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Marchesi" To: Yonghong Song Cc: bpf@vger.kernel.org, david.faust@oracle.com, cupertino.miranda@oracle.com, Eduard Zingerman Subject: Re: [PATCH bpf-next] bpf: avoid uninitialized value in BPF_CORE_READ_BITFIELD In-Reply-To: <263a563a-3abe-4c88-8a1e-e10fb8a6dfad@linux.dev> (Yonghong Song's message of "Tue, 7 May 2024 09:23:31 -0700") References: <20240507113950.28208-1-jose.marchesi@oracle.com> <263a563a-3abe-4c88-8a1e-e10fb8a6dfad@linux.dev> Date: Tue, 07 May 2024 20:13:34 +0200 Message-ID: <87zft1cwxd.fsf@oracle.com> User-Agent: Gnus/5.13 (Gnus v5.13) Content-Type: text/plain X-ClientProxiedBy: LO2P265CA0162.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:9::30) 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_|BN0PR10MB5128:EE_ X-MS-Office365-Filtering-Correlation-Id: acf3afef-d76e-41e3-41b5-08dc6ec16aea X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230031|1800799015|366007|376005; 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Marchesi wrote: >> GCC warns that `val' may be used uninitialized in the >> BPF_CORE_READ_BITFIELD macro, defined in bpf_core_read.h as: >> >> [...] >> unsigned long long val; \ >> [...] \ >> switch (__CORE_RELO(s, field, BYTE_SIZE)) { \ >> case 1: val = *(const unsigned char *)p; break; \ >> case 2: val = *(const unsigned short *)p; break; \ >> case 4: val = *(const unsigned int *)p; break; \ >> case 8: val = *(const unsigned long long *)p; break; \ >> } \ >> [...] >> val; \ >> } \ >> >> This patch initializes `val' to zero in order to avoid the warning, >> and random values to be used in case __builtin_preserve_field_info >> returns unexpected values for BPF_FIELD_BYTE_SIZE. > > In clang, __builtin_preserve_field_info either returns correct value > or caused compilation error. Do you mean for gcc __builtin_preserve_field_info > might return an unexpected value here? The __builtin_preserve_field_info implementation in GCC will emit an error if the size of the bitfield is not a power of two. It doesn't check that the bitfield is 64-bit or smaller, but that should not be a problem. So I would say we are ok there. > BTW, your change makes sense to silent this warning. So Ack below. > > >> >> Tested in bpf-next master. >> No regressions. >> >> Signed-off-by: Jose E. Marchesi >> Cc: david.faust@oracle.com >> Cc: cupertino.miranda@oracle.com >> Cc: Eduard Zingerman >> Cc: Yonghong Song > > Acked-by: Yonghong Song > >> --- >> tools/lib/bpf/bpf_core_read.h | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h >> index b5c7ce5c243a..88d129b5f0a1 100644 >> --- a/tools/lib/bpf/bpf_core_read.h >> +++ b/tools/lib/bpf/bpf_core_read.h >> @@ -89,7 +89,7 @@ enum bpf_enum_value_kind { >> */ >> #define BPF_CORE_READ_BITFIELD(s, field) ({ \ >> const void *p = (const void *)s + __CORE_RELO(s, field, BYTE_OFFSET); \ >> - unsigned long long val; \ >> + unsigned long long val = 0; \ >> \ >> /* This is a so-called barrier_var() operation that makes specified \ >> * variable "a black box" for optimizing compiler. \