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Wed, 12 Aug 2026 09:40:51 +0000 (GMT) Received: from [0.0.0.0] (unknown [9.111.53.227]) by smtpav01.fra02v.mail.ibm.com (Postfix) with ESMTP; Wed, 12 Aug 2026 09:40:51 +0000 (GMT) Message-ID: Date: Wed, 12 Aug 2026 11:40:51 +0200 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH bpf-next v3 5/6] bpf, s390: Clear fetch destination on faulting arena atomic To: Daniel Borkmann , memxor@gmail.com Cc: eddyz87@gmail.com, puranjay@kernel.org, bpf@vger.kernel.org References: <20260811131600.506721-1-daniel@iogearbox.net> <20260811131600.506721-5-daniel@iogearbox.net> Content-Language: en-US From: Ilya Leoshkevich In-Reply-To: <20260811131600.506721-5-daniel@iogearbox.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Authority-Analysis: v=2.4 cv=RsP16imK c=1 sm=1 tr=0 ts=6a7c3fa6 cx=c_pps a=aDMHemPKRhS1OARIsFnwRA==:117 a=aDMHemPKRhS1OARIsFnwRA==:17 a=IkcTkHD0fZMA:10 a=Sv0fKeRqtYgA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=RzCfie-kr_QcCd8fBx8p:22 a=hWMQpYRtAAAA:8 a=VnNF1IyMAAAA:8 a=A3LaVL0a1cGBXct1hZYA:9 a=QEXdDO2ut3YA:10 a=KCsI-UfzjElwHeZNREa_:22 X-Proofpoint-GUID: zTjtQo5tZIBYbIKyHEPOMLENHEmid2Zq X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwODEyMDA3NiBTYWx0ZWRfX79eXsRCQZL2a OSmXM8Z9p8WP2fadElV7seFDz325GuDEfzk0Q3NsjiJypOyJ+SiEqJ9I4/NGh6Iz9xRbqvKBB8o wDNnSzGp3Xv2MCG7C0IpRPO8xkQwq1pxA1MERUDzGd2nM+15/6F8WBPByEVgWxzRSoSHcWBRX45 8Eb+81PfiKW2rJmPLSPAbOWGnzpkkO1nzxeqVJmq9BuGYhT7UKTW6ZxNs/8JLIcGgp8qlbrEMzo HAxb9tXWA0iI+1mlt5FPTviipDgtkNRtFBGW32V2RcogUjDVBdu2YLNPl915Npwbp7swJwhNc5t lpj1N4xeiEq48REVF0LHX2lcXO0fL9TpekyZZtD8roAHbkoiKTznxAZete7V/gW996AZokjfGEO NNkCK+S4Dqbm1kMZLicy9tA2uc0Ty1KNjWbo0GECHykSwBKCihB/4PFUfjYL3+MgJEGyDX2LrIs V/mQgGdDdkZsfMeQB8A== X-Proofpoint-ORIG-GUID: k8wGKye-FNYs7KEHbBHOEQpWGAWBc-8q X-Proofpoint-Spam-Info: AW1haW4tMjYwODEyMDA3NiBTYWx0ZWRfX95P4/hF0IEQy HGojgd/i2VzHkIdJH0w+aH6CmkMD6VFU7eC2VgRcXVDB2y79CaPjxq14rikXS4w8+mEyZmKPCLE e1HRliCrRfNcP7GaYLhUYcyc0QCCokg= X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-08-12_02,2026-08-10_03,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 impostorscore=0 suspectscore=0 clxscore=1011 malwarescore=0 phishscore=0 adultscore=0 lowpriorityscore=0 priorityscore=1501 bulkscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2608120076 On 8/11/26 15:15, Daniel Borkmann wrote: > Same missing register clear as on riscv64. A RMW atomic on an arena pointer > is converted to BPF_PROBE_ATOMIC and gets an exception table entry, but > bpf_jit_probe_atomic_pre() only fills in the arena base and the probe > offset, leaving probe->reg at the -1 that bpf_jit_probe_init() set, which > bpf_jit_probe_post() writes into the entry and ex_handler_bpf() then reads > back as "there is nothing to clear". > > That is right for a plain BPF_{ADD,AND,OR,XOR}, which only writes memory, > but an RMW carrying BPF_FETCH also reads the old value into a register: > src_reg for BPF_{ADD,AND,OR,XOR} | BPF_FETCH and BPF_XCHG, and r0 for > BPF_CMPXCHG. So on a fault over an unmapped arena page the program resumes > at the landing pad with whatever that register held before the atomic > instead of the 0 that every other BPF_PROBE_* access delivers. > > Fill probe->reg in from bpf_atomic_load_reg(). Unlike x86-64 and arm64, > s390x does not report arena violations from its exception handler, so there > is no access direction to correct here, only the missing register clear. > > Fixes: 2f9469484a3b ("s390/bpf: Support arena atomics") > Signed-off-by: Daniel Borkmann > Cc: Ilya Leoshkevich > --- > arch/s390/net/bpf_jit_comp.c | 11 +++++++++++ > 1 file changed, 11 insertions(+) Looks reasonable, thanks! Reviewed-by: Ilya Leoshkevich