Re: [PATCH v2 bpf-next 2/6] bpf: Use bpf_verifier_env buffers for reg_set_min_max

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

On Mon, Mar 30, 2026 at 06:51:04PM -0700, Eduard Zingerman wrote:
> On Mon, 2026-03-30 at 14:05 +0200, Paul Chaignon wrote:
> > On Mon, Mar 23, 2026 at 11:42:11AM -0700, Eduard Zingerman wrote:
> > > On Fri, 2026-03-20 at 17:49 +0100, Paul Chaignon wrote:
> > 
> > [...]
> > 
> > > > @@ -17196,30 +17192,23 @@ static int reg_set_min_max(struct bpf_verifier_env *env,
> > > >  	 * variable offset from the compare (unless they were a pointer into
> > > >  	 * the same object, but we don't bother with that).
> > > >  	 */
> > > > -	if (false_reg1->type != SCALAR_VALUE || false_reg2->type != SCALAR_VALUE)
> > > > -		return 0;
> > > > -
> > > > -	/* We compute branch direction for same SCALAR_VALUE registers in
> > > > -	 * is_scalar_branch_taken(). For unknown branch directions (e.g., BPF_JSET)
> > > > -	 * on the same registers, we don't need to adjust the min/max values.
> > > > -	 */
> > > > -	if (false_reg1 == false_reg2)
> > > 
> > > A side note:
> > > 
> > > The above hunk was added as a part of [1] to mitigate some invariant
> > > violation errors. Surprisingly, none of the tests added in [1] fail
> > > on current master if above hunk is commented out. Probably due to
> > > recent improvements in bounds deduction. Should we remove these
> > > tests as a part of the series?
> > > 
> > > [1] https://lore.kernel.org/all/20251103063108.1111764-3-kafai.wan@linux.dev/
> > 
> > Nice catch! Out of those five new tests, the three "jset on same
> > register, scalar value unknown branch" never fail if you revert the
> > commit they were testing, even at the time they were added.
> 
> When I get back to [1] and revert [2] I see the following test failing:
> 
>   verifier_bounds/jset on same register, scalar value branch taken
> 
> The other two indeed pass.

I think we're saying the same thing :) My always failing tests are those
ending with "value unknown branch [1-3]", while the one ending with
"value branch taken" indeed fails as expected. I also forgot to mention
before that "jset on same register, constant value branch taken" also
never fails.

So to sum up, these never fail:
  - jset on same register, constant value branch taken
  - jset on same register, scalar value unknown branch 1
  - jset on same register, scalar value unknown branch 2
  - jset on same register, scalar value unknown branch 3

While these fail as expected:
  - conditional jump on same register, branch taken
  - jset on same register, scalar value branch taken

Are you suggesting to remove the first four?

> 
> [1] 9f32bfec545c ("selftests/bpf: Add test for conditional jumps on same scalar register")
> [2] d43ad9da8052 ("bpf: Skip bounds adjustment for conditional jumps on same scalar register")
> 
> > I believe
> > these three tests were intended to cover the above "false_reg1 ==
> > false_reg2" check and supposed to fail with an invariant violation when
> > the check is missing.
> > 
> > I believe this check was never actually needed. For an invariant
> > violation to happen, we need regs_refine_cond_op to refine a register
> > based on a incorrectly-detected branch being verified. For jset, that
> > can only happen if one of the two registers is constant. In our case,
> > that would mean both registers are constant. But if both registers are
> > constant, then is_scalar_branch_taken is always able to precisely
> > deduce the outcome of the jset. Hence, we wouldn't even reach this
> > "false_reg1 == false_reg2" check.
> > 
> > I think I'll remove this check in a preparatory commit, along with the
> > related selftests and an explanation why it's all not-needed. Cc'ing
> > KaFai Wan in case I missed something.
> > 
> > > 
> > > [...]