From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [216.205.24.124]) by sourceware.org (Postfix) with ESMTP id 79AAD385042B for ; Tue, 19 Jan 2021 16:54:51 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 79AAD385042B Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-245-dCNpkQBlM4unybsuajhXmg-1; Tue, 19 Jan 2021 11:54:48 -0500 X-MC-Unique: dCNpkQBlM4unybsuajhXmg-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 486A41005504; Tue, 19 Jan 2021 16:54:47 +0000 (UTC) Received: from ovpn-112-159.phx2.redhat.com (ovpn-112-159.phx2.redhat.com [10.3.112.159]) by smtp.corp.redhat.com (Postfix) with ESMTP id 5BD6360D06; Tue, 19 Jan 2021 16:54:46 +0000 (UTC) Message-ID: <93b9dd73b2cac2b53c49dbe4f76d4f8645591328.camel@redhat.com> Subject: Re: [PATCH] add support for -Wmismatched-dealloc From: David Malcolm To: Florian Weimer , Martin Sebor via Libc-alpha Cc: Martin Sebor , Joseph Myers Date: Tue, 19 Jan 2021 11:54:42 -0500 In-Reply-To: <87sg76led0.fsf@oldenburg2.str.redhat.com> References: <74efece7-9a4b-83ee-7fdd-475c0d514378@gmail.com> <758e723b-67cf-a211-7bc2-2ccd3fc744e5@gmail.com> <2555516b-4583-21fc-e844-fd44619488cd@gmail.com> <655918b2-16c6-74b1-6a49-505a7607007f@gmail.com> <87mtxok7ob.fsf@oldenburg2.str.redhat.com> <0aae9006-6001-8fc8-ad6d-c8e3ee60f82c@gmail.com> <87turwiqqw.fsf@oldenburg2.str.redhat.com> <87czybsuoe.fsf@oldenburg2.str.redhat.com> <87sg76led0.fsf@oldenburg2.str.redhat.com> User-Agent: Evolution 3.36.5 (3.36.5-1.fc32) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-7.9 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, KAM_SHORT, RCVD_IN_DNSWL_LOW, RCVD_IN_MSPIKE_H3, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 19 Jan 2021 16:54:52 -0000 On Tue, 2021-01-12 at 09:59 +0100, Florian Weimer wrote: > * Martin Sebor via Libc-alpha: > > > > realpath only returns a pointer to the heap if RESOLVED is null, > > > so > > > the annotation is wrong here. > > This is intentional. When realpath() returns the last argument > > (when it's nonnull) passing the returned pointer to free will not > > be diagnosed but passing it to some other deallocator not > > associated > > with the function will be. That means for example that passing > > a pointer allocated by C++ operator new() to realpath() and then > > deleting the pointer returned from the function as opposed to > > the argument will trigger a false positive. I decided this was > > an okay trade-off because unless the function allocates memory > > I expect the returned pointer to be ignored (similarly to how > > the pointer returned from memcpy is ignored). If you don't like > > the odds I can remove the attribute from the function until we > > have one that captures this conditional return value (I'd like > > to add one in GCC 12). > > Maybe David can comment on how this interacts with his static > analyzer > work. BTW, the -fanalyzer part of support for __attribute__((malloc(deallocator))) is in gcc 11 as of yesterday: https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=c7e276b869bdeb4a95735c1f037ee1a5f629de3d > In all other cases, the attribute means that the pointer needs to > be freed to avoid a resource leak. Indeed, the analyzer doesn't have any special knowledge of realpath and the conditional behavior. Given that annotation it will assume that the returned value is either a pointer that needs to be freed, or NULL on a failure, with no knowledge that the 2nd argument could be returned. I haven't tested Martin's patch, but I tried this example: $ cat t.c #include #define PATH_MAX 4096 void free(void *ptr); char *realpath(const char *path, char *resolved_path) __attribute__ ((malloc (free))) __attribute__ ((__warn_unused_result__)); void test_1 (const char *path) { char buf[PATH_MAX]; char *result = realpath (path, buf); printf ("result: %s\n", result); } void test_2 (const char *path) { char buf[PATH_MAX]; char *result = realpath (path, buf); printf ("result: %s\n", result); free (result); } I believe test_1 is correct (although redundant in its use of "result" rather than buf, and can output a truncated path). I believe test_2 is a crasher bug: a "free" of on-stack "buf". Compiling with GCC 11 (with the __attribute__((malloc (DEALLOCATOR))) support: $ ./xgcc -B. -c -fanalyzer t.c t.c: In function ‘test_1’: t.c:14:1: warning: leak of ‘result’ [CWE-401] [-Wanalyzer-malloc-leak] 14 | } | ^ ‘test_1’: events 1-2 | | 12 | char *result = realpath (path, buf); | | ^~~~~~~~~~~~~~~~~~~~ | | | | | (1) allocated here | 13 | printf ("result: %s\n", result); | 14 | } | | ~ | | | | | (2) ‘result’ leaks here; was allocated at (1) | Here it falsely complains about test_1; it doesn't "know" that buf is returned by realpath and assumes that result needs to be freed. In test_2, there's a free of result == buf i.e. a free of an on-stack buffer, which it doesn't complain about, treating "result" as a malloc- ed pointer (as specified by the attribute). So I don't think this attribute should be applied to realpath. If we suddenly apply it pointers > which can only conditionally be freed, that reduces the value of > those > annotations, I think. FWIW, the analyzer already special-cases some functions; see the various region_model::impl_call_* functions in: https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/analyzer/region-model-impl-calls.cc There's a case for doing this for stuff in POSIX, which would apply here, I think. As noted above, I haven't tested Martin's glibc patch (I don't think I'm subscribed to this list). > Thanks, > Florian Hope this is constructive Dave