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([2620:10d:c090:500::4:d279]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-39d9d8cd962sm102522a91.2.2026.09.10.14.49.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 10 Sep 2026 14:49:45 -0700 (PDT) Message-ID: <87c1060d5233c9b7a73fcaf1b6d96bc1a700c49e.camel@gmail.com> Subject: Re: [PATCH bpf 1/2] bpf: Fix divide-by-zero in btf_struct_walk() From: Eduard Zingerman To: Jiayuan Chen , bpf@vger.kernel.org Cc: Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Kumar Kartikeya Dwivedi , Martin KaFai Lau , Song Liu , Yonghong Song , Jiri Olsa , Emil Tsalapatis , Ihor Solodrai , Shuah Khan , Amery Hung , Tejun Heo , Matt Bobrowski , linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Date: Thu, 10 Sep 2026 14:49:42 -0700 In-Reply-To: <20260910122316.186384-1-jiayuan.chen@linux.dev> References: <20260910122316.186384-1-jiayuan.chen@linux.dev> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.2 (3.60.2-1.fc44) Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Thu, 2026-09-10 at 20:22 +0800, Jiayuan Chen wrote: > When an access goes past the struct and the last member is a flexible > array, btf_struct_walk() folds the offset back into a single element with > (off - moff) % t->size, but never checks that the element type has a size= . >=20 > BTF takes an empty struct, so this in program BTF >=20 > /* event could be empty */ > struct event { > #ifdef HAVE_TIMESTAMP > __u64 ts; > #endif > }; >=20 > struct batch { > int nr; > struct event events[]; > }; >=20 > divides by zero at prog load time. Getting there needs a PTR_TO_BTF_ID th= at > is not MEM_ALLOC, e.g. a plain read of a local kptr stashed in a map from= a > sleepable program. >=20 > Oops: divide error: 0000 [#1] SMP KASAN PTI > RIP: 0010:btf_struct_walk+0x53f/0x1570 > Call Trace: > > btf_struct_access+0x42a/0xcd0 > check_ptr_to_btf_access+0x4dc/0x1160 > check_mem_access+0x3a45/0x8740 > check_load_mem+0x36a/0xd10 > do_check_common+0x3ef0/0xb210 > bpf_check+0x6d3b/0x8580 > bpf_prog_load+0xf7c/0x2720 > __sys_bpf+0xa83/0x3690 > __x64_sys_bpf+0xc7/0x150 > x64_sys_call+0x1f3f/0x27e0 > do_syscall_64+0xe5/0x610 > entry_SYSCALL_64_after_hwframe+0x76/0x7e > >=20 > Reject a zero-sized element type. The fixed array path in the same functi= on > already bails out on the same thing: >=20 > btf_struct_walk() > ... > /* skip empty array */ > if (moff =3D=3D mtrue_end) > continue; >=20 > msize /=3D total_nelems; >=20 > Fixes: 9c5f8a1008a1 ("bpf: Support variable length array in tracing progr= ams") > Signed-off-by: Jiayuan Chen > --- Acked-by: Eduard Zingerman