bpf: exact states comparison for iterator convergence checks

Convergence for open coded iterators is computed in is_state_visited()
by examining states with branches count > 1 and using states_equal().
states_equal() computes sub-state relation using read and precision marks.
Read and precision marks are propagated from children states,
thus are not guaranteed to be complete inside a loop when branches
count > 1. This could be demonstrated using the following unsafe program:

     1. r7 = -16
     2. r6 = bpf_get_prandom_u32()
     3. while (bpf_iter_num_next(&fp[-8])) {
     4.   if (r6 != 42) {
     5.     r7 = -32
     6.     r6 = bpf_get_prandom_u32()
     7.     continue
     8.   }
     9.   r0 = r10
    10.   r0 += r7
    11.   r8 = *(u64 *)(r0 + 0)
    12.   r6 = bpf_get_prandom_u32()
    13. }

Here verifier would first visit path 1-3, create a checkpoint at 3
with r7=-16, continue to 4-7,3 with r7=-32.

Because instructions at 9-12 had not been visitied yet existing
checkpoint at 3 does not have read or precision mark for r7.
Thus states_equal() would return true and verifier would discard
current state, thus unsafe memory access at 11 would not be caught.

This commit fixes this loophole by introducing exact state comparisons
for iterator convergence logic:
- registers are compared using regs_exact() regardless of read or
  precision marks;
- stack slots have to have identical type.

Unfortunately, this is too strict even for simple programs like below:

    i = 0;
    while(iter_next(&it))
      i++;

At each iteration step i++ would produce a new distinct state and
eventually instruction processing limit would be reached.

To avoid such behavior speculatively forget (widen) range for
imprecise scalar registers, if those registers were not precise at the
end of the previous iteration and do not match exactly.

This a conservative heuristic that allows to verify wide range of
programs, however it precludes verification of programs that conjure
an imprecise value on the first loop iteration and use it as precise
on the second.

Test case iter_task_vma_for_each() presents one of such cases:

        unsigned int seen = 0;
        ...
        bpf_for_each(task_vma, vma, task, 0) {
                if (seen >= 1000)
                        break;
                ...
                seen++;
        }

Here clang generates the following code:

<LBB0_4>:
      24:       r8 = r6                          ; stash current value of
                ... body ...                       'seen'
      29:       r1 = r10
      30:       r1 += -0x8
      31:       call bpf_iter_task_vma_next
      32:       r6 += 0x1                        ; seen++;
      33:       if r0 == 0x0 goto +0x2 <LBB0_6>  ; exit on next() == NULL
      34:       r7 += 0x10
      35:       if r8 < 0x3e7 goto -0xc <LBB0_4> ; loop on seen < 1000

<LBB0_6>:
      ... exit ...

Note that counter in r6 is copied to r8 and then incremented,
conditional jump is done using r8. Because of this precision mark for
r6 lags one state behind of precision mark on r8 and widening logic
kicks in.

Adding barrier_var(seen) after conditional is sufficient to force
clang use the same register for both counting and conditional jump.

This issue was discussed in the thread [1] which was started by
Andrew Werner <awerner32@gmail.com> demonstrating a similar bug
in callback functions handling. The callbacks would be addressed
in a followup patch.

[1] https://lore.kernel.org/bpf/97a90da09404c65c8e810cf83c94ac703705dc0e.camel@gmail.com/

Co-developed-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Co-developed-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231024000917.12153-4-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>