 6 | if (getenv("SKIP")) | | ~ | | | | | (3) following ‘true’ branch... | 7 | goto target; | | ~~~~ | | | | | (4) ...to here | 8 | | 9 | char *foo = malloc(30); | | ^~~ | | | | | (1) region created on stack here | | (2) capacity: 8 bytes |...... | 12 | free(foo); | | ~~~~~~~~~ | | | | | (5) use of uninitialized value ‘foo’ here ...but -fanalyzer isn't something we have enabled by default, it is opt-in. I'm also not sure how comprehensive the flow control analysis of -fanalyzer is ? Can we be sure it'll catch these mistakes in large complex functions with many code paths ? Even if the compiler does reliably warn, I think the code pattern remains misleading to contributors, as the flow control flaw is very non-obvious. Rather than accept the status quo and remove the coding guideline, I think we should strengthen the guidelines, such that it is explicitly forbidden in any method that uses 'goto'. Personally I'd go all the way to -Werror=declaration-after-statement, as while C99 mixed decl is appealing, it isn't exactly a game changer in improving code maintainability. > Signed-off-by: Stefan Hajnoczi > --- > docs/devel/style.rst | 20 -------------------- > 1 file changed, 20 deletions(-) > > diff --git a/docs/devel/style.rst b/docs/devel/style.rst > index 2f68b50079..80c4e4df52 100644 > --- a/docs/devel/style.rst > +++ b/docs/devel/style.rst > @@ -199,26 +199,6 @@ Rationale: a consistent (except for functions...) bracing style reduces > ambiguity and avoids needless churn when lines are added or removed. > Furthermore, it is the QEMU coding style. > > -Declarations > -============ > - > -Mixed declarations (interleaving statements and declarations within > -blocks) are generally not allowed; declarations should be at the beginning > -of blocks. To avoid accidental re-use it is permissible to declare > -loop variables inside for loops: > - > -.. code-block:: c > - > - for (int i = 0; i < ARRAY_SIZE(thing); i++) { > - /* do something loopy */ > - } > - > -Every now and then, an exception is made for declarations inside a > -#ifdef or #ifndef block: if the code looks nicer, such declarations can > -be placed at the top of the block even if there are statements above. > -On the other hand, however, it's often best to move that #ifdef/#ifndef > -block to a separate function altogether. > - > Conditional statements > ====================== > > -- > 2.43.0 > With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|