Re: BPF: bug without effect in BPF_RSH case of adjust_scalar_min_max_vals()

On Mon, Dec 4, 2017 at 6:03 PM, Jann Horn [off-list ref] wrote:

As far as I can tell, commit b03c9f9fdc37 ("bpf/verifier: track signed
and unsigned min/max values") introduced the following effectless bug
in the BPF_RSH case of adjust_scalar_min_max_vals() (unless that's
intentional):

[...]

=======================
        BPF_LD_MAP_FD(BPF_REG_ARG1, mapfd),

        BPF_MOV64_REG(BPF_REG_TMP, BPF_REG_FP),
        BPF_ALU64_IMM(BPF_ADD, BPF_REG_TMP, -4), // allocate 4 bytes stack
        BPF_MOV32_IMM(BPF_REG_ARG2, 1),
        BPF_STX_MEM(BPF_W, BPF_REG_TMP, BPF_REG_ARG2, 0),
        BPF_MOV64_REG(BPF_REG_ARG2, BPF_REG_TMP),
        BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
        BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 2),
        BPF_MOV64_REG(BPF_REG_0, 0), // prepare exit
        BPF_EXIT_INSN(), // exit
        BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_0, 0),

        BPF_ALU64_IMM(BPF_AND, BPF_REG_3, 0xf),
        BPF_MOV64_IMM(BPF_REG_1, -42),
        BPF_ALU64_REG(BPF_ADD, BPF_REG_1, BPF_REG_3),
        BPF_MOV64_IMM(BPF_REG_2, 2),
        BPF_ALU64_REG(BPF_RSH, BPF_REG_1, BPF_REG_2),
        BPF_EXIT_INSN()
=======================

For using the eBPF bytecode in selftests:

Signed-off-by: Jann Horn <jannh@google.com>