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Marchesi" To: Eduard Zingerman Cc: Yonghong Song , bpf@vger.kernel.org, david.faust@oracle.com, cupertino.miranda@oracle.com, Yonghong Song Subject: Re: BPF selftests and strict aliasing In-Reply-To: <6819204566bfae73c140938920eeb389d27abad8.camel@gmail.com> (Eduard Zingerman's message of "Mon, 29 Jan 2024 18:15:39 +0200") References: <87plxmsg37.fsf@oracle.com> <87a5opskz0.fsf@oracle.com> <04efa2a3-ca81-42c3-883f-5b91917f2bde@linux.dev> <6819204566bfae73c140938920eeb389d27abad8.camel@gmail.com> Date: Mon, 29 Jan 2024 18:05:03 +0100 Message-ID: <87sf2gnk8w.fsf@oracle.com> User-Agent: Gnus/5.13 (Gnus v5.13) Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: LNXP265CA0033.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:5c::21) To DM6PR10MB3113.namprd10.prod.outlook.com (2603:10b6:5:1a7::12) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM6PR10MB3113:EE_|SA1PR10MB6638:EE_ X-MS-Office365-Filtering-Correlation-Id: 5f313a5d-d38c-48e8-793b-08dc20ec7173 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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Maybe this require= s >> latest gcc? Or test C file is not sufficient enough to trigger the warni= ng? >>=20 >> [~/tmp1]$ cat t.c >> struct t { >> =C2=A0 char a; >> =C2=A0 short b; >> =C2=A0 int c; >> }; >> void init(struct t *); >> long foo() { >> =C2=A0 struct t dummy; >> =C2=A0 init(&dummy); >> =C2=A0 return *(int *)&dummy; >> } >> [~/tmp1]$ gcc -Wall -Werror -O2 -g -Wno-compare-distinct-pointer-types -= c t.c >> [~/tmp1]$ gcc --version >> gcc (GCC) 11.4.1 20230605 (Red Hat 11.4.1-2) > > I managed to trigger this warning for gcc 13.2.1: > > $ gcc -fstrict-aliasing -Wstrict-aliasing=3D1 -c test-punning.c -o /d= ev/null > test-punning.c: In function =E2=80=98foo=E2=80=99: > test-punning.c:10:19: warning: dereferencing type-punned pointer migh= t break strict-aliasing rules [-Wstrict-aliasing] > 10 | return *(int *)&dummy; > | ^~~~~~ > =20 > Note the -Wstrict-aliasing=3D1 option, w/o =3D1 suffix it does not trigge= r. > > Grepping words "strict-aliasing", "strictaliasing", "strict_aliasing" > through clang code-base does not show any diagnostic related tests or > detection logic. It appears to me clang does not warn about strict > aliasing violations at all and -Wstrict-aliasing=3D* are just stubs at > the moment. Detecting strict aliasing violations can only be done by looking at particular code constructions (casts immediately followed by dereferencing for example) so GCC provides these three levels: 1, 2, and 3 which is the default. Level 1 can result in false positives (hence the "might" in the warning message) while higher levels have less false positives, but will likely miss lots of real positives. In this case, it seems to me clear that a pointer to int does not alias a pointer to struct t. So I would say, in this little program strict-aliasing=3D1 catches a real positive, while strict-aliasing=3D3 misses a real positive.