Re: [syzbot] [bpf?] WARNING in reg_bounds_sanity_check

[Paul Chaignon <paul.chaignon@gmail.com>]

On Mon, Jul 07, 2025 at 05:57:32PM -0700, Eduard Zingerman wrote:
> On Mon, 2025-07-07 at 17:51 -0700, Alexei Starovoitov wrote:
> > On Mon, Jul 7, 2025 at 5:37 PM Eduard Zingerman <eddyz87@gmail.com> wrote:
> > >
> > > On Mon, 2025-07-07 at 16:29 -0700, Eduard Zingerman wrote:
> > > > On Tue, 2025-07-08 at 00:30 +0200, Paul Chaignon wrote:

[...]

> > > But I think the program below would still be problematic:
> > >
> > > SEC("socket")
> > > __success
> > > __retval(0)
> > > __naked void jset_bug1(void)
> > > {
> > >         asm volatile ("                                 \
> > >         call %[bpf_get_prandom_u32];                    \
> > >         if r0 < 2 goto 1f;                              \
> > >         r0 |= 1;                                        \
> > >         if r0 & -2 goto 1f;                             \
> > > 1:      r0 = 0;                                         \
> > >         exit;                                           \
> > > "       :
> > >         : __imm(bpf_get_prandom_u32)
> > >         : __clobber_all);
> > > }
> > >
> > > The possible_r0 would be changed by `if r0 & -2`, so new rule will not hit.
> > > And the problem remains unsolved. I think we need to reset min/max
> > > bounds in regs_refine_cond_op for JSET:
> > > - in some cases range is more precise than tnum
> > > - in these cases range cannot be compressed to a tnum
> > > - predictions in jset are done for a tnum
> > > - to avoid issues when narrowing tnum after prediction, forget the
> > >   range.
> >
> > You're digging too deep. llvm doesn't generate JSET insn,
> > so this is syzbot only issue. Let's address it with minimal changes.
> > Do not introduce fancy branch taken analysis.
> > I would be fine with reverting this particular verifier_bug() hunk.

Ok, if LLVM doesn't generate JSETs, I agree there's not much point
trying to reduce false positives. I like Eduard's solution below
because it handles the JSET case without removing the warning. Given
the number of crashes syzkaller is generating, I suspect this isn't
only about JSET, so it'd be good to keep some visibility into invariant
violations.

> 
> My point is that the fix should look as below (but extract it as a
> utility function):
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 53007182b46b..b2fe665901b7 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -16207,6 +16207,14 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state
>                         swap(reg1, reg2);
>                 if (!is_reg_const(reg2, is_jmp32))
>                         break;
> +               reg1->u32_max_value = U32_MAX;
> +               reg1->u32_min_value = 0;
> +               reg1->s32_max_value = S32_MAX;
> +               reg1->s32_min_value = S32_MIN;
> +               reg1->umax_value = U64_MAX;
> +               reg1->umin_value = 0;
> +               reg1->smax_value = S64_MAX;
> +               reg1->smin_value = S32_MIN;

Looks like __mark_reg_unbounded :)

I can send a test case + __mark_reg_unbounded for BPF_JSET | BPF_X in
regs_refine_cond_op. I suspect we may need the same for the BPF_JSET
case as well, but I'm unable to build a repro for that so far.

>                 val = reg_const_value(reg2, is_jmp32);
>                 if (is_jmp32) {
>                         t = tnum_and(tnum_subreg(reg1->var_off), tnum_const(~val));
> 
> ----
> 
> Because of irreconcilable differences in what can be represented as a
> tnum and what can be represented as a range.