On Thu, 2023-12-14 at 17:10 +0200, Eduard Zingerman wrote: > [...] > > 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. Here is the minimized version of the test: https://gist.github.com/eddyz87/fb4d3c7d5aabdc2ae247ed73fefccd32 If executed several times: ./test_progs -vvv -a verifier_and/pruning_test it eventually crashes VM with the following error: [ 2.039066] divide error: 0000 [#1] PREEMPT SMP NOPTI ... [ 2.039987] Call Trace: [ 2.039987] <TASK> [ 2.039987] ? die+0x36/0x90 [ 2.039987] ? do_trap+0xdb/0x100 [ 2.039987] ? bpf_prog_32cfdb2c00b08250_pruning_test+0x4d/0x60 [ 2.039987] ? do_error_trap+0x7d/0x110 [ 2.039987] ? bpf_prog_32cfdb2c00b08250_pruning_test+0x4d/0x60 [ 2.039987] ? exc_divide_error+0x38/0x50 [ 2.039987] ? bpf_prog_32cfdb2c00b08250_pruning_test+0x4d/0x60 [ 2.039987] ? asm_exc_divide_error+0x1a/0x20 [ 2.039987] ? bpf_prog_32cfdb2c00b08250_pruning_test+0x4d/0x60 [ 2.039987] bpf_test_run+0x1b5/0x350 [ 2.039987] ? bpf_test_run+0x115/0x350 ... I'll continue debugging this a bit later today.
