Re: [Bug Report] bpf: incorrectly pruning runtime execution path

[Eduard Zingerman <eddyz87@gmail.com>]

[...]
> The reason why retval checks fails is that the way you disable dead
> code removal pass is not complete. Disable opt_remove_dead_code()
> just prevent the instruction #30 from being removed, but also note
> opt_hard_wire_dead_code_branches(), which convert conditional jump
> into unconditional one, so #30 is still skipped.
> 
> > Note that I tried this test with two functions:
> > - bpf_get_current_cgroup_id, with this function I get retval 2, not 4 :)
> > - bpf_get_prandom_u32, with this function I get a random retval each time.
> > 
> > What is the expectation when 'bpf_get_current_cgroup_id' is used?
> > That it is some known (to us) number, but verifier treats it as unknown scalar?
> > 
> 
> Either one would work, but to make #30 always taken, r0 should be
> non-zero.

Oh, thank you, I made opt_hard_wire_dead_code_branches() a noop,
replaced r0 = 0x4 by r0 /= 0 and see "divide error: 0000 [#1] PREEMPT SMP NOPTI"
error in the kernel log on every second or third run of the test
(when using prandom).

Working to minimize the test case will share results a bit later.