Re: [PATCH bpf-next] bpf: make the verifier trace the "not qeual" for regs

[Menglong Dong]

On Tue, Dec 12, 2023 at 3:16 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Sun, Dec 10, 2023 at 5:00 AM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
> >
> > We can derive some new information for BPF_JNE in regs_refine_cond_op().
> > Take following code for example:
> >
> >   /* The type of "a" is u16 */
> >   if (a > 0 && a < 100) {
> >     /* the range of the register for a is [0, 99], not [1, 99],
> >      * and will cause the following error:
> >      *
> >      *   invalid zero-sized read
> >      *
> >      * as a can be 0.
> >      */
> >     bpf_skb_store_bytes(skb, xx, xx, a, 0);
> >   }
> >
> > In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the
> > TRUE branch, the dst_reg will be marked as known to 0. However, in the
> > fallthrough(FALSE) branch, the dst_reg will not be handled, which makes
> > the [min, max] for a is [0, 99], not [1, 99].
> >
> > For BPF_JNE, we can reduce the range of the dst reg if the src reg is a
> > const and is exactly the edge of the dst reg.
> >
> > Signed-off-by: Menglong Dong <menglong8.dong@xxxxxxxxx>
> > ---
> >  kernel/bpf/verifier.c | 45 ++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 44 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 727a59e4a647..7b074ac93190 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -1764,6 +1764,40 @@ static void __mark_reg_const_zero(struct bpf_reg_state *reg)
> >         reg->type = SCALAR_VALUE;
> >  }
> >
> > +#define CHECK_REG_MIN(value)                   \
> > +do {                                           \
> > +       if ((value) == (typeof(value))imm)      \
> > +               value++;                        \
> > +} while (0)
> > +
> > +#define CHECK_REG_MAX(value)                   \
> > +do {                                           \
> > +       if ((value) == (typeof(value))imm)      \
> > +               value--;                        \
> > +} while (0)
> > +
> > +static void mark_reg32_not_equal(struct bpf_reg_state *reg, u64 imm)
> > +{
> > +               CHECK_REG_MIN(reg->s32_min_value);
> > +               CHECK_REG_MAX(reg->s32_max_value);
> > +               CHECK_REG_MIN(reg->u32_min_value);
> > +               CHECK_REG_MAX(reg->u32_max_value);
> > +}
> > +
> > +static void mark_reg_not_equal(struct bpf_reg_state *reg, u64 imm)
> > +{
> > +               CHECK_REG_MIN(reg->smin_value);
> > +               CHECK_REG_MAX(reg->smax_value);
> > +
> > +               CHECK_REG_MIN(reg->umin_value);
> > +               CHECK_REG_MAX(reg->umax_value);
> > +
> > +               CHECK_REG_MIN(reg->s32_min_value);
> > +               CHECK_REG_MAX(reg->s32_max_value);
> > +               CHECK_REG_MIN(reg->u32_min_value);
> > +               CHECK_REG_MAX(reg->u32_max_value);
> > +}
>
> please don't use macros for this, this code is tricky enough without
> having to jump around double-checking what exactly macros are doing.
> Just code it explicitly.
>

Okay!

> Also I don't see the need for mark_reg32_not_equal() and
> mark_reg_not_equal() helper functions, there is just one place where
> this logic is going to be called from, so let's add code right there.
>

Yeah, you are right. And I just found that you have already
implemented the test case for this logic in reg_bounds.c/range_cond().
I wonder why this logic is not implemented in the verifier yet?
Am I missing something?

Thanks!
Menglong Dong

> > +
> >  static void mark_reg_known_zero(struct bpf_verifier_env *env,
> >                                 struct bpf_reg_state *regs, u32 regno)
> >  {
> > @@ -14332,7 +14366,16 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state
> >                 }
> >                 break;
> >         case BPF_JNE:
> > -               /* we don't derive any new information for inequality yet */
> > +               /* try to recompute the bound of reg1 if reg2 is a const and
> > +                * is exactly the edge of reg1.
> > +                */
> > +               if (is_reg_const(reg2, is_jmp32)) {
> > +                       val = reg_const_value(reg2, is_jmp32);
> > +                       if (is_jmp32)
> > +                               mark_reg32_not_equal(reg1, val);
> > +                       else
> > +                               mark_reg_not_equal(reg1, val);
> > +               }
> >                 break;
> >         case BPF_JSET:
> >                 if (!is_reg_const(reg2, is_jmp32))
> > --
> > 2.39.2
> >