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Mon, 13 Apr 2026 14:43:28 -0700 (PDT) Message-ID: <5258cc4b95c2c835b699598f0ce24b3a318b5bc3.camel@gmail.com> Subject: Re: [PATCH] bpf: btf: reject division by zero in btf_struct_walk for zero-size flex array elements From: Eduard Zingerman To: Dudu Lu , bpf@vger.kernel.org Cc: martin.lau@linux.dev, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org Date: Mon, 13 Apr 2026 14:43:25 -0700 In-Reply-To: <20260413085033.71952-1-phx0fer@gmail.com> References: <20260413085033.71952-1-phx0fer@gmail.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.58.3 (3.58.3-1.fc43) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Mon, 2026-04-13 at 16:50 +0800, Dudu Lu wrote: > btf_struct_walk() computes `off =3D (off - moff) % t->size` when handling > access to a flexible array member. If the element type is a zero-size > struct (e.g., `struct B {}`), t->size is 0 and this causes a division- > by-zero exception. On x86 this triggers a #DE fault resulting in a > kernel panic. >=20 > The BTF validator does not reject zero-size structs: btf_struct_check_met= a() > passes trivially for vlen=3D0 structs, and btf_array_resolve() skips the > overflow check for flex arrays (nelems=3D0). This allows a user to load > crafted BTF containing `struct B {}; struct A { int x; struct B flex[]; }= ` > and then trigger the crash when the verifier calls btf_struct_walk() > during program load. Could you please add a selftest as well? [...]