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

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Date:   Thu, 14 Dec 2023 18:26:13 +0200
From:   Eduard Zingerman <eddyz87@...il.com>
To:     Hao Sun <sunhao.th@...il.com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Bug Report] bpf: incorrectly pruning runtime execution path

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.