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Thu, 03 Sep 2026 10:15:55 -0700 (PDT) Received: from ezingerman-fedora-PF4V722J.thefacebook.com ([2620:10d:c090:500::6:dba3]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-3339a534d25sm354893eec.5.2026.09.03.10.15.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 03 Sep 2026 10:15:54 -0700 (PDT) From: Eduard Zingerman To: bpf@vger.kernel.org, ast@kernel.org, andrii@kernel.org Cc: daniel@iogearbox.net, martin.lau@linux.dev, kernel-team@fb.com, yonghong.song@linux.dev, eddyz87@gmail.com, memxor@gmail.com, npc@anthropic.com Subject: [PATCH bpf 3/3] selftests/bpf: check zero extension of an arena 32-bit cmpxchg Date: Thu, 3 Sep 2026 10:15:41 -0700 Message-ID: <20260903171542.1438050-3-eddyz87@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260903171542.1438050-1-eddyz87@gmail.com> References: <20260903171542.1438050-1-eddyz87@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Add a test to verify that destination register of a 32-bit cmpxchg operating on an arena pointer is explicitly zero extended. W/o patch #1 this did not happen. Reported-by: Nicholas Carlini Suggested-by: Nicholas Carlini Signed-off-by: Eduard Zingerman --- .../selftests/bpf/progs/verifier_zext.c | 26 +++++++++++++++++++ 1 file changed, 26 insertions(+) diff --git a/tools/testing/selftests/bpf/progs/verifier_zext.c b/tools/testing/selftests/bpf/progs/verifier_zext.c index 8f2362da91d6..572017fe28fb 100644 --- a/tools/testing/selftests/bpf/progs/verifier_zext.c +++ b/tools/testing/selftests/bpf/progs/verifier_zext.c @@ -356,6 +356,32 @@ __naked void arena_ptr(void) : __clobber_all); } +/* + * Result of a 32-bit cmpxchg is always explicitly zero extended. + * Check that this holds for arenas (BPF_PROBE_ATOMIC instruction flavor). + */ +SEC("socket") +__success +__xlated("probe r0 = atomic_cmpxchg((u32 *)(r1 +0), r0, r2)") +__xlated("w0 = w0") +__naked void zext_arena_cmpxchg32(void) +{ + asm volatile (" \ + r9 = %[arena] ll; /* associate the arena with the program */ \ + r1 = 0; \ + r1 = addr_space_cast(r1, 0, 1); \ + r0 = 0; \ + r2 = 0; \ + .8byte %[cmpxchg32]; \ + r0 >>= 32; /* make the upper half live */ \ + exit; \ +" : + : __imm_addr(arena), + __imm_insn(cmpxchg32, + BPF_ATOMIC_OP(BPF_W, BPF_CMPXCHG, BPF_REG_1, BPF_REG_2, 0)) + : __clobber_all); +} + #endif /* Check if probe mem loads keep their zero extension. */ -- 2.55.0