[...] > 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.
