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Sat, 17 Oct 2026 07:35:41 -0700 (PDT) Content-Type: text/plain; charset=UTF-8 Date: Sat, 17 Oct 2026 14:35:40 +0000 Message-Id: Subject: Re: [PATCH bpf] bpf: Zero-extend the index in inlined bpf_get_func_arg() From: "Alexei Starovoitov" To: "Woojin Ji" , In-Reply-To: <179221432098.1890560.17043109037413683447.get-func-arg-index@gmail.com> References: <179221432098.1890560.17043109037413683447.get-func-arg-index@gmail.com> X-Mailer: mkdraft (claude review draft; edit before sending) Content-Transfer-Encoding: 8bit Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Sat, Oct 17, 2026 at 02:42 PM Woojin Ji wrote: > + /* > + * The helper takes a u32 index; zero-extend it before > + * the bounds check and the 64-bit address math below. > + */ > + insn_buf[cnt++] = BPF_MOV32_REG(BPF_REG_2, BPF_REG_2); > insn_buf[cnt++] = BPF_JMP32_REG(BPF_JGE, BPF_REG_2, BPF_REG_0, 6); > insn_buf[cnt++] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_2, 3); After JMP32 the low 32 bits of R2 are below nr_args, which is at most 255, so a 32-bit shift cannot overflow and it clears the upper bits. can we just make the shift 32-bit ? Then no need for the extra insn and the comment. The selftest stays as is. Something like this (compile tested only): diff --git a/kernel/bpf/fixups.c b/kernel/bpf/fixups.c index d6f8352..ec62df6 100644 --- a/kernel/bpf/fixups.c +++ b/kernel/bpf/fixups.c @@ -2256,7 +2256,7 @@ patch_map_ops_generic: cnt = 2; } insn_buf[cnt++] = BPF_JMP32_REG(BPF_JGE, BPF_REG_2, BPF_REG_0, 6); - insn_buf[cnt++] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_2, 3); + insn_buf[cnt++] = BPF_ALU32_IMM(BPF_LSH, BPF_REG_2, 3); insn_buf[cnt++] = BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1); insn_buf[cnt++] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_2, 0); insn_buf[cnt++] = BPF_STX_MEM(BPF_DW, BPF_REG_3, BPF_REG_0, 0); pw-bot: cr