On Fri, Sep 20, 2024 at 03:05:35PM GMT, Eduard Zingerman wrote:
> On Sun, 2024-09-15 at 21:45 -0600, Daniel Xu wrote:
> > This commit allows progs to elide a null check on statically known map
> > lookup keys. In other words, if the verifier can statically prove that
> > the lookup will be in-bounds, allow the prog to drop the null check.
> >
> > This is useful for two reasons:
> >
> > 1. Large numbers of nullness checks (especially when they cannot fail)
> > unnecessarily pushes prog towards BPF_COMPLEXITY_LIMIT_JMP_SEQ.
> > 2. It forms a tighter contract between programmer and verifier.
> >
> > For (1), bpftrace is starting to make heavier use of percpu scratch
> > maps. As a result, for user scripts with large number of unrolled loops,
> > we are starting to hit jump complexity verification errors. These
> > percpu lookups cannot fail anyways, as we only use static key values.
> > Eliding nullness probably results in less work for verifier as well.
> >
> > For (2), percpu scratch maps are often used as a larger stack, as the
> > currrent stack is limited to 512 bytes. In these situations, it is
> > desirable for the programmer to express: "this lookup should never fail,
> > and if it does, it means I messed up the code". By omitting the null
> > check, the programmer can "ask" the verifier to double check the logic.
>
> Nit: maybe add a few lines why tools/testing/selftests/bpf/progs/iters.c
> has to be changed.
Ack.
>
> [...]
>
> > +/* 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 >= state->allocated_stack)
> > + /* Stack uninitialized */
> > + return -1;
>
> I'm not sure verifier guarantees that key->off is negative.
> E.g. the following simple program:
>
> 0: (b7) r1 = 16 ; R1_w=16
> 1: (bf) r2 = r10 ; R2_w=fp0 R10=fp0
> 2: (0f) r2 += r1
> mark_precise: frame0: last_idx 2 first_idx 0 subseq_idx -1
> mark_precise: frame0: regs=r1 stack= before 1: (bf) r2 = r10
> mark_precise: frame0: regs=r1 stack= before 0: (b7) r1 = 16
> 3: R1_w=16 R2_w=fp16
>
> => I think 'slot' should be checked to be >= 0.
Ah, so in case stack grows "up" right? Which seems invalid but probably
good to check.
>
> > +
> > + 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;
> > +}
> > +
> > static int get_helper_proto(struct bpf_verifier_env *env, int func_id,
> > const struct bpf_func_proto **ptr)
> > {
> > @@ -10511,6 +10557,15 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
> > env->insn_aux_data[insn_idx].storage_get_func_atomic = true;
> > }
> >
> > + /* Logically we are trying to check on key register state before
> > + * the helper is called, so process here. Otherwise argument processing
> > + * may clobber the spilled key values.
> > + */
> > + regs = cur_regs(env);
> > + if (func_id == BPF_FUNC_map_lookup_elem)
> > + meta.const_map_key = get_constant_map_key(env, ®s[BPF_REG_2]);
>
> Nit: there is a long 'switch (func_id)' slightly below this point,
> maybe move this check there?
I had that initially but discovered that verifier marks the stack value
as unknown as part of check_func_arg(). I _think_ it was:
if (is_spilled_reg(&state->stack[spi]) &&
(state->stack[spi].spilled_ptr.type == SCALAR_VALUE ||
env->allow_ptr_leaks)) {
if (clobber) {
__mark_reg_unknown(env, &state->stack[spi].spilled_ptr);
for (j = 0; j < BPF_REG_SIZE; j++)
scrub_spilled_slot(&state->stack[spi].slot_type[j]);
}
goto mark;
}
I remember spending some time debugging it. Which is why I left that
comment above this code.
Thanks for reviewing!
