Re: [PATCH bpf-next v3 1/2] bpf: verifier: Support eliding map lookup nullness

On Tue, Sep 24, 2024 at 12:40 PM Daniel Xu dxu@dxuuu.xyz wrote:

+/* Returns constant key value if possible, else -1 */ +static long get_constant_map_key(struct bpf_verifier_env *env,

•                            struct bpf_reg_state *key)
  

+{

•   struct bpf_func_state *state = func(env, key);
  

•   struct bpf_reg_state *reg;
  

•   int stack_off;
  

•   int slot;
  

•   int spi;
  

•   if (key->type != PTR_TO_STACK)
  

•           return -1;
  

•   if (!tnum_is_const(key->var_off))
  

•           return -1;
  

•   stack_off = key->off + key->var_off.value;
  

•   slot = -stack_off - 1;
  

•   if (slot < 0)
  

•           /* Stack grew upwards */
  

The comment is misleading. The verifier is supposed to catch this. It's just this helper was called before the stack bounds were checked? Maybe the call can be done later?

•           return -1;
  

•   else if (slot >= state->allocated_stack)
  

•           /* Stack uninitialized */
  

•           return -1;
  

•   spi = slot / BPF_REG_SIZE;
  

•   reg = &state->stack[spi].spilled_ptr;
  

•   if (!tnum_is_const(reg->var_off))
  

•           /* Stack value not statically known */
  

•           return -1;
  

•   return reg->var_off.value;
  

+}

Looks like the code is more subtle than it looks.

I think it's better to guard it all with CAP_BPF.

pw-bot: cr